Home / Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules

Stereochemistry and Chirality

By James Ashenhurst

  • Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules

Last updated: February 20th, 2024 |

Assigning  R and  S Configurations With the Cahn-Ingold-Prelog (CIP) Rules

  • Enantiomers are stereoisomers that are non-superimposable mirror images of each other (by the way, molecules that are superimposable mirror images of each other are considered to be identical molecules). 
  • Enantiomers rotate plane-polarized light in equal and opposite directions, but there is no straightforward way to trace back the absolute configuration of a molecule to the direction of optical rotation
  • A tetrahedral atom with four different substituents (a chiral center) can have two different configurations. A naming scheme developed by Cahn, Ingold and Prelog (CIP) is used for assigning the terms  R or  S  to each chiral center.  (When all the (R,S) designations of a molecule are specified, this is referred to as its “ absolute configuration”.)
  • In the CIP protocol, each atom attached to the chiral center is ranked in order of atomic number (highest = #1, lowest = #4).  (We go further down the chain in the event of ties. )
  • With the #4 substituent in the back: if #1, #2, and #3 trace a clockwise path, the chiral center is assigned (R) . If they trace a counterclockwise path the chiral center is (S) .
  • When #4 is in the front or on the side, some useful tips and tricks can be used to avoid having to rotate the whole molecule ( See also: How To Draw The Enantiomer of A Chiral Molecule ).
  • For breaking ties, it’s useful to keep track of which carbons you’re working on with the “dot method”.

Table of Contents

  • Chiral Centers And The Problem Of Naming
  • The Cahn-Ingold-Prelog System For Naming Chiral Centers
  • Oh No! What Do We Do When #4 Is Not In The Back?
  • Assigning R/S When #4 Is In The Front: A Short Cut
  • Assigning R/S When #4 Is In The Plane Of The Page
  • Breaking Ties With The “Dot Technique”
  • Conclusion: Assigning R and S With CIP

(Advanced) References and Further Reading

This post was co-authored by Matt Pierce of Organic Chemistry Solutions .  Ask Matt about scheduling an online tutoring session here .

1. Chiral Centers And The Problem Of Naming

Previously on MOC we’ve described enantiomers : molecules that are non-superimposable mirror images of each other. Perhaps the most memorable example is these “enantiocats”.

drawing-of-enantiocats-master-organic-chemistry-graeme-mackay-look-at-the-legs

Each of these cats is said to be “chiral”: they lack a plane of symmetry.

What causes molecules to have chirality?

The most common source of chirality is a “chiral centre”: typically a  tetrahedral carbon attached to four different “groups”, or “substituents”.  For each chiral centre there are  two  (and only two!) different ways of arranging the 4 different substituents, which gives rise to two different configurations.  [If you don’t believe there are only two, see Single Swap Rule ].

The purpose of this post is to introduce and describe the nomenclature we use to describe these configurations: the (R)/(S) notation, or Cahn-Ingold Prelog Rules.

Let’s look at a simple example.

Both of these molecules are 1-bromo-1-chloroethane. But they are not  exactly the same molecule, in the same way that your left shoe is not exactly the same as your right. They are non-superimposable mirror images of each other. How do we communicate this difference?

One way would be to describe their physical properties. For example, although these two molecules have the same boiling point, melting point, and share many other physical properties, they rotate plane-polarized light in equal and opposite directions, a property called  optical rotation ( See Optical Rotation and Optical Activity ) We could use (+)-1-bromo-1-chloroethane to refer to the isomer that rotates polarized light to the right (clockwise, or “dextrorotatory”) and use (-)-1-bromo-1-chloroethane to refer to the isomer that rotates polarized light to the left (counterclockwise, or “levorotatory”).

However this nomenclature suffers from a serious problem. There is no simple correlation between the arrangement of substituents around a chiral centre and the direction in which polarized light is rotated . Another solution is needed.

2. The Cahn Ingold Prelog (CIP) System For Naming Chiral Centers

A solution to this quandary was proposed by Robert Cahn, Chris Ingold, and Vladimir Prelog in 1966. The resulting “CIP” protocol works as follows:

  • Prioritize the four groups around a chiral center according to atomic number . The highest atomic number is assigned priority #1, and the lowest atomic number is assigned priority #4.
  • Orient the chiral centre such that the #4 priority substituent is pointing away from the viewer. For our purposes, it’s enough for it merely to be attached to a “dashed” bond.
  • If the path traced from 1-2-3   is clockwise , the chiral center is assigned ( R ) (from Latin,  rectus )
  • If the path traced is counter clockwise , the chiral center is assigned ( S ) (from the Latin  sinister)
  • Now we have a better way to describe molecules [A] and [B] shown above. Molecule [A] is named ( R )-1-bromo-1-chloroethane, and molecule [B] is named ( S )-1-bromo-1-chloroethane:

We should reiterate that the designations (R) and (S) bear no relationship to whether a molecule rotates plane-polarized light clockwise (+) or counterclockwise (-). For example the most common naturally occurring configuration of the amino acid alanine is (S), but its optical rotation (in aqueous acid solution) is (+).

3. What About When #4 Is Not In The Back?

That seems simple enough! “Is that it?”, you might ask.

Uh, no. As it happens, there’s a few bumps in the road toward determining (R)/(S) once we get beyond the simple example above.

These “trickier cases” fall into three main categories.

  • What if the #4 substituent is not helpfully pointing away from the viewer , as it was in our example above? What if it’s in the “front” (i.e. attached to a “wedged” bond) or, heaven forbid, in the plane of the page?
  • Assigning priorities in complex situations . What do we do in situations where a chiral centre has two or more identical atoms attached? In other words, how do we break ties? 
  • What do we do if the molecule is drawn a peculiar way , such as in  Fischer or Newman projections ?

We’re not going to be able to fully address all of these issues in this post. But we can certainly deal with #1 and make some headway with #2. For #3, see How To Determine R/S On A Fischer Projection and How to Determine R/S on a Newman Projection

4. Determining R/S When The #4 Substituent Is In Front (i.e. on a “Wedge”): A Short Cut

Let’s first consider the molecule below. The name of this molecule is ( R )-1-fluoroethanol. It is listed below with priorities assigned based on atomic number. In this case F>O>C>H. So F is #1 and H is #4. The tricky part here is that the #4 priority is pointing out of the page (on a “wedge”).

How do we determine (R)/(S) in this case?  There are two ways to do it.

Many instructors will tell you to “simply” rotate the molecule in your head so that the #4 priority is on a dash. Then you can assign R or S in the traditional way. This “simple” advice is not always an easy task for beginners.

Thankfully, it is technically unnecessary to perform such a mental rotation.

Here’s  a way around this. When the #4 priority is on a wedge you can just reverse the rules. So now we have two sets of rules:

If the #4 priority is on a dash :

  • Clockwise = R
  • Counterclockwise = S

If the #4 priority is on a wedge , reverse the typical rules:

  • Clockwise = S
  • Counterclockwise = R

R and S can easily be assigned to either picture of the molecule. I still encourage you to use a model kit and learn how to do so, however. Organic chemistry is much easier to understand, and much more beautiful, if you can master how to visualize a tetrahedral carbon atom.

See also, How To Draw The Enantiomer of A Chiral Molecule

5. Determining R/S When The #4 Group Is In The Plane Of The Page?

What if the #4 priority is in the plane of the paper, for example on a line? In this case it’s impossible to assign R and S in the traditional way. You’d have a 50:50 shot of getting it correct: not good odds. Again, if you can redraw the molecule in your head, then it’s better to just do that. If you can’t do this reliably then you need to learn the “single swap” concept.

Here’s how it works.  Swapping any two groups on a chiral centre will flip the configuration of the chiral centre from R to S (and vice versa). [ We previously talked about the “single swap rule” here ]

Knowing this, we can do a nifty trick.

  • Take the #4 substituent (in the plane of the page) and  swap it with the substituent in the back [If the configuration is R, this will switch it to S. If the configuration is S, this will flip it to R. We’ll need to account for this in step #3].
  • Redraw the chiral centre, and determine R/S on the new chiral centre which now has group #4 in the back.
  •  Whatever result you got,  flip it to its opposite to account for the fact that you did a single swap in step #1.

Here’s an example. Note that here  I first switched #4 and #3, but the main point is to switch two groups so that #4 is out of the plane of the paper.

This method always works, assuming you’ve determined the four priorities accurately. (It also works for cases when #4 is on a wedge).

However, sometimes we’re not in the position of dealing with 4 different atoms attached to a chiral carbon. For instance, it’s possible to have chiral carbons which are attached to 4  carbons . So how do we break the ties in these cases?

6. Determining CIP Priorities: Breaking “Ties” With The “Dot Technique”

The quick answer is to use the “dot technique”. Here’s how it works. Let’s do it for 4-ethyl-4-methyloctane, above.

  • Go outward from the chiral centre to each of the surrounding 4 atoms and assign priorities (based on atomic number) to each of these atoms. [Sometimes it’s helpful to draw  dots on each of these atoms.]

3. Compare each list, atom by atom. In our example, since C>H, (C,H,H) takes priority over (H,H,H) so the CH 3 group is assigned priority #4.

4. If there is still a tie, move the dots to the highest ranking atom in the list (i.e. the atom with highest atomic number). The dots are helpful because they help you to keep track of where you are, which can be important in complex examples.

5. In this case, we keep moving along the chain. By the way, if you ever reach the end of the chain without determining a difference, that means that the groups are identical and it isn’t a chiral centre after all.

6. By this point we have enough information to assign (R)/(S). Since priority #4 is in the front, we can also break out our “opposite rule” for good measure:

7. Conclusion: The Cahn-Ingold-Prelog Rules For Assigning R and S Configurations

In the next post we’ll go into some trickier examples with determining R/S, including how to deal with double bonds, rings, and isotopes. In a future post, we’ll get into determining R/S in the Fischer and Newman projections.

Thanks to Matt Pierce for making major contributions to this article.  

Ask Matt about scheduling an online tutoring session  here .

Related Articles

  • Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) – The Method of Dots
  • How To Draw The Enantiomer Of A Chiral Molecule
  • How To Determine R and S Configurations On A Fischer Projection
  • Assigning R/S To Newman Projections (And Converting Newman To Line Diagrams)
  • Types of Isomers: Constitutional Isomers, Stereoisomers, Enantiomers, and Diastereomers
  • On Cats, Part 4: Enantiocats
  • On Cats, Part 6: Stereocenters
  • Stereochemistry Practice Problems and Quizzes (MOC Membership)
  • How To Draw A Bond Rotation
  • Specification of Molecular Chirality R. S. Cahn, Sir Christopher Ingold, V. Prelog Angew. Chem. Int. Ed. 1966, 5 (4), 385-415 DOI: 10.1002/anie.196603851 This is not the first paper on the topic by the authors (see Refs. 4 and 5), but it is a major publication and an attempt to consolidate all the information on chirality in a single place. This paper discusses the various types of chirality possible in chemistry (not just at tetrahedral carbons!) and how to assign chirality unambiguously.
  • Basic Principles of the CIP‐System and Proposals for a Revision Dr. Vladlmir Prelog and Prof. Dr. Günter Helmchen Angew. Chem. Int. Ed. 1982 , 21 (8), 567-583 DOI: 10.1002/anie.198205671 An update to Ref. #1, which addresses a lot of edge cases that may come up in complex stereochemical assignments.
  • CHIRALITY IN CHEMISTRY Vladimir Prelog Nobel Lecture, 1975 Prelog’s Nobel Lecture. Nobel Lectures are fascinating to read as they give insight into the life of scientists and the path to discovery, which is rarely linear.
  • “Absolutely” simple stereochemistry Philip S. Beauchamp Journal of Chemical Education 1984, 61 (8), 666 DOI : 10.1021/ed061p666 This paper describes a simple method for determining stereochemistry of tetrahedral carbons using the hands, suitable for undergraduate students of organic chemistry.
  • A simple hand method for Cahn-Ingold-Prelog assignment of R and S configuration to chiral carbons Martin P. Aalund and James A. Pincock Journal of Chemical Education 1986, 63 (7), 600 DOI : 10.1021/ed063p600 A follow-up to the previous paper (Ref #4), but sadly it is incomplete!
  • A Web-Based Stereochemistry Tool to Improve Students’ Ability to Draw Newman Projections and Chair Conformations and Assign R/S Labels Nimesh Mistry, Ravi Singh, and Jamie Ridley Journal of Chemical Education 2020, 97 (4), 1157-1161 DOI : 10.1021/acs.jchemed.9b00688 This paper discusses a web-based tool that helps students with visualization of chiral compounds and assignment of stereochemistry as per the Cahn-Ingold-Prelog (CIL) rules. See ref. 34 in the paper for the link.

00 General Chemistry Review

  • Lewis Structures
  • Ionic and Covalent Bonding
  • Chemical Kinetics
  • Chemical Equilibria
  • Valence Electrons of the First Row Elements
  • How Concepts Build Up In Org 1 ("The Pyramid")

01 Bonding, Structure, and Resonance

  • How Do We Know Methane (CH4) Is Tetrahedral?
  • Hybrid Orbitals and Hybridization
  • How To Determine Hybridization: A Shortcut
  • Orbital Hybridization And Bond Strengths
  • Sigma bonds come in six varieties: Pi bonds come in one
  • A Key Skill: How to Calculate Formal Charge
  • The Four Intermolecular Forces and How They Affect Boiling Points
  • 3 Trends That Affect Boiling Points
  • How To Use Electronegativity To Determine Electron Density (and why NOT to trust formal charge)
  • Introduction to Resonance
  • How To Use Curved Arrows To Interchange Resonance Forms
  • Evaluating Resonance Forms (1) - The Rule of Least Charges
  • How To Find The Best Resonance Structure By Applying Electronegativity
  • Evaluating Resonance Structures With Negative Charges
  • Evaluating Resonance Structures With Positive Charge
  • Exploring Resonance: Pi-Donation
  • Exploring Resonance: Pi-acceptors
  • In Summary: Evaluating Resonance Structures
  • Drawing Resonance Structures: 3 Common Mistakes To Avoid
  • How to apply electronegativity and resonance to understand reactivity
  • Bond Hybridization Practice
  • Structure and Bonding Practice Quizzes
  • Resonance Structures Practice

02 Acid Base Reactions

  • Introduction to Acid-Base Reactions
  • Acid Base Reactions In Organic Chemistry
  • The Stronger The Acid, The Weaker The Conjugate Base
  • Walkthrough of Acid-Base Reactions (3) - Acidity Trends
  • Five Key Factors That Influence Acidity
  • Acid-Base Reactions: Introducing Ka and pKa
  • How to Use a pKa Table
  • The pKa Table Is Your Friend
  • A Handy Rule of Thumb for Acid-Base Reactions
  • Acid Base Reactions Are Fast
  • pKa Values Span 60 Orders Of Magnitude
  • How Protonation and Deprotonation Affect Reactivity
  • Acid Base Practice Problems

03 Alkanes and Nomenclature

  • Meet the (Most Important) Functional Groups
  • Condensed Formulas: Deciphering What the Brackets Mean
  • Hidden Hydrogens, Hidden Lone Pairs, Hidden Counterions
  • Don't Be Futyl, Learn The Butyls
  • Primary, Secondary, Tertiary, Quaternary In Organic Chemistry
  • Branching, and Its Affect On Melting and Boiling Points
  • The Many, Many Ways of Drawing Butane
  • Wedge And Dash Convention For Tetrahedral Carbon
  • Common Mistakes in Organic Chemistry: Pentavalent Carbon
  • Table of Functional Group Priorities for Nomenclature
  • Summary Sheet - Alkane Nomenclature
  • Organic Chemistry IUPAC Nomenclature Demystified With A Simple Puzzle Piece Approach
  • Boiling Point Quizzes
  • Organic Chemistry Nomenclature Quizzes

04 Conformations and Cycloalkanes

  • Staggered vs Eclipsed Conformations of Ethane
  • Conformational Isomers of Propane
  • Newman Projection of Butane (and Gauche Conformation)
  • Introduction to Cycloalkanes (1)
  • Geometric Isomers In Small Rings: Cis And Trans Cycloalkanes
  • Calculation of Ring Strain In Cycloalkanes
  • Cycloalkanes - Ring Strain In Cyclopropane And Cyclobutane
  • Cyclohexane Conformations
  • Cyclohexane Chair Conformation: An Aerial Tour
  • How To Draw The Cyclohexane Chair Conformation
  • The Cyclohexane Chair Flip
  • The Cyclohexane Chair Flip - Energy Diagram
  • Substituted Cyclohexanes - Axial vs Equatorial
  • Ranking The Bulkiness Of Substituents On Cyclohexanes: "A-Values"
  • Cyclohexane Chair Conformation Stability: Which One Is Lower Energy?
  • Fused Rings - Cis-Decalin and Trans-Decalin
  • Naming Bicyclic Compounds - Fused, Bridged, and Spiro
  • Bredt's Rule (And Summary of Cycloalkanes)
  • Newman Projection Practice
  • Cycloalkanes Practice Problems

05 A Primer On Organic Reactions

  • The Most Important Question To Ask When Learning a New Reaction
  • Learning New Reactions: How Do The Electrons Move?
  • The Third Most Important Question to Ask When Learning A New Reaction
  • 7 Factors that stabilize negative charge in organic chemistry
  • 7 Factors That Stabilize Positive Charge in Organic Chemistry
  • Nucleophiles and Electrophiles
  • Curved Arrows (for reactions)
  • Curved Arrows (2): Initial Tails and Final Heads
  • Nucleophilicity vs. Basicity
  • The Three Classes of Nucleophiles
  • What Makes A Good Nucleophile?
  • What makes a good leaving group?
  • 3 Factors That Stabilize Carbocations
  • Equilibrium and Energy Relationships
  • What's a Transition State?
  • Hammond's Postulate
  • Learning Organic Chemistry Reactions: A Checklist (PDF)
  • Introduction to Free Radical Substitution Reactions
  • Introduction to Oxidative Cleavage Reactions

06 Free Radical Reactions

  • Bond Dissociation Energies = Homolytic Cleavage
  • Free Radical Reactions
  • 3 Factors That Stabilize Free Radicals
  • What Factors Destabilize Free Radicals?
  • Bond Strengths And Radical Stability
  • Free Radical Initiation: Why Is "Light" Or "Heat" Required?
  • Initiation, Propagation, Termination
  • Monochlorination Products Of Propane, Pentane, And Other Alkanes
  • Selectivity In Free Radical Reactions
  • Selectivity in Free Radical Reactions: Bromination vs. Chlorination
  • Halogenation At Tiffany's
  • Allylic Bromination
  • Bonus Topic: Allylic Rearrangements
  • In Summary: Free Radicals
  • Synthesis (2) - Reactions of Alkanes
  • Free Radicals Practice Quizzes

07 Stereochemistry and Chirality

  • Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) - The Method of Dots
  • Enantiomers vs Diastereomers vs The Same? Two Methods For Solving Problems
  • The Meso Trap
  • Optical Rotation, Optical Activity, and Specific Rotation
  • Optical Purity and Enantiomeric Excess
  • What's a Racemic Mixture?
  • Chiral Allenes And Chiral Axes
  • Stereochemistry Practice Problems and Quizzes

08 Substitution Reactions

  • Introduction to Nucleophilic Substitution Reactions
  • Walkthrough of Substitution Reactions (1) - Introduction
  • Two Types of Nucleophilic Substitution Reactions
  • The SN2 Mechanism
  • Why the SN2 Reaction Is Powerful
  • The SN1 Mechanism
  • The Conjugate Acid Is A Better Leaving Group
  • Comparing the SN1 and SN2 Reactions
  • Polar Protic? Polar Aprotic? Nonpolar? All About Solvents
  • Steric Hindrance is Like a Fat Goalie
  • Common Blind Spot: Intramolecular Reactions
  • The Conjugate Base is Always a Stronger Nucleophile
  • Substitution Practice - SN1
  • Substitution Practice - SN2

09 Elimination Reactions

  • Elimination Reactions (1): Introduction And The Key Pattern
  • Elimination Reactions (2): The Zaitsev Rule
  • Elimination Reactions Are Favored By Heat
  • Two Elimination Reaction Patterns
  • The E1 Reaction
  • The E2 Mechanism
  • E1 vs E2: Comparing the E1 and E2 Reactions
  • Antiperiplanar Relationships: The E2 Reaction and Cyclohexane Rings
  • Bulky Bases in Elimination Reactions
  • Comparing the E1 vs SN1 Reactions
  • Elimination (E1) Reactions With Rearrangements
  • E1cB - Elimination (Unimolecular) Conjugate Base
  • Elimination (E1) Practice Problems And Solutions
  • Elimination (E2) Practice Problems and Solutions

10 Rearrangements

  • Introduction to Rearrangement Reactions
  • Rearrangement Reactions (1) - Hydride Shifts
  • Carbocation Rearrangement Reactions (2) - Alkyl Shifts
  • Pinacol Rearrangement
  • The SN1, E1, and Alkene Addition Reactions All Pass Through A Carbocation Intermediate

11 SN1/SN2/E1/E2 Decision

  • Identifying Where Substitution and Elimination Reactions Happen
  • Deciding SN1/SN2/E1/E2 (1) - The Substrate
  • Deciding SN1/SN2/E1/E2 (2) - The Nucleophile/Base
  • SN1 vs E1 and SN2 vs E2 : The Temperature
  • Deciding SN1/SN2/E1/E2 - The Solvent
  • Wrapup: The Quick N' Dirty Guide To SN1/SN2/E1/E2
  • Alkyl Halide Reaction Map And Summary
  • SN1 SN2 E1 E2 Practice Problems

12 Alkene Reactions

  • E and Z Notation For Alkenes (+ Cis/Trans)
  • Alkene Stability
  • Addition Reactions: Elimination's Opposite
  • Stereoselective and Stereospecific Reactions
  • Regioselectivity In Alkene Addition Reactions
  • Stereoselectivity In Alkene Addition Reactions: Syn vs Anti Addition
  • Hydrohalogenation of Alkenes and Markovnikov's Rule
  • Hydration of Alkenes With Aqueous Acid
  • Rearrangements in Alkene Addition Reactions
  • Halogenation of Alkenes and Halohydrin Formation
  • Oxymercuration Demercuration of Alkenes
  • Hydroboration Oxidation of Alkenes
  • m-CPBA (meta-chloroperoxybenzoic acid)
  • OsO4 (Osmium Tetroxide) for Dihydroxylation of Alkenes
  • Palladium on Carbon (Pd/C) for Catalytic Hydrogenation of Alkenes
  • Cyclopropanation of Alkenes
  • A Fourth Alkene Addition Pattern - Free Radical Addition
  • Alkene Reactions: Ozonolysis
  • Summary: Three Key Families Of Alkene Reaction Mechanisms
  • Synthesis (4) - Alkene Reaction Map, Including Alkyl Halide Reactions
  • Alkene Reactions Practice Problems

13 Alkyne Reactions

  • Acetylides from Alkynes, And Substitution Reactions of Acetylides
  • Partial Reduction of Alkynes With Lindlar's Catalyst
  • Partial Reduction of Alkynes With Na/NH3 To Obtain Trans Alkenes
  • Alkyne Hydroboration With "R2BH"
  • Hydration and Oxymercuration of Alkynes
  • Hydrohalogenation of Alkynes
  • Alkyne Halogenation: Bromination, Chlorination, and Iodination of Alkynes
  • Alkyne Reactions - The "Concerted" Pathway
  • Alkenes To Alkynes Via Halogenation And Elimination Reactions
  • Alkynes Are A Blank Canvas
  • Synthesis (5) - Reactions of Alkynes
  • Alkyne Reactions Practice Problems With Answers

14 Alcohols, Epoxides and Ethers

  • Alcohols - Nomenclature and Properties
  • Alcohols Can Act As Acids Or Bases (And Why It Matters)
  • Alcohols - Acidity and Basicity
  • The Williamson Ether Synthesis
  • Ethers From Alkenes, Tertiary Alkyl Halides and Alkoxymercuration
  • Alcohols To Ethers via Acid Catalysis
  • Cleavage Of Ethers With Acid
  • Epoxides - The Outlier Of The Ether Family
  • Opening of Epoxides With Acid
  • Epoxide Ring Opening With Base
  • Making Alkyl Halides From Alcohols
  • Tosylates And Mesylates
  • PBr3 and SOCl2
  • Elimination Reactions of Alcohols
  • Elimination of Alcohols To Alkenes With POCl3
  • Alcohol Oxidation: "Strong" and "Weak" Oxidants
  • Demystifying The Mechanisms of Alcohol Oxidations
  • Protecting Groups For Alcohols
  • Thiols And Thioethers
  • Calculating the oxidation state of a carbon
  • Oxidation and Reduction in Organic Chemistry
  • Oxidation Ladders
  • SOCl2 Mechanism For Alcohols To Alkyl Halides: SN2 versus SNi
  • Alcohol Reactions Roadmap (PDF)
  • Alcohol Reaction Practice Problems
  • Epoxide Reaction Quizzes
  • Oxidation and Reduction Practice Quizzes

15 Organometallics

  • What's An Organometallic?
  • Formation of Grignard and Organolithium Reagents
  • Organometallics Are Strong Bases
  • Reactions of Grignard Reagents
  • Protecting Groups In Grignard Reactions
  • Synthesis Problems Involving Grignard Reagents
  • Grignard Reactions And Synthesis (2)
  • Organocuprates (Gilman Reagents): How They're Made
  • Gilman Reagents (Organocuprates): What They're Used For
  • The Heck, Suzuki, and Olefin Metathesis Reactions (And Why They Don't Belong In Most Introductory Organic Chemistry Courses)
  • Reaction Map: Reactions of Organometallics
  • Grignard Practice Problems

16 Spectroscopy

  • Degrees of Unsaturation (or IHD, Index of Hydrogen Deficiency)
  • Conjugation And Color (+ How Bleach Works)
  • Introduction To UV-Vis Spectroscopy
  • UV-Vis Spectroscopy: Absorbance of Carbonyls
  • UV-Vis Spectroscopy: Practice Questions
  • Bond Vibrations, Infrared Spectroscopy, and the "Ball and Spring" Model
  • Infrared Spectroscopy: A Quick Primer On Interpreting Spectra
  • IR Spectroscopy: 4 Practice Problems
  • 1H NMR: How Many Signals?
  • Homotopic, Enantiotopic, Diastereotopic
  • Diastereotopic Protons in 1H NMR Spectroscopy: Examples
  • C13 NMR - How Many Signals
  • Liquid Gold: Pheromones In Doe Urine
  • Natural Product Isolation (1) - Extraction
  • Natural Product Isolation (2) - Purification Techniques, An Overview
  • Structure Determination Case Study: Deer Tarsal Gland Pheromone

17 Dienes and MO Theory

  • What To Expect In Organic Chemistry 2
  • Are these molecules conjugated?
  • Conjugation And Resonance In Organic Chemistry
  • Bonding And Antibonding Pi Orbitals
  • Molecular Orbitals of The Allyl Cation, Allyl Radical, and Allyl Anion
  • Pi Molecular Orbitals of Butadiene
  • Reactions of Dienes: 1,2 and 1,4 Addition
  • Thermodynamic and Kinetic Products
  • More On 1,2 and 1,4 Additions To Dienes
  • s-cis and s-trans
  • The Diels-Alder Reaction
  • Cyclic Dienes and Dienophiles in the Diels-Alder Reaction
  • Stereochemistry of the Diels-Alder Reaction
  • Exo vs Endo Products In The Diels Alder: How To Tell Them Apart
  • HOMO and LUMO In the Diels Alder Reaction
  • Why Are Endo vs Exo Products Favored in the Diels-Alder Reaction?
  • Diels-Alder Reaction: Kinetic and Thermodynamic Control
  • The Retro Diels-Alder Reaction
  • The Intramolecular Diels Alder Reaction
  • Regiochemistry In The Diels-Alder Reaction
  • The Cope and Claisen Rearrangements
  • Electrocyclic Reactions
  • Electrocyclic Ring Opening And Closure (2) - Six (or Eight) Pi Electrons
  • Diels Alder Practice Problems
  • Molecular Orbital Theory Practice

18 Aromaticity

  • Introduction To Aromaticity
  • Rules For Aromaticity
  • Huckel's Rule: What Does 4n+2 Mean?
  • Aromatic, Non-Aromatic, or Antiaromatic? Some Practice Problems
  • Antiaromatic Compounds and Antiaromaticity
  • The Pi Molecular Orbitals of Benzene
  • The Pi Molecular Orbitals of Cyclobutadiene
  • Frost Circles
  • Aromaticity Practice Quizzes

19 Reactions of Aromatic Molecules

  • Electrophilic Aromatic Substitution: Introduction
  • Activating and Deactivating Groups In Electrophilic Aromatic Substitution
  • Electrophilic Aromatic Substitution - The Mechanism
  • Ortho-, Para- and Meta- Directors in Electrophilic Aromatic Substitution
  • Understanding Ortho, Para, and Meta Directors
  • Why are halogens ortho- para- directors?
  • Disubstituted Benzenes: The Strongest Electron-Donor "Wins"
  • Electrophilic Aromatic Substitutions (1) - Halogenation of Benzene
  • Electrophilic Aromatic Substitutions (2) - Nitration and Sulfonation
  • EAS Reactions (3) - Friedel-Crafts Acylation and Friedel-Crafts Alkylation
  • Intramolecular Friedel-Crafts Reactions
  • Nucleophilic Aromatic Substitution (NAS)
  • Nucleophilic Aromatic Substitution (2) - The Benzyne Mechanism
  • Reactions on the "Benzylic" Carbon: Bromination And Oxidation
  • The Wolff-Kishner, Clemmensen, And Other Carbonyl Reductions
  • More Reactions on the Aromatic Sidechain: Reduction of Nitro Groups and the Baeyer Villiger
  • Aromatic Synthesis (1) - "Order Of Operations"
  • Synthesis of Benzene Derivatives (2) - Polarity Reversal
  • Aromatic Synthesis (3) - Sulfonyl Blocking Groups
  • Birch Reduction
  • Synthesis (7): Reaction Map of Benzene and Related Aromatic Compounds
  • Aromatic Reactions and Synthesis Practice
  • Electrophilic Aromatic Substitution Practice Problems

20 Aldehydes and Ketones

  • What's The Alpha Carbon In Carbonyl Compounds?
  • Nucleophilic Addition To Carbonyls
  • Aldehydes and Ketones: 14 Reactions With The Same Mechanism
  • Sodium Borohydride (NaBH4) Reduction of Aldehydes and Ketones
  • Grignard Reagents For Addition To Aldehydes and Ketones
  • Wittig Reaction
  • Hydrates, Hemiacetals, and Acetals
  • Imines - Properties, Formation, Reactions, and Mechanisms
  • All About Enamines
  • Breaking Down Carbonyl Reaction Mechanisms: Reactions of Anionic Nucleophiles (Part 2)
  • Aldehydes Ketones Reaction Practice

21 Carboxylic Acid Derivatives

  • Nucleophilic Acyl Substitution (With Negatively Charged Nucleophiles)
  • Addition-Elimination Mechanisms With Neutral Nucleophiles (Including Acid Catalysis)
  • Basic Hydrolysis of Esters - Saponification
  • Transesterification
  • Proton Transfer
  • Fischer Esterification - Carboxylic Acid to Ester Under Acidic Conditions
  • Lithium Aluminum Hydride (LiAlH4) For Reduction of Carboxylic Acid Derivatives
  • LiAlH[Ot-Bu]3 For The Reduction of Acid Halides To Aldehydes
  • Di-isobutyl Aluminum Hydride (DIBAL) For The Partial Reduction of Esters and Nitriles
  • Amide Hydrolysis
  • Thionyl Chloride (SOCl2)
  • Diazomethane (CH2N2)
  • Carbonyl Chemistry: Learn Six Mechanisms For the Price Of One
  • Making Music With Mechanisms (PADPED)
  • Carboxylic Acid Derivatives Practice Questions

22 Enols and Enolates

  • Keto-Enol Tautomerism
  • Enolates - Formation, Stability, and Simple Reactions
  • Kinetic Versus Thermodynamic Enolates
  • Aldol Addition and Condensation Reactions
  • Reactions of Enols - Acid-Catalyzed Aldol, Halogenation, and Mannich Reactions
  • Claisen Condensation and Dieckmann Condensation
  • Decarboxylation
  • The Malonic Ester and Acetoacetic Ester Synthesis
  • The Michael Addition Reaction and Conjugate Addition
  • The Robinson Annulation
  • Haloform Reaction
  • The Hell–Volhard–Zelinsky Reaction
  • Enols and Enolates Practice Quizzes
  • The Amide Functional Group: Properties, Synthesis, and Nomenclature
  • Basicity of Amines And pKaH
  • 5 Key Basicity Trends of Amines
  • The Mesomeric Effect And Aromatic Amines
  • Nucleophilicity of Amines
  • Alkylation of Amines (Sucks!)
  • Reductive Amination
  • The Gabriel Synthesis
  • Some Reactions of Azides
  • The Hofmann Elimination
  • The Hofmann and Curtius Rearrangements
  • The Cope Elimination
  • Protecting Groups for Amines - Carbamates
  • The Strecker Synthesis of Amino Acids
  • Introduction to Peptide Synthesis
  • Reactions of Diazonium Salts: Sandmeyer and Related Reactions
  • Amine Practice Questions

24 Carbohydrates

  • D and L Notation For Sugars
  • Pyranoses and Furanoses: Ring-Chain Tautomerism In Sugars
  • What is Mutarotation?
  • Reducing Sugars
  • The Big Damn Post Of Carbohydrate-Related Chemistry Definitions
  • The Haworth Projection
  • Converting a Fischer Projection To A Haworth (And Vice Versa)
  • Reactions of Sugars: Glycosylation and Protection
  • The Ruff Degradation and Kiliani-Fischer Synthesis
  • Isoelectric Points of Amino Acids (and How To Calculate Them)
  • Carbohydrates Practice
  • Amino Acid Quizzes

25 Fun and Miscellaneous

  • A Gallery of Some Interesting Molecules From Nature
  • Screw Organic Chemistry, I'm Just Going To Write About Cats
  • On Cats, Part 1: Conformations and Configurations
  • On Cats, Part 2: Cat Line Diagrams
  • Organic Chemistry Is Shit
  • The Organic Chemistry Behind "The Pill"
  • Maybe they should call them, "Formal Wins" ?
  • Why Do Organic Chemists Use Kilocalories?
  • The Principle of Least Effort
  • Organic Chemistry GIFS - Resonance Forms
  • Reproducibility In Organic Chemistry
  • What Holds The Nucleus Together?
  • How Reactions Are Like Music
  • Organic Chemistry and the New MCAT

26 Organic Chemistry Tips and Tricks

  • Common Mistakes: Formal Charges Can Mislead
  • Partial Charges Give Clues About Electron Flow
  • Draw The Ugly Version First
  • Organic Chemistry Study Tips: Learn the Trends
  • The 8 Types of Arrows In Organic Chemistry, Explained
  • Top 10 Skills To Master Before An Organic Chemistry 2 Final
  • Common Mistakes with Carbonyls: Carboxylic Acids... Are Acids!
  • Planning Organic Synthesis With "Reaction Maps"
  • Alkene Addition Pattern #1: The "Carbocation Pathway"
  • Alkene Addition Pattern #2: The "Three-Membered Ring" Pathway
  • Alkene Addition Pattern #3: The "Concerted" Pathway
  • Number Your Carbons!
  • The 4 Major Classes of Reactions in Org 1
  • How (and why) electrons flow
  • Grossman's Rule
  • Three Exam Tips
  • A 3-Step Method For Thinking Through Synthesis Problems
  • Putting It Together
  • Putting Diels-Alder Products in Perspective
  • The Ups and Downs of Cyclohexanes
  • The Most Annoying Exceptions in Org 1 (Part 1)
  • The Most Annoying Exceptions in Org 1 (Part 2)
  • The Marriage May Be Bad, But the Divorce Still Costs Money
  • 9 Nomenclature Conventions To Know
  • Nucleophile attacks Electrophile

27 Case Studies of Successful O-Chem Students

  • Success Stories: How Corina Got The The "Hard" Professor - And Got An A+ Anyway
  • How Helena Aced Organic Chemistry
  • From a "Drop" To B+ in Org 2 – How A Hard Working Student Turned It Around
  • How Serge Aced Organic Chemistry
  • Success Stories: How Zach Aced Organic Chemistry 1
  • Success Stories: How Kari Went From C– to B+
  • How Esther Bounced Back From a "C" To Get A's In Organic Chemistry 1 And 2
  • How Tyrell Got The Highest Grade In Her Organic Chemistry Course
  • This Is Why Students Use Flashcards
  • Success Stories: How Stu Aced Organic Chemistry
  • How John Pulled Up His Organic Chemistry Exam Grades
  • Success Stories: How Nathan Aced Organic Chemistry (Without It Taking Over His Life)
  • How Chris Aced Org 1 and Org 2
  • Interview: How Jay Got an A+ In Organic Chemistry
  • How to Do Well in Organic Chemistry: One Student's Advice
  • "America's Top TA" Shares His Secrets For Teaching O-Chem
  • "Organic Chemistry Is Like..." - A Few Metaphors
  • How To Do Well In Organic Chemistry: Advice From A Tutor
  • Guest post: "I went from being afraid of tests to actually looking forward to them".

Comment section

22 thoughts on “ introduction to assigning (r) and (s): the cahn-ingold-prelog rules ”.

In a chiral molecule, two groups are attached to it with the normal line bond ,the third is shown through a wedge and hydrogen is not shown..can I conclude that the hydrogen is a dash ?

Yes! The dashed hydrogen is implied!

Thanks. Move the dots. Could not find this before.

Glad you found it useful James!

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During my studies for 11th grade and 12th grade, we had a brilliant Organic Chemistry teacher who taught the concepts beautifully. In addition, I had a passion (more of a “study crush”) on Chemistry in general and Organic Chemistry in particular. To such an extent that this topic of R and S enantiomers is still ingrained in memory. Though I am in a completely different area now of Machine Learning and Analytics in the Healthcare space in Industry, primarily a Software Engg job. Out of sheer curiosity, I googled “Chirality Detection Machine Learning” and voila !! such cool, intereesting papers I came across where they combine Bayesian Learning and Convolutional Neural Networks (Advanced ML Theory) to detect chirality in Nanoparticles. So application of ML in cutting edge Physics. Amazing stuff :!

Most people don’t learn chirality until 2nd year university in north america, so you are ahead of the curve

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I just only want to know the CIP system of Nomenclature

Man this website proved to be a boon for me in quarantine…keep it up🔥🔥 The best content of organic chem I could get in such an incredible way

Thank you so much!! :) This was a great refresher on chirality and you explained it in such a straightforward manner. Appreciate it!

What to do if the compound is not denoted using the dash and wedge but simple bond line notation or expanded notation ?

Can you show an example? There has to be some kind of indicator. If all four bonds from the chiral center are shown as simple line notation there is no way to tell if it is R or S. It’s ambiguous.

Thank you so much, you are a true life saver???

I have a lot of trouble rotating molecules in my head, so these tips feel like magic to me!!! Thank you soooo much :DDDD Btw I also go to McGill!

The molecule used to explain the dot technique is labelled as 3-ethyl-3-methyloctane, however shouldn’t the molecule be named as 4-ethyl-4-methyloctane? The branches are on the fourth carbon…

Shoot. You are right. Thanks for the catch. Fixed!

Thank You so much :)

Thanks!! You saved my org chem exam

I was having trouble with this when 4 was in the plane of the page. This technique is so easy. Thanks

Kindly take my work into consideration in your website.

Abstract:- “The Keval’s Method” is developed for the determination of absolute configuration of a chiral carbon in a Fisher Projection and Wedge-Dash Projection just by simple calculations. This method is easily applicable over both Fisher as well as Wedge-Dash Projection. Various methods for determining absolute configuration have been developed and published till now, some of them used fingers and hands and other used exchanging elements. “Keval’s Method” is the first method in which a chiral carbon is taken to be an origin and the branches to axes, also it is purely calculation based method where absolute configuration is found based on the nature of calculated answer without using fingers and hands and also without exchanging elements.

Your’ Thankfully Keval Chetanbhai Purohit 5th-Computer Engineering, Vishwakarma Government Engineering College, Mo- 7226953531

Thank you very much, I now understand the R/S, its not easy to rotate a compound in your mind……

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  • Finding R and S for Chiral Centers

Key Questions

It is a stereochemical label to indicate the relative spatial orientation of each atom in a molecule with a non-superimposable mirror image .

R indicates that a clockwise circular arrow that goes from higher priority to lower priority crosses over the lowest priority substituent and that lowest-priority substituent is in the back.

The R and S stereoisomers are non-superimposable mirror images , which means if you reflect them on a mirror plane, they do not become the exact same molecule when you overlay them.

When you label a molecule as R or S , you consider the priorities of each substituent on the chiral carbon (connected to four different functional groups).

Let's take this chiral amino acid for example:

http://www.nobelprize.org/

Some general ways you could determine the priorities are:

  • HIgher atomic number of the directly-attached atom gives higher priority
  • Atomic number of the atom attached to the one is considered in step 1 if two substituents have the same first atom
  • Higher number of same-atom branches determines greater priority if the overall substituents are too similar (e.g. isopropyl has higher priority than ethyl)

With ( R )-alanine:

  • #"NH"_2# has priority 1 due to highest atomic number for #"N"# .
  • #"COOH"# has priority 2 due to the higher atomic number of #"O"# vs. #"H"# in #"CH"_3#
  • #"CH"_3# has priority 3 as a result.
  • #"H"# has priority 4 .

Now, if you draw a circular arrow starting at #"NH"_2# , going to #"COOH"# , crossing over #"H"# since it is in the back, and to #"CH"_3# , then you would have gone clockwise.

https://upload.wikimedia.org/

Since the lowest priority atom is in the back, the clockwise arrow corresponds to the R configuration.

If you had started from the same R configuration but oriented #"H"# in the front and #"CH"_3# in the back , it would have been S configuration. Let's call this S configuration A, where you just nudge two substituents to flip them from front/back to back/front.

If you reflect the same R configuration over a mirror plane, keeping the orientations of #"H"# in the back and #"CH"_3# in the front after the flip, the configuration is also S . Let's call this S configuration B, where you've actually done a reflection.

If you start from S configuration B, and flipped it over a vertical axis (literally rotating #180^o# in space), you would get S configuration A.

assign r/s configurations to the labeled chiral centers in this carbohydrate molecule

  • How many stereoisomers of a heptulose are possible? How many are D and how many are L sugars? How many names will be needed for all the isomers?
  • What does R configuration mean?
  • What does S configuration mean?
  • How can I identify and draw the optical isomers for the isomers of #[Cr(H_2O)_3Cl_3 ]^+#?
  • How can I identify and draw the geometric isomers for the isomers of #[Ni(CN)_2Cl_2]^(2-)#?
  • How can I identify and draw the optical isomers for all the isomers of #[Co(NH_3 )_2Cl_2 ]^-#
  • Determine whether the cis or trans isomers in #[Cr(H_2O)_3Cl_3]^+#?
  • Is ( R )-lactic acid dextrorotatory or levorotatory?
  • Is ( R )-sodium lactate dextrorotatory or levorotatory?
  • What is a dextrorotatory compound?
  • When is a compound optically active?
  • When is a compound optically inactive?
  • Why is it called the Cahn-Ingold-Prelog system?
  • How does the Cahn-Ingold-Prelog system work?
  • How can I assign relative priorities to the groups or atoms in each of the following: #-CH_2OH#, #-CH_3#, #-H# and #-CH_2 CH_2OH#?
  • How can I show the R-configuration of the molecule bromochlorofluoroiodomethane?
  • Question #27b9d
  • How do structural isomers differ from stereoisomers?
  • Question #16b44
  • Question #255f6
  • What are the rules for defining E-Z configuration?
  • How do we determine #D# and #L# terms, and how do they relate to absolute configuration?

Stereochemistry (R and S), Isomers, and Optical Activity

  • Introduction to Chirality and Chiral Centers
  • Chiral and Achiral Molecules
  • Finding R and S for Tricky Examples
  • Enantiomers
  • Diastereomers
  • Meso Compounds

Assign sequence priorities to the four substituents by looking at the atoms attached directly to the chiral center.

The Viewing Rule

Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Example 10 Example 11 Example 12 S     R     Achiral A response to your selection will appear here. A sequence assignment will be shown above.

Configurational drawings of chiral molecules sometimes display structures in a way that does not permit an easy application of the viewing rule. In the example of carvone, shown above, the initial formula directed the lowest priority substituent (H) toward the viewer, requiring either a reorientation display or a very good sense of three-dimensional structure on the part of the reader. The Fischer projection formulas, described later , are another example of displays that challenge even experienced students. A useful mnemonic, suggested by Professor Michael Rathke, is illustrated below. Here a stereogenic tetrahedral carbon has four different substituents, designated 1, 2, 3 & 4 . If we assume that these numbers represent the sequence priority of these substituents (1 > 2 > 3 > 4), then the R and S configurations are defined.

The viewing rule states that when the lowest priority substituent (4) is oriented behind the triangular face defined by the three higher priority substituents (shaded light gray here), a clockwise sequential arrangement of these substituents (1, 2 & 3) is defined as R , and a counter-clockwise sequence as S . Now a tetrahedral structure may be viewed from any of the four triangular faces, and the symmetry of the system is such that a correct R/S assignment is made if the remote out-of plane group has an even number sequence priority (2 or 4), whereas the wrong assignment results when the out-of plane group has an odd priority (1 or 3). Once one recognizes this relationship, the viewing options are increased and a configurational assignment is more easily achieved. For an example, click on the diagram to see the 1:3:4-face, shaded light gray. oriented in front of substituent 2. Note that the R/S assignment is unchanged.

Ephedrine from Ma Huang: m.p. 35 - 40 º C,   [α] D = –41º,   moderate water solubility [this isomer may be referred to as (–)-ephedrine] Pseudoephedrine from Ma Huang: m.p. 119 º C,   [α] D = +52º,   relatively insoluble in water [this isomer may be referred to as (+)-pseudoephedrine]

For an interesting example illustrating the distinction between a chiral center and an asymmetric carbon Click Here .

(+)-tartaric acid: [α] D = +13º m.p. 172 ºC (–)-tartaric acid: [α] D = –13º m.p. 172 ºC meso -tartaric acid: [α] D = 0º m.p. 140 ºC

To learn more about chemical procedures for achieving resolution Click Here .

Conformations of meso-Tartaric Acid Fischer Projection A eclipsed, achiral B staggered, chiral C staggered, achiral D staggered, chiral

Conformations of Biphenyls

The 1,2-Dichlorocyclohexanes The 1,3-Dichlorocyclohexanes Examine Conformations of cis-1,2-Dichlorocyclohexane Examine Conformations of trans-1,2-Dichlorocyclohexane   Examine Conformations of cis-1,3-Dichlorocyclohexane Examine Conformations of trans-1,3-Dichlorocyclohexane

The 1,4-Dichlorocyclohexanes

This page is the property of William Reusch.   Comments, questions and errors should be sent to [email protected] . These pages are provided to the IOCD to assist in capacity building in chemical education. 05/05/2013

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5.3 Chirality and the R/S Naming System

Other than geometric isomers, there is another type of stereoisomer that is related to a special property called chirality. We will start with the basic concepts of chirality, then expand the topic further from there.

5.3.1 Chiral and Chirality

T o talk about chirality, let’s first take a closer look at our left hand and our right hand. The left hand can be regarded as a mirror image of the right hand, and vice versa. Now, let’s try to superimpose (overlay) the left hand on the right hand. Can you do that?

No ! No matter how hard you try, the left hand can not be superimposed on the right hand. This is because of the special property of the hand that is called chirality . Both the left and right hand are chiral (ky-ral) and show chirality . Chiral is derived from the Greek word cheir , which means “hand”, and chirality means “handedness”.

""

The definition of chirality is the property of any object (molecule) being non-superimposable on its mirror image . The left and right hand are mirror images of each other, and they are not superimposable, so both the left hand and right hand are chiral. You can also find many other objects in daily life that show chirality as well.

""

If an object is superimposable on its mirror image (in such a case, the object and its mirror image are exactly identical), then this object is not chiral, and it is referred to as achiral .

""

In organic chemistry, we are interested in organic molecules that are chiral. Let’s see the following molecular models that represent a molecule and its mirror image.

""

In the models here, the four balls with different colors represent four different substituents, and the two structures are mirror images of each other. The effort of trying to superimpose one structure on the other does not work. Therefore, according to the definition of chiral/chirality, both molecules are non-superimposable on the mirror image, so they are both chiral and show chirality.

""

The chirality of the molecule results from the structure of the central carbon. When the central carbon is sp3 carbon and bonded with four different groups (represented by four different colors in the model), the molecule is chiral. The central carbon is called the chirality center (or asymmetric center). A molecule with one chirality center must be chiral. The chirality center can also be called the asymmetric center. We will use the term chirality center in this book.

In summary, a chirality (asymmetric) center should meet two requirements:

  • sp 3 carbon;
  • bonded with four different groups. 

For following compounds, label each of the chirality center with a star.

""

  • The carbons in CH 3 or CH 2 are NEVER chirality centers. The chirality center must be the carbon bonded with a branch (or branches).
  • sp 2 double bond carbon is NEVER a chirality center.
  • Carbon in a ring can also be chirality center as long as it meet the two requirements.
  • Not all the above compounds have a chirality center.

""

Exercises 5.2

  • Draw the structure of following compounds, determine which one has an chirality center and label it with a star.

a) 1-bromobutane,

b) 1-pentanol,

c) 2-pentanol,

d) 3-pentanol,

e) 2-bromopropanoic acid

f) 2-methyl cyclohexanone

      2.  Label all the chirality centers in the following molecules.

Nicotine & cholesterol

Answers to Chapter 5 Practice Questions

5.3.2 Stereoisomer with One Chirality Center — Enantiomers

For 2-butanol, we are able to recognize that C2 is the chirality center.

""

The perspective formula shows the 3D structure of 2-butanol in two different ways, and they are non-superimposable mirror images of each other.

""

The two mirror images are different molecules. They have the same bonding but differ in the way the atoms arranged in space. So, the two molecules are stereoisomers . This specific type of stereoisomer is defined as an enantiomer . Molecules that are a pair of non-superimposable mirror images of each other are called  enantiomers .

Important Properties of Enantiomers:

  • Enantiomers are a pair of non-superimposable mirror images.
  • Enantiomers are a pair of molecules that are both chiral and show chirality ( Enantiomers must be chiral ).
  • For any chiral molecule, it must have its enantiomer, that is, the mirror image of the molecule.
  • Achiral molecules do not have enantiomers. The mirror image of an achiral molecule is an identical molecule to itself.

To draw the 3D structure of any enantiomer, we need to use perspective formula  with solid and dashed wedges to show the tetrahedral arrangements of groups around the sp 3 carbon (refer to  section 2.11 ). Out of the four bonds on tetrahedral carbon, two bonds lie within the paper plane are shown as ordinary lines, the solid wedge represent a bond that point out of the paper plane, and the dashed wedge represent a bond that point behind the paper plane. For the first enantiomer, you can draw the four groups with any arrangement, then draw the other enantiomer by drawing the mirror image  of the first one. Please note, although it seems there are different ways to show the enantiomers, there are only total two  enantiomers, we will learn in next section how to identify and designate each of them.

Several possible ways to show the structures are included in the answer here. However, your answer can be different to any of them, as long as a pair of mirror images are shown.

""

Exercises 5.3

Draw the pair of enantiomers of  2-chloro-1-propanol .

5.3.3 R/S Naming System of the Chirality Center

The two enantiomers are different compounds, though they are very similar; therefore, we need a nomenclature system to distinguish between them, to give each one a different designation so that we know which one we are talking about. That is the R/S naming system defined in IUPAC. The R/S designation can be determined by following the Cahn-Ingold-Prelog rule, the rule devised by R. S Cahn, C. Ingold and V. Prelog.

Cahn-Ingold-Prelog Rule:

  • Assign priorities of the groups (or atoms) bonded to the chirality center by following the same priority rules as for the E/Z system ( section 5.2 ).  The highest priority group is labelled as #1, and lowest priority group is labelled as #4 in this book.
  • Orient the molecule in a way that the lowest  priority group  (#4) is pointing away  from you.
  • Look at the direction in which the priority decreases for the other three groups, that is 1→ 2 → 3. 

For the co unterclockwise direction, the designation is S – ,  sinister , which means “left” in Latin.

Let’s take the following molecule as an example to practice the rule:

C in the center and Cl, OH,H, & CH3 around

Step 1: The priorities are assigned.

Cl (1), OH (2), CH3 (3), & H (4)

Step 2: Re-orient the molecule, so H (#4, lowest priority) is on the position away from us. Then, the other three groups will be arranged in this way:

Cl (1) then OH (2), then CH3 (3)

Step 3: Go along the direction from #1→#2→#3; it is in the clockwise direction, so this enantiomer is assigned an R configuration, and the complete name of the molecule is ( R )-1-chloroethanol .

Now, let’s assign the configuration of the other enantiomer:

""

Following the same steps, put H away from us, and the arrangement of the other three groups is:

""

The counterclockwise direction gives the S configuration, and the complete name of the molecule is ( S )-1-chloroethanol .

Examples:  Assign R/S configuration of the chirality center.

C center, CH3, F, H, & CH2CH2Cl around

More practical hints  about R/S assignment with Cahn-Ingold-Prelog rule:

  • Assigning priority is the first possible challenge for applying the C.I.P. rule. Review and practice the guidelines in section 5.2 .
  • The second challenge is to re-orient the molecule (to arrange the #4 group away from you). The molecule model will be very helpful for this purpose . Assemble a molecular model with four different colors connected on the carbon. Compare your model to the given structure and match the assigned priority to each color; for example, red is #1, blue is #2, etc. Then, rotate the model to arrange the lowest (#4) group away from you and see how the other groups locate to get the answer.

For the perspective formula of enantiomers, it is important to know the following properties:

  • One (odd number of) switch (interchanging) for a pair of groups inverts the configuration of the chirality center.
  • Two (even number of) switches get the original configuration back.

""

For the structures above:

  • One switch of A  leads to B , A is R and B is S , so A and B are enantiomers.
  • One switch of B  leads to C , B is S and C is R , so  B and C are enantiomers.
  • Two switches of C  lead to A , and both C and A are R , so  C and A are identical .

Exercises 5.4

Determine the R/S configuration of the chirality center in following compounds.

""

Answers to Chapter 5 Practice Questions 

Exercises 5.5

Determine the relationship for each pair of molecules: enantiomers, identical, constitutional isomers, non-isomer:

""

Organic Chemistry I Copyright © 2021 by Xin Liu is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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4.7: R and S Assignments in Compounds with Two or More Stereogenic Centers

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  • Layne Morsch
  • University of Illinois Springfield

The Chinese shrub Ma Huang (Ephedra vulgaris) contains two physiologically active compounds ephedrine and pseudoephedrine. Both compounds are stereoisomers of 2-methylamino-1-phenyl-1-propanol, and both are optically active, one being levorotatory and the other dextrorotatory. Since the properties of these compounds (see below) are significantly different, they cannot be enantiomers. How, then, are we to classify these isomers and others like them?

  • Ephedrine: m.p. 35 - 40 º C, [α]D = –41º, moderate water solubility [this isomer may be referred to as (–)-ephedrine]
  • Pseudoephedrine: m.p. 119 º C, [α]D = +52º, relatively insoluble in water [this isomer may be referred to as (+)-pseudoephedrine]

Since these two compounds are optically active, each must have an enantiomer. Although these missing stereoisomers were not present in the natural source, they have been prepared synthetically and have the expected identical physical properties and opposite-sign specific rotations with those listed above. The structural formula of 2-methylamino-1-phenylpropanol has two stereogenic carbons (#1 & #2). Each may assume an R or S configuration, so there are four stereoisomeric combinations possible. These are shown in the following illustration, together with the assignments that have been made on the basis of chemical interconversions

We turn our attention next to molecules which have more than one stereocenter. We will start with a common four-carbon sugar called D-erythrose.

image126.png

A note on sugar nomenclature: biochemists use a special system to refer to the stereochemistry of sugar molecules, employing names of historical origin in addition to the designators ' D ' and ' L '. You will learn about this system if you take a biochemistry class. We will use the D/L designations here to refer to different sugars, but we won't worry about learning the system.

As you can see, D -erythrose is a chiral molecule: C 2 and C 3 are stereocenters, both of which have the R configuration. In addition, you should make a model to convince yourself that it is impossible to find a plane of symmetry through the molecule, regardless of the conformation. Does D-erythrose have an enantiomer? Of course it does – if it is a chiral molecule, it must. The enantiomer of erythrose is its mirror image, and is named L-erythrose (once again, you should use models to convince yourself that these mirror images of erythrose are not superimposable).

image128.png

Notice that both chiral centers in L-erythrose both have the S configuration. In a pair of enantiomers, all of the chiral centers are of the opposite configuration .

What happens if we draw a stereoisomer of erythrose in which the configuration is S at C 2 and R at C 3 ? This stereoisomer, which is a sugar called D-threose, is not a mirror image of erythrose. D-threose is a diastereomer of both D-erythrose and L-erythrose.

image129.png

The definition of diastereomers is simple: if two molecules are stereoisomers (same molecular formula, same connectivity, different arrangement of atoms in space) but are not enantiomers, then they are diastereomers by default. In practical terms, this means that at least one - but not all - of the chiral centers are opposite in a pair of diastereomers. By definition, two molecules that are diastereomers are not mirror images of each other.

L-threose, the enantiomer of D-threose, has the R configuration at C 2 and the S configuration at C 3 . L-threose is a diastereomer of both erythrose enantiomers.

In general, a structure with n stereocenters will have 2 n different stereoisomers. (We are not considering, for the time being, the stereochemistry of double bonds – that will come later). For example, let's consider the glucose molecule in its open-chain form (recall that many sugar molecules can exist in either an open-chain or a cyclic form). There are two enantiomers of glucose, called D-glucose and L-glucose. The D-enantiomer is the common sugar that our bodies use for energy. It has n = 4 stereocenters, so therefore there are 2 n = 2 4 = 16 possible stereoisomers (including D-glucose itself).

image130.png

In L-glucose, all of the stereocenters are inverted relative to D -glucose. That leaves 14 diastereomers of D-glucose: these are molecules in which at least one, but not all, of the stereocenters are inverted relative to D-glucose. One of these 14 diastereomers, a sugar called D -galactose, is shown above: in D-galactose, one of four stereocenters is inverted relative to D-glucose. Diastereomers which differ in only one stereocenter (out of two or more) are called epimers . D-glucose and D-galactose can therefore be refered to as epimers as well as diastereomers.

Exercise 5.9.1

Draw the structure of L-galactose, the enantiomer of D-galactose.

Solutions (3.10)

Exercise 5.9.2

Draw the structure of two more diastereomers of D-glucose. One should be an epimer.

Solutions (3.11)

Contributors

  • Organic Chemistry With a Biological Emphasis by Tim Soderberg (University of Minnesota, Morris)
  • William Reusch, Professor Emeritus ( Michigan State U. ), Virtual Textbook of Organic Chemistry

IMAGES

  1. Solved A B O HO H Assign R S configurations to the labeled

    assign r/s configurations to the labeled chiral centers in this carbohydrate molecule

  2. [Solved] assign r, s configurations to each indicated chirality center

    assign r/s configurations to the labeled chiral centers in this carbohydrate molecule

  3. Solved Assign R and S configuration at the chiral centers in

    assign r/s configurations to the labeled chiral centers in this carbohydrate molecule

  4. [Solved] assign r, s configurations to each indicated chirality center

    assign r/s configurations to the labeled chiral centers in this carbohydrate molecule

  5. OneClass: Assign R/S configurations to each chiral center in the

    assign r/s configurations to the labeled chiral centers in this carbohydrate molecule

  6. OneClass: Assign R, S configuration to each chirality center in the

    assign r/s configurations to the labeled chiral centers in this carbohydrate molecule

VIDEO

  1. Organic Chemistry

  2. PG-POLY TRB Stereochemistry- How to assign R/S configuration( wedge-dash & fischer projection )

  3. How to assign R and S configurations in telugu

  4. How to assign R & S configuration in perspective formula / Sinhala

  5. Assign R/S configuration in the following compounds • Stereoisomerism • Organic Chemistry

  6. Assigning (R) and (S) Configurations to Chiral Molecules

COMMENTS

  1. Solved Assign R/S configurations to the labeled chiral

    Science Chemistry Chemistry questions and answers Assign R/S configurations to the labeled chiral centers in this carbohydrate molecule. Please include an explanation as to why my answer is incorrect! This problem has been solved! You'll get a detailed solution from a subject matter expert that helps you learn core concepts. See Answer

  2. 7.2: Naming chiral centers: the R and S system

    Rules for assigning an R/S designation to a chiral center. 1: Assign priorities to the four substituents, with #1 being the highest priority and #4 the lowest. Priorities are based on the atomic number. 2: Trace a circle from #1 to #2 to #3. 3: Determine the orientation of the #4 priority group.

  3. How to Assign R / S Configurations to Chiral Centers

    Step 1: Prioritizing the substituents The first step is to prioritize all the substituents from one to four. Bromine is the atom with the largest atomic number, so this substituent is given the highest priority; hydrogen has the smallest atomic number, so it's given the lowest priority.

  4. 6.3: Absolute Configuration and the (R) and (S) System

    The stereocenters are labeled as R or S. ... Rules for assigning an R/S designation to a chiral center. 1: Assign priorities to the four substituents, with #1 being the highest priority and #4 the lowest. ... which by step 4a tells us that this carbon has the 'R' configuration, and that this molecule is (R)-glyceraldehyde. Its enantiomer ...

  5. Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules

    1. Chiral Centers And The Problem Of Naming Previously on MOC we've described enantiomers: molecules that are non-superimposable mirror images of each other. Perhaps the most memorable example is these "enantiocats". Each of these cats is said to be "chiral": they lack a plane of symmetry. What causes molecules to have chirality?

  6. 5.2 How to Assign R and S

    In this lesson on stereochemistry, Chad comprehensively covers how to assign R and S to chiral centers using the Cahn-Ingold-Prelog rules and works several R...

  7. Finding R and S for Chiral Centers

    Truong-Son N. · 1 · Sep 7 2015 What does S configuration mean? R and S are used to describe the configuration of a chirality center. Chirality center meaning that there are 4 different groups attached to one carbon. To determine whether the chirality center is R or S you have to first prioritize all four groups connected to the chirality center.

  8. Stereoisomers

    The resulting nomenclature system is sometimes called the CIP system or the R-S system. In the CIP system of nomenclature, each chiral center in a molecule is assigned a prefix (R or S), according to whether its configuration is right- or left-handed. No chemical reactions or interrelationship are required for this assignment.

  9. Naming Chiral Centers R and S Configurations

    This videos discusses how to name chiral centers and designate chiral carbons as R or SSupport us!: https://www.patreon.com/learningsimplyTwitter: https://tw...

  10. 5.3 Chirality and the R/S Naming System

    Draw the structure of following compounds, determine which one has an chirality center and label it with a star. a) 1-bromobutane, b) 1-pentanol, c) 2-pentanol, d) 3-pentanol, e) 2-bromopropanoic acid. f) 2-methyl cyclohexanone 2. Label all the chirality centers in the following molecules.

  11. 3.5: Naming chiral centers- the R and S system

    Rules for assigning an R/S designation to a chiral center. 1: Assign priorities to the four substituents, with #1 being the highest priority and #4 the lowest. Priorities are based on the atomic number. 2: Trace a circle from #1 to #2 to #3. 3: Determine the orientation of the #4 priority group.

  12. Assigning R/S Configuration to Chiral Centers (Part 1)

    In this video I will show you the steps to assign R/S configuration to Chiral Centers, using different examples (linear and cyclic molecules, molecules given in different projections....

  13. 5.3: Absolute Configuration: R-S Sequence Rules

    The molecule posed in this question has an ( S) configuration so the remaining substituents are added in a counterclockwise fashion. Exercise 5.3.1 5.3. 1. 1) Orient the following so that the least priority (4) atom is paced behind, then assign stereochemistry ( ( R) or ( S )). 2) Draw ( R )-2-bromobutan-2-ol.

  14. R and S configuration in organic chemistry

    How to assign R and S configuration to a compound with two asymmetric centers? Here we can use sequence rules to rank the groups attached around each chiral ...

  15. 4.7: R and S Assignments in Compounds with Two or More Stereogenic Centers

    The structural formula of 2-methylamino-1-phenylpropanol has two stereogenic carbons (#1 & #2). Each may assume an R or S configuration, so there are four stereoisomeric combinations possible. These are shown in the following illustration, together with the assignments that have been made on the basis of chemical interconversions.

  16. Assign R/S configurations to the labeled chiral centers in this

    To assign R/S configurations to the labeled chiral centers in the carbohydrate molecule, we need to assign priority numbers to the groups attached to each chiral center. The group with the highest atomic number gets the highest priority (1), and the priority decreases as the atomic numbers decrease.

  17. How to assign R and S configuration to multiple chiral centers

    This video shows how to assign R and S configuration to multiple chiral centers in molecules

  18. Assign R/S configurations to the labeled chiral centers in this

    The R/S configuration of a chiral center is determined by assigning priorities to the substituents, orienting the lowest priority group away from the viewer, and then tracing a path from the highest to the third highest priority. If the path is clockwise, the configuration is R; if it is counterclockwise, the configuration is S.

  19. Solved Assigning R and S Configuration 17. Assign the R/S

    Science Chemistry Chemistry questions and answers Assigning R and S Configuration 17. Assign the R/S configuration to each chiral center below. If you have difficulty, try building a molecule. OH Br H NH2 Q Br Br НО , H This problem has been solved! You'll get a detailed solution from a subject matter expert that helps you learn core concepts.

  20. Solved Assign R or S configuration to the chirality centers

    Assign R or S configuration to the chirality centers in the molecule below. НѕС... ОН 1 Сл 2 4 3 нас ОН Lower number chiral carbon: S Higher number chiral carbon: R у This problem has been solved! You'll get a detailed solution from a subject matter expert that helps you learn core concepts. See Answer