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Open Pedagogy Assignments are assignments in which students use their agency and creativity to create knowledge artifacts that can support their own learning, their classmates’ learning, and the learning of students around the world. (See this peer-reviewed article for more details.) The assignment on this page is aligned to a learning outcome of Biology for Non-Majors I and we’ve identified the module where the reading appears. The assignment can be created with basic web and computing tools, a cell phone camera or any video recording device, Google or Word documents, and your learning management system.

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• Identify the principles of chemistry that are integral to biology.

In the module on Chemistry, we provide a general overview of how the principles of chemistry form the basis of biology. For this assignment, you are going to reflect upon which concept(s) in this module were most difficult for you to learn.

The product of your work will help future students to learn about some of the most difficult concepts in the course. Thus, think of your audience as friends who are taking Biology for Nonmajors in the next term. You want to help them understand a concept in the course that was particularly difficult for you.

First, identify a concept from this module that you struggled to learn regarding the basic principles of chemistry in biology. Review the content in the module and anything further you learned in class. Then, think of how you would explain the concept in your own words to a friend who is also taking the course. Keep these questions in mind:

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Second, using your cell phone or any other recording device, create a short video explaining the topic in your own words. You don’t have to edit or create a professional-grade film. You’ve most likely have done this type of recording already on social media, so feel free to use the same informal conversational tone. Use additional images or tips that would have been helpful for you.

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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”.

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
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• 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
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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 Key Factors For Determining 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
• Alkene Addition Reactions: "Regioselectivity" and "Stereoselectivity" (Syn/Anti)
• Stereoselective and Stereospecific Reactions
• 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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Balancing Chemical Equations | Assignment for OpenStax Chemistry: Atoms First 2e | Chapter 7: Stoichiometry of Chemical Reactions

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Module 13: Fundamental Equilibrium Concepts

Assignment: equilibrium.

• For each of the following reactions, determine whether the value of the equilibrium constant favors the formation of reactants, products, or both sides equally.

a) Br 2 (g) + Cl 2 (g)  ↔ 2 BrCl(g)              K eq = 3 x 10 6

b) H 2 (g) + Br 2 (g) ↔   2 HBr(g)            K eq = 1.15

c) I 2 (g) ↔   I(g) + I(g)                          K eq = 4.5 x 10 -7

2. Molecular chlorine decomposes into atoms according to the reaction:

Cl 2 (g) ↔   2 Cl (g)

The equilibrium constant for the reaction at 25°C is 1.4 x 10 -38 .  Would many chlorine atoms be present at this temperature?  Explain how you can determine this.

3. For the following reaction at equilibrium at 2000°C, the concentration of N2 and O2 are both 4.8 M.

N 2 (g) + O 2 (g) ↔   2 NO(g)       K eq = 6.3 x 10 -4

Calculate the concentration of NO at equilibrium. Show your work; pay careful attention to exponents.

4. CaCO 3 (chalk) can produce solid CaO and CO 2 gas when heated. If 6.0 moles of carbon dioxide forms in a 4.500 L reaction vessel, what is the equilibrium constant for this reaction?

5.  Le Chatelier’s Principle states that if a stress is applied to a reversible reaction at equilibrium, the reaction will undergo a shift in order to re-establish its equilibrium. Consider the following exothermic reversible reaction at equilibrium: 2 A ↔B + C In which direction (left or right) would the following stresses cause the system to shift? a. decrease the concentration of A b. increase the concentration of B c. lower the temperature

6. Chemical engineers use Le Chatêlier’s principle to predict shifts in chemical system at equilibrium resulting from changes in reaction conditions. Predict the changes needed to maximize the yield of product in each of the following industrial chemical systems:

• a) the production of ethene (ethylene)

C 2 H 6 (g) + energy ↔C 2 H 4 (g) + H 2 (g)

• b) the production of methanol

CO(g) + 2H 2 (g) ↔ CH 3 OH(g) + energy

• Acetic acid, HC 2 H 3 O 2 , is in equilibrium with its ions:

HC 2 H 3 O 2 (aq) ↔ H+(aq) + C 2 H 3 O 2 – (aq)     Keq = 1.8 x 10 -5

At equilibrium, the concentration of the ions are:                  [H+] = 1.33 x 10 -3 M

[C 2 H 3 O 2 – ] = 1.33 x10 -3 M

Calculate the concentration of the acid, HC 2 H 3 O 2 .

8.Sulfuric acid is the most common commercial acid, with millions of tons produced each year. The second step in the production involves the oxidation of sulfur dioxide gas catalyzed with V 2 O 5 (s),

2SO 2 (g) + O 2 (g) ↔2 SO 3 (g)      D H ° = -198 kJ

a) Set-up the equilibrium law for this reaction.

b) List three stresses that could shift the equilibrium to the products side of the reaction.

c) If the equilibrium constant, K eq , for the above reaction at a given temperature is 4 x 10 3 , what is the constant (K eq ) for the reverse reaction (the decomposition of sulfur trioxide)?

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Absolute Configuration - R-S Sequence Rules

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To name the enantiomers of a compound unambiguously, their names must include the "handedness" of the molecule. The method for this is formally known as R/S nomenclature.

Introduction

The method of unambiguously assigning the handedness of molecules was originated by three chemists: R.S. Cahn, C. Ingold, and V. Prelog and is also often called the Cahn-Ingold-Prelog rules . In addition to the Cahn-Ingold system, there are two ways of experimentally determining the absolute configuration of an enantiomer:

• X-ray diffraction analysis. Note that there is no correlation between the sign of rotation and the structure of a particular enantiomer.
• Chemical correlation with a molecule whose structure has already been determined via X-ray diffraction.

However, for non-laboratory purposes, it is beneficial to focus on the R/S system. The sign of optical rotation , although different for the two enantiomers of a chiral molecule, at the same temperature, cannot be used to establish the absolute configuration of an enantiomer; this is because the sign of optical rotation for a particular enantiomer may change when the temperature changes.

Stereocenters are labeled R or S

The "right hand" and "left hand" nomenclature is used to name the enantiomers of a chiral compound. The stereocenters are labeled as R or S.

Consider the first picture: a curved arrow is drawn from the highest priority ( 1 ) substituent to the lowest priority (4 ) substituent. If the arrow points in a counterclockwise direction ( left when leaving the 12 o' clock position), the configuration at stereocenter is considered S ("Sinister" → Latin= "left"). If, however, the arrow points clockwise,( Right when leaving the 12 o' clock position) then the stereocenter is labeled R ("Rectus" → Latin= "right"). The R or S is then added as a prefix, in parenthesis, to the name of the enantiomer of interest. For example: ( R )-2-Bromobutane and ( S )-2,3- Dihydroxypropanal.

Sequence rules to assign priorities to substituents

Before applying the R and S nomenclature to a stereocenter, the substituents must be prioritized according to the following rules:

First, examine at the atoms directly attached to the stereocenter of the compound. A substituent with a higher atomic number takes precedence over a substituent with a lower atomic number. Hydrogen is the lowest possible priority substituent, because it has the lowest atomic number.

• When dealing with isotopes, the atom with the higher atomic mass receives higher priority.
• When visualizing the molecule, the lowest priority substituent should always point away from the viewer (a dashed line indicates this). To understand how this works or looks, imagine that a clock and a pole. Attach the pole to the back of the clock, so that when when looking at the face of the clock the pole points away from the viewer in the same way the lowest priority substituent should point away.
• Then, draw an arrow from the highest priority atom to the 2nd highest priority atom to the 3rd highest priority atom. Because the 4th highest priority atom is placed in the back, the arrow should appear like it is going across the face of a clock. If it is going clockwise, then it is an R-enantiomer; If it is going counterclockwise, it is an S-enantiomer.

When looking at a problem with wedges and dashes, if the lowest priority atom is not on the dashed line pointing away, the molecule must be rotated. Remember that

• Wedges indicate coming towards the viewer.
• Dashes indicate pointing away from the viewer.

If there are two substituents with equal rank, proceed along the two substituent chains until there is a point of difference. First, determine which of the chains has the first connection to an atom with the highest priority (the highest atomic number). That chain has the higher priority.

If the chains are similar, proceed down the chain, until a point of difference.

For example : an ethyl substituent takes priority over a methyl substituent. At the connectivity of the stereocenter, both have a carbon atom, which are equal in rank. Going down the chains, a methyl has only has hydrogen atoms attached to it, whereas the ethyl has another carbon atom. The carbon atom on the ethyl is the first point of difference and has a higher atomic number than hydrogen; therefore the ethyl takes priority over the methyl.

If a chain is connected to the same kind of atom twice or three times, check to see if the atom it is connected to has a greater atomic number than any of the atoms that the competing chain is connected to.

• If none of the atoms connected to the competing chain(s) at the same point has a greater atomic number: the chain bonded to the same atom multiple times has the greater priority
• If however, one of the atoms connected to the competing chain has a higher atomic number: that chain has the higher priority.

A 1-methylethyl substituent takes precedence over an ethyl substituent. Connected to the first carbon atom, ethyl only has one other carbon, whereas the 1-methylethyl has two carbon atoms attached to the first; this is the first point of difference. Therefore, 1-methylethyl ranks higher in priority than ethyl, as shown below:

Remember that being double or triple bonded to an atom means that the atom is connected to the same atom twice. In such a case, follow the same method as above.

Caution!! Keep in mind that priority is determined by the first point of difference along the two similar substituent chains. After the first point of difference, the rest of the chain is irrelevant.

After all your substituents have been prioritized in the correct manner, you can now name/label the molecule R or S.

• Put the lowest priority substituent in the back (dashed line).
• Proceed from 1 to 2 to 3. (it is helpful to draw or imagine an arcing arrow that goes from 1--> 2-->3)
• Determine if the direction from 1 to 2 to 3 clockwise or counterclockwise.

i) If it is clockwise it is R. ii) if it is counterclockwise it is S .

USE YOUR MODELING KIT: Models assist in visualizing the structure. When using a model, make sure the lowest priority is pointing away from you. Then determine the direction from the highest priority substituent to the lowest: clockwise (R) or counterclockwise (S).

IF YOU DO NOT HAVE A MODELING KIT : remember that the dashes mean the bond is going into the screen and the wedges means that bond is coming out of the screen. If the lowest priority bond is not pointing to the back, mentally rotate it so that it is. However, it is very useful when learning organic chemistry to use models.

If you have a modeling kit use it to help you solve the following practice problems.

Exercise \(\PageIndex{1}\)

Are the following R or S?

• S: I > Br > F > H. The lowest priority substituent, H, is already going towards the back. It turns left going from I to Br to F, so it's a S.
• R: Br > Cl > CH 3 > H. You have to switch the H and Br in order to place the H, the lowest priority, in the back. Then, going from Br to Cl, CH 3 is turning to the right, giving you a R.
• Neither R or S : This molecule is achiral. Only chiral molecules can be named R or S.
• R: OH > CN > CH 2 NH 2 > H. The H, the lowest priority, has to be switched to the back. Then, going from OH to CN to CH 2 NH 2 , you are turning right, giving you a R. (5)
• S: \(\ce{-COOH}\) > \(\ce{-CH_2OH}\) > \(\ce{C#CH}\) > \(\ce{H}\). Then, going from \(\ce{-COOH}\) to \(\ce{-CH_2OH}\) to \(\ce{-C#CH}\) you are turning left, giving you a S configuration.
• Schore and Vollhardt. Organic Chemistry Structure and Function. New York:W.H. Freeman and Company, 2007.
• McMurry, John and Simanek, Eric. Fundamentals of Organic Chemistry . 6th Ed. Brooks Cole, 2006.

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100 Hard Words to Spell in English

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English is a great language with both basic and complicated vocabulary. Certain words in the language even have various spellings but the same pronunciation. In addition, several words sound different than how they should be spelled. Similarly, there are many hard words to spell in English. In this blog, we have shared a list of some of the hardest words to spell in the English language, especially for those who are always caught making spelling mistakes.

In the era of autocorrect, many people have overlooked the importance of spelling and only struggle with it while writing an essay or academic paper. If you are participating in a Spelling Bee competition, you must have a thorough understanding of English words and how to spell them. Typically, in the competition, you will be asked to write hard-to-spell words. So, feel free to explore the list and expand your vocabulary.

Why is it Difficult to Spell Certain English Words?

Have you ever thought about why some English words are misspelled often and are even hard to spell? Generally, in English, there are some spelling rules. If you are unaware of it, then chances are there for you to get locked in spelling errors out of confusion. Find here, some common reasons why certain words are hard to spell.

• Distinctions between British and American English
• Long words containing a lot of vowels
• Words that contain silent letters
• Two consonants together
• Homophones [words with similar sounds but distinct spellings and meanings]
• Homographs [words that have the same spelling but a distinct origin, pronunciation, and meaning]
• Homonyms [words with the same phoneme and spelling but distinct meanings]
• Confusion with ‘e’ and ‘i’
• The ‘c’ and ‘k’ sounds are similar
• A difficulty with the sounds “y,” “ee,” and “lee”
• Ambiguity between “ea” and “ae”

List of Hardest Words To Spell in English

The English language typically combines pronunciations and spellings from many other languages including Latin, Greek, French, and German. Moreover, different countries spell some words differently. As a result, confusion often prevails among users.

To assist you, below we have organized and listed some of the most difficult words to spell in English. Feel free to take a look.

Also Read: Top 12 Longest Words in English Language

Easy Words That Are Hard to Spell

These are some easy English words that many people find hard to spell or pronounce.

1. Necessary

One of the reasons English is difficult to spell is that some letters like c and s can generate the same sounds but do not always do so. This, paired with the usage of double consonants that do not affect how the word is spoken makes it difficult to spell. How do you recall where and how many c or s letters you will need? So have a look at the word. Do you say “c” with two s’s? Ask yourself this to ensure that a single c followed by a double “s” is necessary.

2. Narcissistic

Narcissistic, like necessary, is difficult to spell because the letters “c” and “s” sound the same way. It can be difficult to remember where to place the double “s”. It might help to know that the word narcissistic is derived from the Greek nárkissos, a plant name linked with opiates.

3. Accommodate

The word accommodate contains a double “c” and a double “m” for good measure. However, the vowels, not the consonants, are what make writing this term difficult. The word “accommodate” [uh-kom-uh-deyt] sounds like it should be spelled with three o’s, or is there a “u” in there? But there are no u’s, and the first letter is an “a”, of all things.

Speaking of words with a double “c”, the word vacuum is one that you might anticipate having this spelling but does not. Vacuum has the unusual double “u” instead of the common double c (other words with this peculiar combination are muumuu and continuum). The word’s root is vacuus, which is Latin for “empty.”

Silent g’s can come at the beginning and conclusion of words. One example is phlegm [flem]. This term is especially difficult to pronounce because it combines the letters ph and ff. This style of expressing the ff sound appears in Greek words such as phlegm and phone.

6. Spaghetti

A word with Italian origins that is difficult to spell is spaghetti. The letter “i” at the end of a word in Italian denotes that it is plural. The nearly-silent “h” may also confuse you while spelling this word.

7. Entrepreneur

Entrepreneur [ahn-truh-pruh-nur] is a French-spelled word that English speakers find difficult to pronounce. As it begins with an ah sound, you might assume it contains an “a”, but this is not the case. Next time you write this word, remind yourself that the majority of the vowel sounds are “e”, with the exception of the “eu” at the very end for the “oor” sound.

When it comes to silent letters, we employ an astonishing amount of them in English while spelling. Unless you know the word, there is no way to know these letters should be there. Asthma is a prime example of this, with a silent “th”. Yes, English will occasionally insert a silent “th” to keep you on your toes.

Another word that uses a deceptive silent letter is “indict” [in-dahyt]. You do not pronounce the letter c in this word, thus you may overlook it when spelling. The letter “c” comes from the late Latin word indictāre, which is connected to the English verb dictate.

10. Acquiesce

Another word that means “to assent tacitly; agree,” acquiesce [ak-wee-es], too has some odd letter combinations associated with its pronunciation. The Latin verb acquiēscere, which means “to find rest in,” is the source of the name. When spelling this word, bear in mind that the prefix ac-means “toward” or “to.” This may help you spell the word correctly because it indicates that it breaks down as ac-qui-esce.

Simple Words That Are Hard to Spell

Certain simple English words are frequently misspelled. Here, let us have a look at the list of the easy words that are difficult to spell.

• Accommodate
• Intelligence
• Pronunciation
• Handkerchief

Hard Words to Spell for School Students

The following are some English words that many students write incorrectly.

• Defalcation

Tough to Spell English Words

The following are some hard words to spell in English.

• Bureaucracy

Also Read: What is the Importance of Grammar in English?

Hard Words to Spell for College Students

Here is a list of hard words that college students struggle to spell.

• Contemptuous
• Superintendent
• Discernible
• Resplendent

Frequently Misspelled Words

The following are some of the most regularly misspelled English terms.

• Chronological
• Acknowledgment

Hard to Spell Long Words

Even for native speakers of the language, English is a lovely yet challenging language. In addition to errors, persons with strong fluency occasionally have trouble spelling lengthy words correctly. Some lengthy words that are challenging to spell are listed here.

• Hepaticocholangiogastrostomy
• Pneumonoultramicroscopicsilicovolcanokoniosis
• Pseudopseudohypoparathyroidism
• Hippopotomonstrosesquippedaliophobia
• Psychoneuroendocrinological
• Spectrophotofluorometrically
• Pneumonoultramicroscopicsilicovolcanoconiosis
• Antidisestablishmentarianism
• Supercalifragilisticexpialidocious
• Floccinaucinihilipilification

Tricky Words to Spell in the English Language

There are a few English words that are challenging to spell. The spelling and pronunciation of certain words may not always match. So, there is a chance that you will get into trouble. Here is a list of hard words to spell.

• Chiaroscurist
• Autochthonous
• Gobbledegook
• Paraphernalia
• Chiaroscuro
• Gubernatorial
• Pochemuchka
• Kierkegaardian
• Sacrilegious

Spelling rules in English can be difficult, with many catches and exceptions. You can avoid making mistakes if you correctly follow the guidelines. Silent letters, homophones, regional variances, and double consonants are certain traps to avoid. But keep in mind that you will become more proficient at spelling words in the English language the more you study and practice.

If you are worried about receiving bad grades on your assignment due to spelling errors, approach us immediately. We have native English speakers as assignment helpers to provide you with high-quality English assignment help online . In addition, we offer proofreading as a component of our online assignment help services, so you can get aid from our language assistants to fix any spelling or grammar mistakes in your papers.

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