Ch. 5 - ChiralitySee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins
Johnny Betancourt

An achiral molecule is superimposable on its mirror image. In other words, if you can flip it and/or rotate it to make it look like its mirror image the molecule is not chiral. 

Quick Overview: 

Here are some of the ways a molecule could be achiral:

  • Has a plane of symmetry
  • Doesn’t have 4 unique groups
  • Is a meso compound
  • Is a non-quaternary amine


Achiral Molecules:

There are a few types of achiral molecules, and there are even some achiral molecules with chiral centers (also called stereogenic/inversion centers or stereocenters) in them. Achiral molecules don't have optical activity, that is they don’t rotate light like chiral molecules do. Let’s break down some examples of how a molecule might look chiral but actually be achiral:

Unique Groups:

Unique-groups-of-2,2-dichlorobutaneUnique groups of 2,2-dichlorobutane

Just because a molecule has wedge and dash drawn does not mean that it’s chiral. Remember that we need 4 unique groups attached. This molecule above only has 3 unique groups, so it’s achiral. Molecules without 4 unique groups aren’t really locked into a specific spatial arrangement. 

Plane of Symmetry:

Plane of symmetryPlane of symmetry

This molecule has 3 unique groups, and it’s also got a plane of symmetry, sometimes called a mirror plane or axis of symmetry. If a molecule’s got just one potential chiral center and a plane of symmetry, it’s achiral.

Non-meso achiral molecules can’t have stereoisomers! That’s because stereoisomers, by definition, have at least one chiral center. Things get a bit more complicated when there’s more than one potential chiral center.

Meso Compounds:

cis-1,2-dichlorocyclohexane is a meso compoundcis-1,2-dichlorocyclohexane is a meso compound

When dealing with more than one chiral center, make sure to watch out for meso compounds! Check out my videos on them for the surefire way to identify them! Even though they’ve got chiral centers, they’re overall achiral. They’re not so hard to find as rings, but they can get a bit trickier in straight-chain molecules. 

Rotating a straight-chain molecule to visually identify a meso compoundRotating a straight-chain molecule to visually identify a meso compound

With straight-chain molecules, the easiest way to visually identify a meso compound is to rotate the molecule so that the chiral centers’ substituents are facing the same direction. 


Amines:

Amine chiralityAmine chirality

Amines can be tricky! Basically, only quaternary amines can be chiral. If it’s quaternary, we can follow the same old chirality rules. Check out my videos on non-carbon chiral centers for more details!

That’s it for this quick look at achiral molecules! Remember that I’ve got tons of videos on this topic and else everything you need in Organic Chemistry. Good luck studying! 


Johnny Betancourt

Johnny got his start tutoring Organic in 2006 when he was a Teaching Assistant. He graduated in Chemistry from FIU and finished up his UF Doctor of Pharmacy last year. He now enjoys helping thousands of students crush mechanisms, while moonlighting as a clinical pharmacist on weekends.