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Enantiomers, Diastereomers
& Meso Compounds

Master the relationships between stereoisomers: learn to identify mirror images, non-mirror-image stereoisomers, and molecules with internal symmetry planes.

Organic Chemistry 1

What Are Stereoisomers?

Stereoisomers are molecules with the same molecular formula and the same connectivity of atoms, but they differ in the 3D arrangement of those atoms in space. Unlike constitutional isomers, you cannot convert one stereoisomer into another by simply rearranging bonds.

Chirality and Stereocenters

A molecule is chiral if it is non-superimposable on its mirror image: just like your left and right hands. The most common source of chirality is a stereocenter (chiral center): a carbon bonded to four different groups.

C* A B C D
(\(R\))-config
 mirror C* A B C D
(\(S\))-config

Defining Enantiomers

Mirror Images
Enantiomers are stereoisomers that are non-superimposable mirror images of each other. Every stereocenter has the opposite R/S configuration.
Identical Physical Properties
Same melting point, boiling point, solubility, and density. They differ only in the direction they rotate plane-polarized light: \( (+) ) vs \( (-) \).

Defining Diastereomers

Diastereomers are stereoisomers that are NOT mirror images of each other. They arise when a molecule has two or more stereocenters and some but not all configurations are inverted.

(\(R, R\))
 C1 OH H C2 OH H
(\(S, S\))
 C1 H OH C2 H OH
(\(R, S\))
 C1 OH H C2 H OH Enantiomers (all inverted) Diastereomers (some inverted) Diastereomers

Meso Compounds

A meso compound has two or more stereocenters but is achiral overall because it possesses an internal plane of symmetry. The stereocenters effectively cancel each other out, producing net zero optical rotation.

COOH
C1 (\(S\))
 OH H symmetry plane
C2 (\(R\))
 H OH COOH

The Stereoisomer Decision Tree

Same connectivity? Yes No Constitutional Isomers Mirror images? Yes No Superimposable? Yes No Identical (Meso) Enantiomers Diastereomers (includes cis/trans)

Maximum Number of Stereoisomers

The maximum number of stereoisomers for a molecule with \( n \) stereocenters is given by:

1
\( n = 1 \): \( 2^1 = 2 \) stereoisomers (one enantiomeric pair)
2
\( n = 2 \): \( 2^2 = 4 \) maximum, but a meso form may reduce this to 3
3
\( n = 3 \): \( 2^3 = 8 \) maximum stereoisomers (four enantiomeric pairs)

Enantiomers vs Diastereomers

Property Enantiomers Diastereomers
Mirror Images? Yes No
Stereocenters Inverted ALL inverted SOME inverted
Physical Properties Identical (except optical rotation) Different
Optical Activity Equal but opposite (\(+\) vs \(-\)) Different magnitudes
Separable by Chromatography? Only with chiral column Yes, standard methods
Includes cis/trans? No Yes

How to Identify the Relationship

1
Find all stereocenters. Identify every carbon bonded to four different groups. Assign R/S to each.
2
Compare R/S at each center. Write out the configuration of both molecules side by side.
3
ALL inverted? They are enantiomers. Some inverted? They are diastereomers.
4
Check for meso. Look for an internal plane of symmetry. If present, the molecule is achiral despite having stereocenters.
5
Check superimposability. If two structures are mirror images AND superimposable, they are identical (the same compound, likely meso).

Example 1: 2-Bromobutane

2-Bromobutane has one stereocenter at C2. With \( n = 1 \), we expect \( 2^1 = 2 \) stereoisomers: the (R) and (S) forms, which are enantiomers.

(\(R\))-2-Bromobutane
 CH₃ C2* CH₂CH₃ Br H mirror
(\(S\))-2-Bromobutane
 CH₃ C2* CH₂CH₃ Br H

Example 2: Tartaric Acid Stereoisomers

Tartaric acid has two stereocenters (\( n = 2 \)), so \( 2^2 = 4 \) max. But one form is meso, giving only 3 stereoisomers:

(\(R, R\))
Optically active: rotates light \( (+)\). Enantiomer of (S, S).
(\(S, S\))
Optically active: rotates light \( (-)\). Enantiomer of (R, R).
(\(R, S\)) Meso
Has an internal symmetry plane. Achiral: \( [\alpha] = 0 \). Diastereomer of both.

Example 3: Three Stereocenters

A molecule with 3 stereocenters and no internal symmetry has \( 2^3 = 8 \) stereoisomers arranged as 4 enantiomeric pairs.

Pair 1
(\(R,R,R\))
(\(S,S,S\))
 Pair 2
(\(R,R,S\))
(\(S,S,R\))
 Pair 3
(\(R,S,R\))
(\(S,R,S\))
 Pair 4
(\(R,S,S\))
(\(S,R,R\))
 Within each pair: ENANTIOMERS Between pairs: DIASTEREOMERS
\( (R,R,R) \) vs \( (R,R,S) \): only C3 differs
 = Diastereomers (some, not all, inverted)

Optical Activity Summary

Enantiomers
Rotate light by equal magnitude, opposite sign. A racemic mixture (\( \pm \)) gives \( [\alpha] = 0 \).
Diastereomers
Rotate light by different magnitudes and may rotate in the same or opposite directions.
Meso
No net optical rotation: \( [\alpha] = 0 \). Internal symmetry cancels the contributions of opposite stereocenters.

Common Mistakes

✕ \( R = (+) \) and \( S = (-) \)
R/S describes spatial arrangement. The sign of rotation is determined experimentally and has no direct correlation with \(R\) or \(S\).
✕ Meso = No Stereocenters
Meso compounds do have stereocenters. They are achiral because an internal symmetry plane makes them superimposable on their mirror image.
✕ Forgetting Meso in the Count
The \( 2^n \) formula gives the maximum. When identical substituents create a meso form, the actual count is less than \( 2^n \).

Summary Table

Feature Enantiomers Diastereomers Meso
Mirror images? Yes No Self-mirror
Superimposable? No No Yes (with mirror)
Chiral? Yes Usually yes No
Optically active? \( [\alpha] \neq 0 \) \( [\alpha] \neq 0 \) (usually) \( [\alpha] = 0 \)
R/S relationship All inverted Some inverted Mixed (e.g., \(R, S\))
Physical properties Identical Different Unique

Question 1 of 5

Two molecules have the configurations (\(R, R, S\)) and (\(S, S, R\)) at their three stereocenters. What is their relationship?

Question 2 of 5

A compound has two stereocenters with configurations (\(R, S\)) and an internal plane of symmetry. This compound is best described as:

Question 3 of 5

What is the maximum number of stereoisomers for a molecule with 4 stereocenters and no internal symmetry?

Question 4 of 5

Enantiomers have identical physical properties except for:

Question 5 of 5

Molecules with configurations (\(R, R\)) and (\(R, S\)) at two stereocenters are related as: