Stereochemistry: study guide

Key ideas

Stereochemistry is about the three-dimensional arrangement of atoms. Molecules that have the same connectivity but different arrangements in space can behave very differently, especially with chiral reagents and in living cells.

• A stereocenter is a carbon with four different groups. A molecule with n stereocenters has at most 2ⁿ stereoisomers.
• Enantiomers are non-superimposable mirror images, with equal and opposite optical rotation. Diastereomers are stereoisomers that are not mirror images.
• A meso compound has stereocenters but an internal mirror plane, so it is achiral and optically inactive, and it reduces the count of isomers.
• Specific rotation is [α] = α ÷ (l × c) with l in dm and c in g/mL. Enantiomeric excess is ee = |R − S| ÷ (R + S) × 100, and a racemic mixture has ee = 0.

Key equations

• stereoisomers ≤ 2ⁿ
• [α] = α ÷ (l × c)
• ee = |R − S| ÷ (R + S) × 100
• optical purity = ee

Worked example

Tartaric acid has two stereocenters, so the maximum is 2² = 4 stereoisomers, but because one of them is meso there are only three: the (R,R) and (S,S) enantiomers and the meso form. A sample with 75 % R and 25 % S has ee = 50 %, so its rotation is half that of the pure R form.

Common mistakes

• Assuming 2ⁿ isomers without checking for a meso form.
• Treating R and S as + and −. The label R or S says nothing about the direction of optical rotation.
• Calling a racemic mixture optically active. The two rotations cancel.

Practice

Test yourself with practice questions on this topic: Stereochemistry