- 2071 Bhadra · 2+3 marks
Enantiomers and diastereomers of 3-bromo-2-butanol
Short answer
Enantiomers are non-superimposable mirror images with identical physical properties; diastereomers are stereoisomers that are not mirror images and have different physical properties. 3-Bromo-2-butanol has four stereoisomers: two enantiomeric pairs, (2S,3R)/(2R,3S) and (2S,3S)/(2R,3R), with diastereomers between these pairs.
Distinguish between enantiomers and diastereomers. Show these isomers in 3-bromo-2-butanol.
Difference
| Basis | Enantiomers | Diastereomers |
|---|---|---|
| Relationship | Stereoisomers that are non-superimposable mirror images | Stereoisomers that are not mirror images |
| Number of chiral centres | One or more (all centres inverted) | Usually two or more (only some inverted); cis–trans isomers are also diastereomers |
| Physical properties | Identical (m.p., b.p., solubility) | Different |
| Chemical properties | Same with achiral reagents | Different |
| Optical rotation | Equal in magnitude, opposite in sign | Different, unrelated values |
| Separation | Difficult (needs resolution) | Easy (crystallisation, distillation) |
| Example | (+)- and (−)-lactic acid; (2R,3R)- and (2S,3S)-tartaric acid | (2R,3R)-tartaric acid and meso-tartaric acid |
In 3-bromo-2-butanol
(2S,3R) (2R,3S) (2S,3S) (2R,3R)
CH₃ CH₃ CH₃ CH₃
H—C—OH HO—C—H H—C—OH HO—C—H
H—C—Br Br—C—H Br—C—H H—C—Br
CH₃ CH₃ CH₃ CH₃
3-Bromo-2-butanol, CH₃–CH(OH)–CH(Br)–CH₃, has two different chiral carbons (C2 and C3), so it has 2² = 4 stereoisomers (there is no meso form, since the two centres are different).
- Enantiomers: (2S,3R) and (2R,3S) form one pair; (2S,3S) and (2R,3R) form another pair. The two members of each pair are mirror images.
- Diastereomers: any member of one pair relative to a member of the other, e.g. (2S,3R) and (2S,3S); they are not mirror images and have different physical properties.
✓ Answer checked by Claude Opus 5.5 (high effort)
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