Isomers of 3-bromo-2-butanol; racemization, resolution

Possible isomers of 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).

• (2S,3R) and (2R,3S) are enantiomers; (2S,3S) and (2R,3R) are another enantiomeric pair. Members of different pairs are diastereomers.

Racemisation

Racemisation is the conversion of an optically active enantiomer into an equimolar mixture of both enantiomers (a racemic mixture, which is optically inactive). It takes place by heat, light or chemical reaction through a planar intermediate such as a carbocation (e.g. in SN1). The product is written (±).

Resolution of the racemic mixture

Resolution is the separation of the (±) mixture into pure enantiomers. The racemic alcohol is converted into a pair of diastereomeric esters by reaction with an optically pure acid (e.g. (+)-tartaric acid); alternatively, its acid phthalate half-ester is made and turned into diastereomeric salts with an optically pure base such as (−)-brucine. The diastereomers differ in solubility and are separated by fractional crystallisation. Hydrolysis then regenerates the pure (+) and (−) alcohols.