---
title: "Isomers of 3-bromo-2-butanol; racemization, resolution"
subject: "Engineering Chemistry"
chapter: 9
key: "semester_2/engineering_chemistry:bromobutanol-isomers-resolution"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/bromobutanol-isomers-resolution
site: BCT Engineering
---

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

Engineering Chemistry · Chapter 9: Stereochemistry

- Asked in: 2070 Bhadra, 3 marks
- Video: [Diastereomers](https://www.youtube.com/watch?v=23-dfkwKCcg) — The Organic Chemistry Tutor
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/bromobutanol-isomers-resolution
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-9

## Question

Write the possible isomers of 3-bromo-2-butanol and specify enantiomers, racemization and resolution of racemic mixture.

## Short answer

3-Bromo-2-butanol has four stereoisomers: (2S,3R) and (2R,3S) are enantiomers; (2S,3S) and (2R,3R) are another pair. Racemisation converts an enantiomer to a racemic mixture via a planar intermediate. Resolution separates the racemate by forming diastereomeric esters with an optically pure acid, then fractional crystallisation and hydrolysis.

## Answer

#### Possible isomers of 3-bromo-2-butanol
```text
 (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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why are there 4 stereoisomers of 3-bromo-2-butanol and no meso?
   A. Two identical chiral centres
   B. One chiral centre
   C. Two different chiral centres
   D. A plane of symmetry
   Answer: C. 2^2 = 4; the centres differ so no meso form. (Key point: Number)

2. What is racemisation?
   A. Conversion of an active enantiomer into an equimolar optically inactive mixture
   B. Separation of enantiomers
   C. Conversion of meso to active
   D. Conversion of cis to trans
   Answer: A. It occurs via planar intermediates like carbocations (SN1). (Key point: Racemisation)

3. How is a racemic alcohol resolved?
   A. Simple distillation
   B. Heating
   C. Convert to diastereomeric esters or salts with an optically pure acid and separate by crystallisation
   D. Adding more racemic mixture
   Answer: C. Hydrolysis then regenerates the pure alcohols. (Key point: Resolution)

## Related in Engineering Chemistry

- [Enantiomers and diastereomers of 3-bromo-2-butanol](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aenantiomers-diastereomers-bromobutanol) — Engineering Chemistry, chapter 9
- [Enantiomers, racemic mixture, meso compound](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aenantiomers-racemic-meso) — Engineering Chemistry, chapter 9
- [Diastereomers and resolution of racemic mixture](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Adiastereomers-resolution-racemic) — Engineering Chemistry, chapter 9
- [Racemic mixture vs meso compound](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aracemic-meso) — Engineering Chemistry, chapter 9