---
title: "Diastereomers and resolution of racemic mixture"
subject: "Engineering Chemistry"
chapter: 9
key: "semester_2/engineering_chemistry:diastereomers-resolution-racemic"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/diastereomers-resolution-racemic
site: BCT Engineering
---

# Diastereomers and resolution of racemic mixture

Engineering Chemistry · Chapter 9: Stereochemistry

- Asked in: 2075 Baisakh, 3+2 marks
- Video: [Resolution of a Racemic Mixture](https://www.youtube.com/watch?v=QiAT2J8m1Do) — Rick Pires
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/diastereomers-resolution-racemic
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-9

## Question

All the diastereomers are stereoisomers but all the stereoisomers are not diastereomers. Explain. Describe the chemical method for resolution of racemic mixture.

## Short answer

Diastereomers are stereoisomers that are not mirror images, while enantiomers are mirror-image stereoisomers. Hence all diastereomers are stereoisomers, but enantiomers are not diastereomers. To resolve a racemic acid, react it with an optically pure base to give diastereomeric salts; separate these by fractional crystallization, then treat each salt with HCl to recover the pure enantiomers.

## Answer

#### Diastereomers and stereoisomers
**Stereoisomers** are isomers with the same constitution but a different arrangement of atoms in space. They are of two types: **enantiomers** (mirror images) and **diastereomers** (not mirror images). So every diastereomer is a stereoisomer, but not every stereoisomer is a diastereomer: a pair of enantiomers are stereoisomers of each other but not diastereomers. Example: for (2R,3R)-tartaric acid, the (2S,3S) form is its enantiomer and the meso form is its diastereomer. Both are stereoisomers of it, but only the meso form is a diastereomer.

#### Chemical resolution of a racemic mixture
**Resolution** is the separation of a racemic mixture into its pure enantiomers. Enantiomers have identical physical properties, so they are first converted into diastereomers.

**Chemical (diastereomeric salt) method** for a racemic acid (±)-A:
1. React it with an optically pure base such as (−)-brucine, strychnine or (R)-1-phenylethylamine:
   (+)-A·(−)-B and (−)-A·(−)-B are formed. These two salts are diastereomers.
2. They have different solubilities, so separate them by fractional crystallisation.
3. Treat each separated salt with a strong acid (HCl) to liberate the pure (+)-A and (−)-A, and recover the base.
```text
(±)-acid + (−)-base --> [(+)acid·(−)base] + [(−)acid·(−)base]
                          fractional crystallisation
                       --HCl--> (+)-acid        --HCl--> (−)-acid
```
A racemic base is resolved in the same way using an optically active acid such as (+)-tartaric acid.

## Check yourself

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

1. Why are not all stereoisomers diastereomers?
   A. Diastereomers are not stereoisomers
   B. Enantiomer pairs are stereoisomers that are mirror images, not diastereomers
   C. Enantiomers are structural isomers
   D. All stereoisomers are enantiomers
   Answer: B. Stereoisomers are enantiomers or diastereomers. (Key point: Stereo vs dia)

2. Why are enantiomers first converted to diastereomers in resolution?
   A. Diastereomers are mirror images
   B. Enantiomers are chemically unreactive
   C. To make the mixture optically active
   D. Enantiomers have identical physical properties, diastereomers differ in solubility
   Answer: D. Diastereomers can be separated by fractional crystallisation. (Key point: Basis of resolution)

3. Which reagent resolves a racemic acid?
   A. A racemic base
   B. An optically pure base such as (-)-brucine
   C. Dilute HCl
   D. Water
   Answer: B. It forms diastereomeric salts. (Key point: Reagent)

4. How are pure enantiomeric acids liberated from the separated salts?
   A. By heating to racemise
   B. By adding more base
   C. By distillation
   D. By treating with a strong acid such as HCl
   Answer: D. Acid frees the pure acid and recovers the base. (Key point: Liberation)

## Related in Engineering Chemistry

- [Racemic mixture vs meso compound](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aracemic-meso) — 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
- [Isomers of 3-bromo-2-butanol; racemization, resolution](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Abromobutanol-isomers-resolution) — Engineering Chemistry, chapter 9

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