---
title: "Isomerism in 2-hydroxypropanoic acid"
subject: "Engineering Chemistry"
chapter: 9
key: "semester_2/engineering_chemistry:lactic-acid-isomerism"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/lactic-acid-isomerism
site: BCT Engineering
---

# Isomerism in 2-hydroxypropanoic acid

Engineering Chemistry · Chapter 9: Stereochemistry

- Asked in: 2069 Poush, 2 marks
- Video: [Isomerism in Lactic acid Part II,CHEMSCOPE](https://www.youtube.com/watch?v=FkrxjAFHiDo) — Chemscope
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/lactic-acid-isomerism
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-9

## Question

What isomerism is shown by 2-hydroxy propanoic acid and why?

## Short answer

2-Hydroxypropanoic acid (lactic acid) shows optical isomerism because the central carbon is chiral, bonded to –H, –OH, –CH₃, and –COOH. It exists as (R)-(−)- and (S)-(+)-lactic acid enantiomers, rotating light equally in opposite directions; a racemic mixture is inactive.

## Answer

2-Hydroxypropanoic acid (lactic acid), CH₃–CH(OH)–COOH, shows **optical isomerism**.

The central carbon (C2) is bonded to four different groups: –H, –OH, –CH₃ and –COOH. It is therefore a chiral (asymmetric) carbon, and the molecule is non-superimposable on its mirror image.
```text
   COOH           COOH
H—C—OH       HO—C—H
   CH₃            CH₃
 (R)-(−)        (S)-(+)
 D-form         L-form
```
It exists as two enantiomers, (S)-(+)-lactic acid (L-form, from muscle; sarcolactic acid) and (R)-(−)-lactic acid (D-form), which rotate the plane of polarised light by equal angles in opposite directions. An equal mixture is racemic (±)-lactic acid, which is optically inactive (found in sour milk).

## Check yourself

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

1. What isomerism does lactic acid show?
   A. Geometrical isomerism
   B. Optical isomerism
   C. Metamerism
   D. Chain isomerism only
   Answer: B. It has a chiral carbon. (Key point: Type)

2. Why is C2 of lactic acid chiral?
   A. It has two identical groups
   B. It is in a double bond
   C. It is bonded to three groups
   D. It is bonded to four different groups: H, OH, CH3, COOH
   Answer: D. Four different groups make it asymmetric. (Key point: Chiral carbon)

3. Why is racemic (±)-lactic acid inactive?
   A. Neither form rotates light
   B. Equal and opposite rotations cancel
   C. It is meso
   D. It is a single enantiomer
   Answer: B. External compensation. (Key point: Racemic)

## Related in Engineering Chemistry

- [Optical isomerism; tartaric acid; maleic/fumaric Z,E](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aoptical-isomerism-tartaric-maleic) — Engineering Chemistry, chapter 9