Skip to main content

Isomerism in 2-hydroxypropanoic acid

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.

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

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.

   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).

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Engineering Chemistry chapter 9 questions →