Which one of the following compounds is achiral?

The correct answer is Option (1) → Propan-2-ol

A molecule is chiral if it is not superimposable on its mirror image. The usual sign is an asymmetric (chiral) carbon atom, a carbon bonded to four different groups. A molecule with no such carbon (and no other source of chirality) is achiral and optically inactive.

In propan-2-ol, \( \ce{CH3-CH(OH)-CH3} \), the central carbon carries H, OH and two identical methyl groups. It is not asymmetric; the molecule has a plane of symmetry and is superimposable on its mirror image. So propan-2-ol is achiral.

Why the other options are incorrect

• Option (2) Butan-2-ol — Incorrect. \( \ce{CH3-CH(OH)-CH2CH3} \): C-2 carries H, OH, \( \ce{CH3} \) and \( \ce{C2H5} \), four different groups, so the molecule is chiral and exists as two enantiomers.

• Option (3) 2-Chlorobutane — Incorrect. \( \ce{CH3-CHCl-CH2CH3} \): C-2 carries H, Cl, \( \ce{CH3} \) and \( \ce{C2H5} \), so it is chiral.

• Option (4) 2,3-Dihydroxybutanal — Incorrect. \( \ce{CH3-CH(OH)-CH(OH)-CHO} \): both C-2 and C-3 carry four different groups, and with different end groups (CHO and \( \ce{CH3} \)) the molecule has no plane of symmetry, so it is chiral.

NCERT (2026-27 edition):
Molecular Asymmetry, Chirality and Enantiomers – “… Let us consider two simple molecules propan-2-ol (Fig.6.5) and butan-2-ol (Fig.6.6) and their mirror images.”