The figure shows propan-2-ol (A) and butan-2-ol (D), each with its mirror image and with that mirror image rotated by 180°. Structure C overlaps structure A completely, but structure F does not overlap structure D.
Which feature of the two molecules, shown by the circled groups, explains this?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The central carbon of butan-2-ol carries four different groups, while that of propan-2-ol carries two identical CH3 groups.
A molecule that cannot be superimposed on its mirror image is chiral, and a carbon carrying four different groups (an asymmetric carbon) makes such a molecule.
Propan-2-ol (A, B, C) — the two circled groups are both CH3. The central carbon is not asymmetric, so the rotated mirror image C falls on A: the molecule is achiral.
Butan-2-ol (D, E, F) — the circled groups are CH3 and C2H5, with H and OH: four different groups. After rotation, CH3 of F lies where C2H5 of D lies, so F does not overlap D: the molecule is chiral.
Why the other options are incorrect
Option (1) Butan-2-ol has a larger molecular mass than propan-2-ol. — Incorrect. Mass has nothing to do with whether a molecule can be superimposed on its mirror image.
Option (2) The –OH group is on a primary carbon in butan-2-ol but on a secondary carbon in propan-2-ol. — Incorrect. In both molecules the –OH group is on carbon 2, which is a secondary carbon.
Option (4) The central carbon of butan-2-ol is sp2 hybridised, while that of propan-2-ol is sp3 hybridised. — Incorrect. The central carbon in both molecules has four single bonds; it is tetrahedral and sp3 hybridised.
NCERT (2026-27 edition):
6.7.1 Reactions of Haloalkanes (stereochemical aspects) – “…propan-2-ol (A) does not contain an asymmetric carbon, as all the four groups attached to the tetrahedral carbon are not different. We rotate the mirror image (B) of the molecule by 180° (structure C) and try to overlap the structure (C) with the structure (A), these structures completely overlap. Thus propan-2-ol is an achiral molecule.”
“Butan-2-ol has four different groups attached to the tetrahedral carbon and as expected is chiral.”
Fig. 6.5: B is mirror image of A; B is rotated by 180° and C is obtained; C is superimposable on A.
Fig. 6.6: E is mirror image of D; E is rotated by 180° to get F and F is non superimposable on its mirror image D.