Discussion
The achiral molecules among the following (I, II, III and IV) are
Answer: D
The achiral molecules are I and IV.
Molecule I is meso-tartaric acid. Its two stereocentres carry the same substituents and are mirror images of each other, so in the zigzag drawing the OH groups sit one on a wedge and one on a hash, on opposite sides of the chain. An internal mirror (centre of inversion) maps one half onto the other, so the molecule is achiral. Molecule IV, 4-tert-butylcyclohexanol, has a mirror plane through C1 and C4, so it is achiral too, whichever way the two groups are placed.
Molecule II has both OH groups on wedges, which gives a chiral (R,R) or (S,S) tartaric acid with no internal mirror. Molecule III is a trans-disubstituted epoxide with two different groups on its two stereocentres, and it has no mirror plane. So II and III are chiral, which rules out the options that contain them.
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