2018 H2 Chem P1 Q23 — Introduction to Organic Solution

The answer is B (1 and 2 only).
Only chiral molecules rotate plane-polarised light. All four molecules contain chiral centres: 1 has one, and 2, 3 and 4 (3,4-dibromohexane) each have two, at C3 and C4.
• 1 is chiral (one chiral centre), so it rotates plane-polarised light.
• 2 is chiral: it has no internal plane of symmetry in any conformation (only an axis of symmetry), so it is not superimposable on its mirror image and rotates plane-polarised light.
• 3 and 4 are both the meso form (the same compound). Rotate one end about the C3-C4 bond into the eclipsed conformation, with the two Br atoms eclipsed and the two ethyl groups eclipsed: the molecule then has an internal plane of symmetry through the middle of the C3-C4 bond. It is not chiral, so it is optically inactive; the rotations caused by the two chiral centres cancel.
• A (1, 2, 3 and 4) is wrong: 3 and 4 are meso and do not rotate plane-polarised light.
• C (3 and 4 only) is wrong: these are the two optically inactive molecules.
• D (none) is wrong: 1 and 2 are chiral and do rotate plane-polarised light.