Which of the following biphenyls is optically active ?

Answer: 2. [structure shown in figure]

Key idea: a biphenyl is optically active (shows atropisomerism) only if rotation about the central bond is restricted and each ring lacks a plane of symmetry. This needs bulky groups at three or four ortho positions, with two different groups on each ring.

• Option 1: no ortho groups, free rotation, not active.
• Option 2: each ring has Br and I at the two ortho positions (2,2'-dibromo-6,6'-diiodo). Rotation is blocked and each ring is unsymmetric, so the molecule is chiral, optically active.
• Option 3: only one ortho I per ring, rotation is not blocked enough.
• Option 4: one ring has two identical ortho \(\mathrm{CH_3}\) groups and the other ring is unsubstituted, so it has a plane of symmetry.