Which of the following bihphenyls is optically active?

Options

A.
B.
C.
D.

Answer

C

Explanation

If at least of the positions `(2,2',6` and `6')` are occupied by sufficiently large groups, free rotation about the single bond joining the two pheny`1` groups is no longer possible. Provided each ring has no vertical plane of symmetry, this restriced rotation gives rise to optical activity due to the molecule being asymmetric as a whole. Thus, compound `(3)` is optically active. Besides this `6`-nitrobiphenty`1-2,2'-` dicarboxylic acid

and `6,6`-diamino-`2,2'`-dimethy`1`bipheny`1`

have been resolved.
If the substituent group is large enough, then only two groups in the `O-`and `O'-`positions will cause restricted rotation, e.g. bipheny`1-2,2'-`disuphonic acid

has been shown to be optically active. the cause of the restricted rotaiton is due mainly to the steric effects of the groups in the `O-` and `O'-`position.
This type of steroisomerism, which arises from restricted roatation about a single bond and where the steroisomers can be isolated, is called atropisomerism, the isomers are known as atropisomers.