Which of the following has lowest `pK_a` value ?

Options

A.
B.
C.
D.

Answer

D

Explanation

Increasing the number of phenyl groups decreases the `pK_a` - this is what we expect, since we can delocalize the charge over all the rings. Notice, however, that each successive phenyl ring has less effect on the `pK_a` : the first ring lowers the `pK_a` by 8 units, the second by 7, and the third by only 1 unit. In order to have effective delocalization, the system must be planar. Three phenyl rings cannot arrange themselves in a plane around one carbon atom because the ortho-hydrogens clash with each other (they want to occupy the same space) and the compound actually adopts a propeller shape where each phenyl ring is slightly twisted relative to the next.

Even though complete delocalization is not possible, each phenyl ring does lower the `pK_a` because the sp2 carbon on the ring is electron - withdrawing. If we force the system to planar, as in the compounds below, the `pK_a` is lowered considerably.