Question

Which of the following compounds has the lowest $p{K}_a$?

• A.
• B.
• C.
• D.

Answer 1

The correct option is D

Increasing the number of phenyl groups decreases the $p{K}_a$ due to the delocalisation of electrons.
In option c, the three phenyl rings are not planar due to steric hinderance. This reduces the extent of delocalisation. If we force the system to be planar, $p{K}_a$ decreases considerably.
In option (d), the conjugate base is resonance stabilised and due to the formation of sigma bonds from ortho positions, rotation is restricted and effective delocalisation takes place.