Which of the following has lowest pK_a value ?

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.