Account for the fact that the central benzene ring of [4]phenylene (Fig. P15.86) undergoes catalytic hydrogenation readily

Question:

Account for the fact that the central “benzene ring” of [4]phenylene (Fig. P15.86) undergoes catalytic hydrogenation readily under conditions usually used for ordinary alkenes, but the other benzene rings do not. [4]phenylene Figure P15.86 + 3H 5% Pd/C THF H H HH H H