Question

Phenyl group is known to exert negative inductive effect, but each phenyl ring in biphenyl (C6H5–C6H5) is more reactive than benzene towards electrophilic substitution. Explain why.

Answer 1

Experimental data show that biphenyl ia more active in the reactions of electrophilic substitution. Electrophyles attack in o- and p- positions of biphenyl, mostly in p-position.

Resonance structures of σ- complex of biphenyl, formed after electrophilic attack is:

Resonance structures of σ- complex of biphenyl, formed after electrophilic attack is:

Resonance structures of σ- complex of benzene, formed after electrophilic attack is:

The reason why biphenyl is more active in the reactions of electrophilic substitution is that the σ - complex of biphenyl is stabilized more effectively due to the participation of the second benzene ring in delocalization of positive charge.