Answer to Question #84053 in Organic Chemistry for Vijay

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.