Question

Give reasons in two or three sentences only for the following.
Phenyl group is known to exert negative inductive effect, but each phenyl ring in biphenyl \[\left( {{C}_{6}}{{H}_{5}}-{{C}_{6}}{{H}_{5}} \right)\] is more reactive than benzene towards electrophilic substitution.

Answer 1

Hint :We know that the electrophilic aromatic substitution reactions are generally organic reactions in which an electrophile replaces an atom which is attached to an aromatic ring. These reactions involve the replacement of a hydrogen atom belonging to a benzene ring with an electrophile.

Complete Step By Step Answer:
Electrophilic substitution reactions are the reactions where the hydrogen atom aromatic ring is replaced by an electrophile. Hydroxyl group in phenol is an electron-donating group so it activates the aromatic ring. Electron deficient species is known as an electrophile. The substitution of the hydrogen atom of the aromatic ring by electrophile is known as electrophilic substitution reaction. The rate of electrophilic substitution reaction depends on the nature of the substituent. Electron donating group increases the electron density at ortho and para position of a benzene ring and so increases the rate of electrophilic substitution reaction. In the case of phenol, there is an interaction between the lone pair of an oxygen atom and delocalized electrons in the aromatic ring.
Electron donating substituent is a ring activating group that increases the rate of electrophilic substitution at ortho and para position. Also due to the formation of oxocation as an intermediate phenol undergoes electrophilic substitution reaction more easily than benzene. In biphenyl, one phenyl group acts as electron donor, while other acts as electron acceptor. Thus, due to this reason, the phenyl ring is more reactive than benzene towards electrophilic substitution.

Additional Information:
There are many types of aromatic substitution out of which the most important electrophilic aromatic substitutions are aromatic nitration, aromatic halogenation, aromatic sulfonation, aromatic alkylation and aromatic acylation Friedel Crafts reaction. The Aromaticity of the aromatic system remains as such in an electrophilic aromatic substitution reaction. The reactivity of the aromatic substitution depends on inductive effect and mesomeric effect like in compound II there is no such type of effect so it is highly reactive as compared to other two.

Note :
Remember that the electron donating substituent is a ring activating group that increases the rate of electrophilic substitution at ortho and para position. Also due to the formation of oxocation as an intermediate phenol undergoes electrophilic substitution reaction more easily than benzene.