Arenes_Concepts

Why do benzenes prefer to undergo substitution rather than addition reactions?

• Delocalisation of the 6π electrons in the continuously overlapping P orbitals confers resonance stability on benzene

• If benzene undergoes addition reactions, overall aromatic character is destroyed

• If benzene undergoes substitution reactions, aromatic character is retained

General steps of electrophilic substitution mechanism

1. Generation of electrophile, E+

2. Electrophilic attack by E+ on the benzene ring to form a carbocation (slow step)

3. Loss of proton from the carbocation to form the product (fast step)

Difference in conditions of nitration of benzene and methylbenzene

Benzene: 50˚C

Methylbenzene: 30˚C

A Bronsted-Lowry acid is a proton ___

Donor

A Bronsted-Lowry base is a proton ___

Acceptor

Eqn of generation of electrophile, NO2+ in nitration of benzene

A Lewis acid is an electron pair ___

Acceptor

A Lewis base is an electron pair ___

Donor

Activating groups

• Electron-donating

• Increases electron density in benzene ring, makes ring more susceptible to electrophilic attack

• Helps to disperse the positive charge in the intermediate carbocation and stabilise carbocation

• Makes substituted benzene more reactive than benzene

Deactivating groups

• Electron withdrawing

• Decreases electron density in benzene ring and makes the ring less susceptible to electrophilic attack

• Intensifies positive charge in intermediate carbocation, destabilise carbocation

• Makes substituted benzene less reactive than benzene

Criteria for oxidation of side chains on benzene

Benzylic C atom must have at least one H or O atom bonded to it