Revision:Nitration of benzene and methylbenzene

Reaction conditions

• Heat (50 - 55^o C) under reflux; temperature control essential to prevent dinitrobenzene formation.
• Use of a nitrating mixture.

Nitrating mixture

• The nitrating mixture consists of concentrated nitric and sulphuric acids. It produces the NO₂⁺ ion, which acts as an electrophile and attacks the benzene ring.

Reaction (and mechanism)

• The reaction is an electrophilic substitution.

1. Two electrons are transferred from the benzene ring to the electrophile and a covalent bond is formed
   • The Carbon atom to which the electrophile is attached is now fully saturated, so the electron cloud is delocalised over the other five carbon atoms.
   • Since the benzene ring donated the pair of electrons to form the bond, it now bears the positive charge.
2. The hydrogen atom leaves as an H⁺ ion, leaving behind nitrobenzene.

Methylbenzene

• The methyl (-CH₃) group inductively donates electrons to the benzene ring. This not only makes electrophilic substitution easier by increasing electron charge density, but also directs further substitution to the 2, 4, and 6 positions.