Preparation of 4-bromoaniline
Introduction
Aromatic compounds tend to undergo electrophilic aromatic substitutions rather than addition reactions. Substitution of a new group for a hydrogen atom takes place via a resonance-stabilized carbocation. As the benzene ring is quite electron-rich, it almost always behaves as a nucleophile in a reaction which means the substitution on benzene occurs by the addition of an electrophile. Substituted benzenes tend to react at predictable positions. Alkyl groups and other electron-donating substituents enhance substitution and direct it toward the ortho and para positions. Electron-withdrawing substituents slow the substitution and direct it toward the meta positions.
Aromatic compounds also undergo…show more content… Here monosubstitution is easily achieved because the acetamino group [HNC(=O)CH3] cannot activate the benzene ring towards electrophilic attack as well as the simple amino group does. The acetamido group is less effective in donating electron density to the benzene ring, because the electron pair on the nitrogen atom is delocalised by both the carbonyl group and the phenyl ring. The N-phenylethanamide was obtained in the form of crystals with a melting point of 78˚C. After this product was purified by recrystallisation from aqueous ethanol it was found to have a melting point of 106˚C which is much closer to the literature range of 113-114˚C. The percentage yield for the N-phenylethanamide was reasonably high at 82%.
The second step in the formation of 4-bromoaniline, involved reacting the acetanilide with bromine in acidic conditions to produce N-(4-bromophenyl) ethanamide or p-bromoactetanilie. Acetanilide is a para-directing group and so an incoming electrophilic attack on the para position will yield p-bromoacetanilide. The reaction occurred via the following electrophilic substitution mechanism: An electron pair from the benzene ring attacks the Br2 forming a new C-Br bond and leaving a non-aromatic carbocation intermediate. The carbocation intermediate loses H+, and the neutral substitution product forms as two electrons from the C-H
