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 reactions in their side-chains, often at the benzylic position next to the aromatic ring. Reactions at benzylic positions are often promoted by resonance stabilization of the intermediate and/or transition state with the aromatic ring. Aromatic amides are formed via electrophilic substitution. The NH2 group is electron-donating and therefore the substituted ring is considered “activated” and will often react even without a catalyst. The nitrogen based activating group increases reactivity by a resonance effect:
The resonance stabilisation from an amide group in an aromatic compound is mainly due to the delocalisation of the nitrogen lone pair onto the amide oxygen (which makes the lone pair less available to the aromatic ring) and the increased steric bulk of the group, favouring substitution at the 4-position over the 2-positions which it shields. Aniline is the simplest aromatic amine and is synthesised by first nitrating benzene using a concentrated mixture of nitric acid and sulphuric acid to give nitrobenzene which is then hydrogenated in the presence of a nickel catalyst to give the final product. Aniline undergoes very readily electrophilic substitution reactions as the aromatic ring of aniline is very electron rich due to the ability of the lone pair on the nitrogen to be delocalised into the π-system. Aniline can be used as a precursor to more complex materials notably acetamide, sulphanilic acid and sulpha drugs. Currently the largest market for aniline is preparation of methylene diphenyl diisocyanate (MDI), with some 85% of aniline serving this market. It is also used in the preparation of diazonium compounds which are used in dye industry and to produce Anil’s (Schiff's bases from aniline) for use as antioxidants in the rubber industry. In this experiment, aniline is first converted to N-phenylethanamide which is then brominated in the 4-position to produce N-(4-bromophenyl) ethanamide. After bromination, the amide group is hydrolysed back to the amine to produce the final product, 4-bromoaniline. Procedure
Acetanilide is synthesised by reacting 10ml of aniline with 25ml of ethanoic acid in a flask followed by 12ml of ethanoic anyhydride. After mixing and allowing the solution to stand for 5 minutes, it is with 100-200ml of water until crystallisation of the product occurred. The crystals of the N-phenylethanamide are filtered off and dried in air. To form the p-bromoacetanilide, the 5g of the acetanilide and 2.1ml are dissolved in separate 25ml portions of ethanoic acid. The bromine solution is then added to the acetanilide solution slowly over 5 minutes, whilst stirring. The mixture was allowed to stad at room temperature for 15 minutes and then poured into 300ml of cold water. 2g of sodium metabisulfate was added to remove any remaining bromine. The product was then filtered off by suction. The 4-bromoaniline was prepared by refluxing the p-bromoacetanilide with hydrochloric acid and then neutralising it with sodium hydroxide solution. The product separates as an oil which solidifies on complete crystallisation of the product. The colourless crystals are collected by centrifugation and then dried in air.
ProductYield of crude product/ gMelting point...