JEE Chemistry Amines Notes Set 02

Aniline: Preparation

1. Reduction of nitrobenzene with Sn/HCl in the lab or Fe/HCl in industry (or \( \ce{H2}/Pd \)): \( \ce{C6H5NO2 + 6[H] -> C6H5NH2 + 2H2O} \).
2. Hofmann degradation of benzamide.
3. Heating phenol with ammonia and \( \ce{ZnCl2} \) at about 300 C.
4. Hydrolysis of phenyl isocyanide with acid.
5. Schmidt reaction of benzoic acid with hydrazoic acid in conc. sulphuric acid.
6. Aniline cannot be made by the Gabriel synthesis, because aryl halides do not undergo SN2 attack.

Aniline: Properties

Aniline is a colourless, oily, toxic liquid (boiling point about 184 C) that turns brown on exposure to air. It is steam volatile, so it is purified by steam distillation. It is a weak base: it forms salts with strong acids such as HCl, \( \ce{H2SO4} \) and chloroplatinic acid, but its aqueous solution does not change the colour of litmus.

Benzenediazonium Chloride

Diazonium salts are made by treating aniline with sodium nitrite and hydrochloric acid at 0-5 C. Above this temperature the salt reacts with water and gives phenol. Aromatic diazonium salts are stable only in the cold because the positive charge is shared with the ring through delocalisation; aliphatic diazonium ions decompose at once, giving nitrogen.

Nitrobenzene

Nitrobenzene is a pale yellow oily liquid with a smell like bitter almonds (oil of mirbane). It is steam volatile, heavier than water and poisonous. It is made by warming benzene with a mixture of conc. nitric and sulphuric acids at 50-60 C. Its main use is for making aniline, and it is used as a solvent in the Friedel-Crafts reaction because it is itself too deactivated to react.

1. Nitro group is strongly electron withdrawing, so the ring is deactivated and the group is meta directing. Nitration with fuming nitric acid and conc. sulphuric acid at higher temperature gives m-dinitrobenzene; sulphonation gives m-nitrobenzenesulphonic acid; chlorination with Cl2/AlCl3 gives m-chloronitrobenzene.
2. Reduction depends on the medium. Acidic (Sn/HCl, Fe/HCl, H2/Pd): aniline. Neutral (Zn/NH4Cl): N-phenylhydroxylamine. Weakly acidic electrolytic reduction: aniline. Strongly acidic electrolytic reduction: N-phenylhydroxylamine, which rearranges to p-aminophenol. Alkaline media (Zn/NaOH or other) give azoxybenzene, azobenzene and hydrazobenzene, in order of increasing reduction.
3. Mullikan-Barker test: nitrobenzene with Zn and NH4Cl gives phenylhydroxylamine, which reduces Tollens' reagent to a grey or black silver deposit.

Cyanides and Isocyanides

Methyl isocyanate, \( \ce{CH3NCO} \), is a different compound: it is the toxic gas involved in the Bhopal disaster.

Distinguishing Tests

Worked Examples

1. Convert aniline to benzene: diazotise at 0-5 C and warm with \( \ce{H3PO2} \).
2. Prepare p-bromoaniline from aniline: acetylate, brominate (para major) and hydrolyse.
3. Why is aniline less basic than methylamine? The lone pair on nitrogen is delocalised into the ring and is less available for protonation. Methylamine's lone pair is made more available by the methyl group.