JEE Chemistry Amines Notes Set 08

Nitro Compounds: Preparation and Reduction

Nitrobenzene is made by nitrating benzene with a mixture of concentrated nitric and sulphuric acids at about 330 K; the electrophile is the nitronium ion \( \ce{NO2+} \). At a higher temperature or with fuming acid, 1,3-dinitrobenzene forms because nitro is a deactivating, meta-directing group. The reduction products depend on the conditions: metals with acid (Sn/HCl, Fe/HCl) and catalytic hydrogenation give aniline; Zn with aqueous \( \ce{NH4Cl} \) gives phenylhydroxylamine (in the Mulliken-Barker test this product reduces Tollens' reagent to silver); in alkaline medium azoxybenzene, azobenzene and hydrazobenzene form in turn. Ammonium sulphide or sodium hydrogen sulphide selectively reduces one nitro group of 1,3-dinitrobenzene to give 3-nitroaniline.

Preparation of Aromatic Primary Amines

Aniline is obtained by reducing nitrobenzene, by ammonolysis of chlorobenzene under severe conditions, and by the Hofmann bromamide degradation of benzamide. The Hofmann reaction converts a primary amide \( \ce{RCONH2} \) into \( \ce{RNH2} \) with one carbon less: \( \ce{RCONH2 + Br2 + 4NaOH -> RNH2 + Na2CO3 + 2NaBr + 2H2O} \). The N-bromoamide is deprotonated, and as bromide leaves the group R migrates from carbon to nitrogen with retention of configuration, giving an isocyanate; hydrolysis and loss of carbon dioxide give the amine. The migration step is rate-determining, so electron-releasing groups on a migrating aryl group speed up the reaction (4-methylbenzamide is faster than benzamide, and 4-nitrobenzamide is slowest). Only primary amides react. In each case four moles of NaOH and one mole of bromine are needed per mole of amide.

The Gabriel phthalimide synthesis makes pure aliphatic primary amines: potassium phthalimide is N-alkylated by a primary alkyl or benzyl halide and then hydrolysed. It fails for aryl halides, which do not undergo nucleophilic substitution with the phthalimide anion. Using KCN instead of the phthalimide anion adds a carbon: the nitrile is reduced to an amine with one more carbon than the halide.

Properties and Reactions of Aniline

Aniline is a weak base (\( pK_b \approx 9.4 \)) because the nitrogen lone pair is delocalised into the ring. The amino group strongly activates the ortho and para positions. Bromine water gives 2,4,6-tribromoaniline at once. To get mono-substitution the amino group is first acetylated (acetanilide), which is only moderately activating and directs para (or ortho if para is blocked); hydrolysis then restores the amine. Direct nitration gives a mixture, including a considerable amount of the meta isomer, because in the strongly acidic mixture aniline is largely protonated to the meta-directing anilinium ion. Aniline does not undergo Friedel-Crafts reactions because the amino group forms a complex with \( \ce{AlCl3} \); the acetylated amine can be alkylated.

Distinguishing Amines

Carbylamine test. A primary amine (aliphatic or aromatic) warmed with chloroform and alcoholic KOH gives an isocyanide, \( \ce{RNC} \), with a very offensive smell; secondary and tertiary amines do not react. Hinsberg test. Benzenesulphonyl chloride in aqueous alkali reacts with a primary amine to give a sulphonamide that dissolves in alkali (it has an acidic N-H); a secondary amine gives an insoluble sulphonamide; a tertiary amine does not react. Nitrous acid. Aromatic primary amines give diazonium salts at 273-278 K; aliphatic primary amines give nitrogen, alcohols and alkenes; secondary amines give yellow N-nitrosoamines; tertiary aromatic amines are nitrosated in the ring (for example N,N-dimethylaniline gives the green 4-nitroso compound).

Benzenediazonium Salts

Aniline is converted into benzenediazonium chloride by sodium nitrite and excess hydrochloric acid at 273-278 K. Excess acid suppresses coupling of the diazonium ion with unreacted aniline. The salt is stable only in the cold. Its reactions are of two kinds: replacement of the \( \ce{N2} \) group (nitrogen is lost) and coupling (nitrogen is retained).

Replacement reactions. CuCl/HCl, CuBr/HBr and CuCN/KCN give chlorobenzene, bromobenzene and benzonitrile (Sandmeyer reaction); with copper powder and HX the same products form (Gattermann reaction). KI gives iodobenzene without a catalyst. \( \ce{HBF4} \) gives the tetrafluoroborate, which on heating gives fluorobenzene (Balz-Schiemann reaction); treating the tetrafluoroborate with \( \ce{NaNO2} \) and copper gives nitrobenzene. Warming with water gives phenol. \( \ce{H3PO2} \) (or ethanol) replaces the diazonium group by hydrogen (deamination), which allows an amino group to be used as a temporary director. The Gomberg reaction with benzene and NaOH gives biphenyl. Mild reduction with \( \ce{SnCl2}/HCl \) or \( \ce{Na2SO3} \) gives phenylhydrazine; a strong reductant (Zn/HCl) gives aniline.

Coupling reactions. The diazonium ion is a weak electrophile and couples only with strongly activated rings, at the para position (ortho if para is occupied). With phenols the medium is mildly alkaline (pH 9-10) so that the phenoxide ion is present; with aromatic amines it is mildly acidic (pH 4-5). In a molecule such as 4-aminophenol, coupling goes ortho to the OH group in alkaline medium and ortho to the amino group in weakly acidic medium. Products are azo dyes: 4-hydroxyazobenzene (from phenol), 4-aminoazobenzene (yellow, from aniline), 4-(dimethylamino)azobenzene (from dimethylaniline) and an orange-red dye with 2-naphthol (coupling at position 1), which is the test for primary aromatic amines.

Worked Examples

Example 1. Benzamide is converted into 4-bromoaniline. Hofmann degradation gives aniline; acetylation gives acetanilide; bromination in ethanoic acid gives 4-bromoacetanilide; acid hydrolysis gives 4-bromoaniline. Brominating aniline directly would give the tribromo compound.

Example 2 (yield chain). Acetophenone (10 mol) is converted by the haloform reaction into benzoic acid (60%), then benzamide (50%), then aniline (50%), then 2,4,6-tribromoaniline (100%). Moles of product = \( 10 \times 0.60 \times 0.50 \times 0.50 = 1.5 \); molar mass of \( \ce{C6H4Br3N} \) is 330 g/mol, so the mass formed is \( 1.5 \times 330 = 495\,\text{g} \).

Example 3. Aniline is diazotised, converted by CuCN into benzonitrile, hydrolysed to benzoic acid and decarboxylated with soda lime to benzene. The molar mass of the final product is 78 g/mol, and it contains no nitrogen.