Preparation of Amines

Why six different routes exist

An amine is ammonia in which one, two or three N–H bonds have been swapped for N–C bonds. The synthetic problem is therefore one of control: how to install exactly the number of carbon groups wanted, with the correct connectivity, while leaving the rest of the molecule untouched. No single reaction does this for every target, which is why NCERT lists six independent methods. Some build the C–N bond from a halide, some unmask a nitrogen already present in a nitro, nitrile or amide group, and one even shortens the chain.

Two questions discriminate the methods at a glance, and both are favourite NEET hooks. First — does the route give a single amine or a mixture? Second — how does the carbon count change between substrate and amine? Keep both in view as each method is developed below; the closing map collects the answers in one table.

Reduction of nitro compounds

Nitro compounds are reduced to amines either by passing hydrogen gas over a finely divided catalyst — nickel, palladium or platinum — or by treating them with a metal in acidic medium. Nitroalkanes give the corresponding alkanamines, while nitrobenzene gives aniline. The carbon skeleton is untouched; only the oxidation state of nitrogen changes, from $+3$ in the nitro group to $-3$ in the amine.

The aromatic case is the one NCERT and NEET return to repeatedly — the conversion of nitrobenzene to aniline:

$$\ce{C6H5NO2 ->[\text{Sn / HCl}][\text{or } H2/\text{Pt}] C6H5NH2}$$

Catalytic hydrogenation does the same job:

$$\ce{C6H5NO2 + 3H2 ->[\text{Ni / Pd / Pt}] C6H5NH2 + 2H2O}$$

NCERT singles out one reagent combination for economy. Reduction with iron scrap and hydrochloric acid is preferred because the $\ce{FeCl2}$ formed is hydrolysed during the reaction to regenerate hydrochloric acid; only a small initiating quantity of acid is therefore needed, since the acid is effectively recycled as the reduction proceeds. This is why industrial aniline manufacture historically leaned on the iron–acid route rather than on stoichiometric tin, which is both costlier and harder to recover. The NIOS supplement (§28.4.4) adds that reducing nitrobenzene under different conditions diverts the product entirely — zinc dust with ammonium chloride (neutral) gives N-phenylhydroxylamine, and zinc with sodium hydroxide (alkaline) gives azobenzene — a reminder that "reduction of nitrobenzene" is not a single answer but a family of outcomes selected by the medium.

Mechanistically the transformation is a six-electron reduction: the nitrogen, formally in the $+3$ oxidation state in $\ce{-NO2}$, is delivered to $-3$ in $\ce{-NH2}$, passing through nitroso and hydroxylamine intermediates that are only isolated when the reduction is deliberately arrested. Because the aromatic ring itself is inert to these mild conditions, the route is exceptionally clean for aryl systems — there is no over-reduction and no competing substitution, which is exactly why it is the standard preparation of aniline and substituted anilines from the corresponding nitroarenes.

Ammonolysis of alkyl halides

The carbon–halogen bond in an alkyl or benzyl halide is cleaved by a nucleophile. Ammonia is that nucleophile here: an ethanolic solution of ammonia displaces halide in a nucleophilic substitution, replacing $\ce{-X}$ with $\ce{-NH2}$. This cleavage of the C–X bond by ammonia is called ammonolysis, and it is run in a sealed tube at 373 K.

$$\ce{R-X + NH3 -> R-NH3^+X^-}$$

The free amine is liberated from the resulting ammonium salt by a strong base:

$$\ce{R-NH3^+X^- + NaOH -> R-NH2 + NaX + H2O}$$

The flaw is built into the chemistry. The primary amine first formed is itself a nucleophile, at least as good as ammonia, so it attacks a second molecule of alkyl halide to give a secondary amine; that reacts again to give a tertiary amine, and finally a quaternary ammonium salt. Ammonolysis therefore yields a mixture of all four products. The reactivity of the halide follows $\ce{R-I > R-Br > R-Cl}$.

$$\ce{R-X ->[NH3] R-NH2 ->[R-X] R2NH ->[R-X] R3N ->[R-X] R4N^+X^-}$$

The mixture can be biased: a large excess of ammonia keeps the alkyl halide statistically more likely to meet ammonia than a partly built amine, so the primary amine is obtained as the major product. This is selectivity by Le-Chatelier-style swamping, not by mechanism — which is exactly why the next two clean methods exist.

Reduction of nitriles and amides

Both nitriles and amides already contain the nitrogen; reduction simply saturates the C–N multiple bonding to give a primary amine. The two methods differ in one decisive respect — the carbon count.

Reduction of nitriles — ascent of the series

A nitrile reduced with lithium aluminium hydride ($\ce{LiAlH4}$) or by catalytic hydrogenation gives a primary amine that has one more carbon than the alkyl group that carried the cyano function, because the nitrile carbon becomes a $\ce{-CH2-}$ next to nitrogen. This stepwise lengthening is called the ascent of the amine series.

$$\ce{R-C#N ->[\text{(i) LiAlH4}][\text{or } H2/\text{Pt}] R-CH2-NH2}$$

The NIOS example (§28.2.2) makes the carbon bookkeeping explicit: propanenitrile, $\ce{CH3CH2C#N}$, reduces to propan-1-amine, $\ce{CH3CH2CH2NH2}$ — three carbons in, three carbons out, but the amine carbon is brand new relative to the two-carbon ethyl group originally bonded to $\ce{-CN}$.

Reduction of amides — carbon count unchanged

An amide reduced with $\ce{LiAlH4}$ also gives a primary amine, but here the carbon count is unchanged: the carbonyl carbon of the amide stays in the molecule and is reduced to a $\ce{-CH2-}$ group.

$$\ce{R-CONH2 ->[\text{LiAlH4}] R-CH2-NH2}$$

Ethanamide ($\ce{CH3CONH2}$, two carbons) thus gives ethanamine ($\ce{CH3CH2NH2}$, two carbons). Pair this with the amide's other fate — Hofmann degradation, below — and one starting amide can be steered to an amine of either the same or one-fewer carbon, a contrast NEET likes to test side by side.

Gabriel phthalimide synthesis

Gabriel synthesis is the method of choice when a pure aliphatic primary amine is required, free of the secondary and tertiary contamination that dogs ammonolysis. The trick is to mask the nitrogen inside phthalimide, where it can be alkylated only once, and then to release it.

Phthalimide is treated with ethanolic potassium hydroxide to form potassium phthalimide. Its nitrogen anion attacks an alkyl halide ($\ce{S_N2}$), placing exactly one alkyl group on nitrogen. Alkaline hydrolysis of the resulting N-alkylphthalimide then liberates the primary amine.

$$\ce{C6H4(CO)2NH ->[KOH] C6H4(CO)2N^-K^+ ->[R-X] C6H4(CO)2N-R ->[OH^-/H2O] R-NH2}$$

Because the nitrogen carries only one replaceable position once it sits inside phthalimide, over-alkylation is impossible — the product is exclusively the primary amine, carbon count equal to that of the alkyl halide. The contrast with ammonolysis is the whole point of the method: where ammonia's nitrogen keeps three reactive N–H bonds and is therefore alkylated repeatedly, the phthalimide nitrogen has its remaining valences locked into two carbonyl groups, so it can accept exactly one alkyl group and no more. The two fused carbonyls also make the imide N–H acidic enough for KOH to deprotonate it cleanly, generating the nucleophilic anion that does the $\ce{S_N2}$ work.

The decisive limitation, and a near-annual NEET statement, is that aromatic primary amines cannot be made by Gabriel synthesis: aryl halides do not undergo nucleophilic substitution with the phthalimide anion, because the carbon–halogen bond in an aryl halide has partial double-bond character and the ring resists backside attack. The alkylation step therefore simply does not occur for an aryl halide, and the synthesis stalls at the potassium phthalimide stage. Aniline and other arylamines must be made by reducing the corresponding nitro compound instead — which is precisely the pairing NEET 2024 tested in its Gabriel statement question.

Hofmann bromamide degradation

Hofmann's method converts an amide into a primary amine using bromine in aqueous or ethanolic sodium hydroxide. It is unique among these routes in that the product loses one carbon: in the course of the reaction an alkyl or aryl group migrates from the carbonyl carbon of the amide to nitrogen, and the original carbonyl carbon departs as carbonate. The amine therefore has one carbon fewer than the amide.

$$\ce{R-CONH2 + Br2 + 4KOH -> R-NH2 + K2CO3 + 2KBr + 2H2O}$$

The migration is the heart of the mechanism. In outline: NaOH/$\ce{Br2}$ first converts the amide to an N-bromoamide, deprotonation gives a bromamide anion, and loss of bromide generates an electron-deficient acyl nitrene–like centre. The R group then migrates from carbon to the adjacent electron-poor nitrogen, producing an isocyanate, $\ce{R-N=C=O}$. Alkaline hydrolysis of this isocyanate cleaves off the carbon as carbonate and unmasks the amine $\ce{R-NH2}$ — accounting precisely for the missing carbon.

The carbon-count map

The single most rewarding pattern to memorise is how each route shifts the carbon count, because NEET conversion questions are built on exactly this arithmetic. NIOS states the rule compactly: reduction of nitriles, amides and nitro compounds gives amines, but only the nitrile route changes the carbon framework, while in Hofmann degradation the amine has one carbon less than the starting amide.

Choosing a method for a target

Read the target backwards. If a clean primary aliphatic amine is wanted with the carbon count fixed, reach for Gabriel synthesis. If the chain must grow by one carbon, a nitrile reduction is the only NCERT route that does it. If it must shrink by one, Hofmann degradation of the corresponding amide is the answer. To make aniline or any aromatic primary amine, reduce the nitro compound — Gabriel is unavailable and Hofmann is reserved for amides. Ammonolysis is acceptable only when a mixture is tolerable or excess ammonia can be used.

NEET 2024 Q.96 strings several of these ideas together: $\ce{CH3CH2CH2I}$ with NaCN forms a nitrile (chain grows by one), and reduction-type work-up delivers propylamine — a multi-step conversion that hinges on knowing the nitrile route adds a carbon. Treat every conversion problem as a search across this small toolbox.