Revise Primary amines for Cambridge 9701 (19.1)

A primary amine has the general formula $R-NH_2$, where R is an alkyl or aryl group. The nitrogen atom has three bonding pairs and one lone pair of electrons, resulting in a trigonal pyramidal geometry around the nitrogen, similar to ammonia. This lone pair is the centre of the amine's reactivity.

Amines are weak bases because the lone pair on the nitrogen can accept a proton ($H^+$) from an acid or water. The strength of the base depends on the availability of this lone pair. Primary aliphatic amines, like ethylamine, are stronger bases than ammonia. This is because the alkyl group has a positive inductive effect, pushing electron density towards the nitrogen atom, making the lone pair more available. In contrast, primary aromatic amines, like phenylamine, are weaker bases than ammonia because the nitrogen's lone pair is delocalised into the benzene ring's pi system, making it less available to accept a proton.

Basicity equilibrium: $RNH_2(aq) + H_2O(l) \rightleftharpoons RNH_3^+(aq) + OH^-(aq)$

There are two principal methods for synthesising primary amines that you need to know for your A-Level examination. Both are reduction reactions, meaning the nitrogen-containing functional group gains hydrogen atoms or loses oxygen atoms.

1. Reduction of Nitriles (R-C≡N): Nitriles can be reduced to form primary amines. This is a useful way to increase the carbon chain length by one carbon atom. The reduction can be achieved using a strong reducing agent like lithium tetrahydridoaluminate ($LiAlH_4$) in dry ether, or by catalytic hydrogenation with hydrogen gas and a nickel catalyst.

General equation: $R-C\equiv N + 4[H] \rightarrow R-CH_2NH_2$

2. Reduction of Nitroarenes (Ar-NO₂): Aromatic amines, such as phenylamine, are prepared by reducing the corresponding nitro compound. A typical laboratory method involves heating the nitroarene (e.g., nitrobenzene) under reflux with tin metal and concentrated hydrochloric acid. The resulting ammonium salt is then treated with a strong base, like sodium hydroxide, to liberate the free amine.

General equation: $Ar-NO_2 + 6[H] \rightarrow Ar-NH_2 + 2H_2O$

The lone pair of electrons on the nitrogen atom dictates the chemistry of primary amines, allowing them to function as both bases and nucleophiles. Their basicity is shown by their reaction with acids to form salts, while their nucleophilicity is demonstrated in reactions with electron-deficient species like acyl chlorides.

Reaction with Acids: As bases, amines readily accept a proton from an acid in a neutralization reaction to form an ammonium salt. For example, ethylamine reacts with hydrochloric acid to form ethylammonium chloride, an ionic solid which is soluble in water.

$CH_3CH_2NH_2(aq) + HCl(aq) \rightarrow CH_3CH_2NH_3^+Cl^-(aq)$

Reaction with Acyl Chlorides: As nucleophiles, primary amines attack the electron-deficient carbonyl carbon of an acyl chloride in a vigorous nucleophilic addition-elimination reaction. This reaction forms an N-substituted amide and produces misty fumes of hydrogen chloride.

$R'COCl + RNH_2 \rightarrow R'CONHR + HCl$

The reaction with nitrous acid ($HNO_2$) is a crucial test used to distinguish between different types of amines. Since nitrous acid is unstable, it is generated in situ by reacting sodium nitrite ($NaNO_2$) with a cold (below 10 °C), dilute strong acid like HCl.

With primary aliphatic amines, the reaction is characterized by the evolution of nitrogen gas (effervescence) as an alcohol is formed. The intermediate diazonium salt is highly unstable and decomposes immediately.

$RNH_2 + HNO_2 \xrightarrow{<10^\circ C} ROH + N_2(g) + H_2O(l)$

In contrast, primary aromatic amines such as phenylamine react under the same cold conditions to form a relatively stable diazonium salt, like benzenediazonium chloride. This diazonium salt is a vital intermediate in the synthesis of azo dyes and remains in solution without decomposing as long as the temperature is kept low.

$ArNH_2 + HNO_2 + HCl \xrightarrow{<10^\circ C} ArN_2^+Cl^-(aq) + 2H_2O(l)$