Part 1: Amine preparation and basicity

Compare routes to primary amines and explain base strength through the availability of the nitrogen lone pair.

Count carbon groups bonded directly to nitrogen

A primary amine is RNH₂, a secondary amine R₂NH and a tertiary amine R₃N, where the groups can differ. This classification counts carbon-containing groups attached to nitrogen, not the number of carbons elsewhere or the classification of a neighbouring carbon. A quaternary ammonium ion, R₄N⁺, has four carbon groups and is not a neutral amine.

CH₃CH₂NH₂ is ethylamine/ethanamine. CH₃CH₂NHCH₃ is N-methylethanamine, a secondary amine. C₆H₅NH₂ is phenylamine, with N directly bonded to the ring; C₆H₅CH₂NH₂ has an intervening CH₂ and does not delocalise the nitrogen lone pair into the ring in the same way.

Ammonia substitution can continue beyond the first amine

Heat a halogenoalkane with excess ammonia dissolved in ethanol using the specified controlled apparatus. The ammonia nitrogen lone pair attacks the carbon attached to halogen and the C–X bond breaks heterolytically. Deprotonation of the initially formed alkylammonium ion gives a primary amine.

The product amine still has a nitrogen lone pair and can react with more halogenoalkane. Excess ammonia favours primary amine formation by increasing the probability that ammonia is the attacking nucleophile, but does not guarantee a single pure product. Further substitution and separation costs limit this route.

Reduction of a nitrile retains its nitrile carbon

A nitrile, R–C≡N, is reduced to R–CH₂NH₂ by hydrogen with a suitable metal catalyst, such as Ni, or LiAlH₄ in dry ether followed by the appropriate separate work-up. Four [H] equivalents are needed per nitrile group. NaBH₄ under the mild conditions used for aldehydes/ketones is not the standard reagent for this nitrile conversion.

If a halogenoalkane is first converted to a nitrile using KCN in aqueous ethanol under reflux, the CN carbon lengthens the chain by one. Reduction then keeps that extra carbon. Direct substitution with NH₃ does not lengthen the chain. A nitrile route avoids repeated alkylation during reduction, but adds a step and uses toxic cyanide in the preceding substitution.

Reduce nitrobenzene, then release the free amine

Heating nitrobenzene with tin and hydrochloric acid, or iron and hydrochloric acid, reduces the nitro group. Under the acidic reaction conditions the amine is protonated as a phenylammonium salt. Adding sufficient alkali after reduction releases phenylamine. Catalytic hydrogenation is another reduction route when appropriate conditions are supplied.

The shorthand reduction equation below shows the neutral amine. It must be combined with protonation when describing the actual acidic mixture. Aromatic amines are useful intermediates in dye manufacture; detailed dye-coupling chemistry is outside the required scope of this section.

A base uses its lone pair to accept H⁺

Amines act as Brønsted–Lowry bases because nitrogen can donate its lone pair to H⁺. In water this establishes an equilibrium generating OH⁻; partial protonation makes the usual amines weak bases. Adding acid forms an ammonium salt, while alkali can regenerate the neutral amine.

In the specified comparison, a primary aliphatic amine is generally more basic than ammonia: the alkyl group increases electron density and makes the nitrogen lone pair more available for proton acceptance. Phenylamine is less basic than ammonia because the lone pair is partly delocalised into the aromatic ring and is less available. Phenylamine remains a weak base; it is not non-basic.

Do not extend this simple comparison into a universal primary/secondary/tertiary ranking. Solvation, steric effects and the exact substituents affect measured basicity. June 2022 Paper 2 Q10.5 required the link between the substituent effect and lone-pair availability, not simply a memorised ordering.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

Sources