Question 1

Why are aromatic amines like phenylamine weaker bases than aliphatic amines?

Accepted Answer

In aromatic amines, the lone pair of electrons on the nitrogen atom is partially delocalised into the π-system of the benzene ring through resonance. This delocalisation reduces the electron density on the nitrogen, making it less available to accept a proton. In contrast, aliphatic amines have no such delocalisation; instead, the alkyl groups donate electron density through the inductive effect, increasing the electron density on nitrogen and enhancing basicity. As a result, phenylamine is a much weaker base than alkylamines such as ethylamine, and even weaker than ammonia.

Question 2

How can I make a primary amine without getting secondary and tertiary amines as side products?

Accepted Answer

When a halogenoalkane reacts with ammonia, the initially formed primary amine can undergo further nucleophilic substitution, leading to secondary and tertiary amines and even quaternary ammonium salts. To favour the primary amine, use a large excess of ammonia. This ensures that the halogenoalkane is far more likely to encounter an ammonia molecule than a primary amine molecule. Practically, using concentrated ammonia solution and slow addition of the halogenoalkane helps, but some over-alkylation is difficult to eliminate entirely. An alternative method that gives only the primary amine is the reduction of a nitrile, using LiAlH₄ or catalytic hydrogenation, which adds two hydrogens but does not introduce extra alkyl groups.

Question 3

What is the test to distinguish between an aliphatic primary amine and an aromatic primary amine using nitrous acid?

Accepted Answer

Both aliphatic and aromatic primary amines react with nitrous acid (HNO₂), but they give very different products under controlled conditions. Aliphatic primary amines react at room temperature to release nitrogen gas, which is observed as effervescence, and an alcohol is formed. Aromatic primary amines, such as phenylamine, react with nitrous acid at low temperatures (below 10 °C) to form a stable diazonium salt without nitrogen evolution. This diazonium salt can then be coupled with an alkaline phenol (e.g., naphthalen-2-ol) to produce a bright orange or red azo dye. Thus, the evolution of nitrogen gas indicates an aliphatic primary amine, while the formation of a dye after coupling confirms an aromatic primary amine.

Question 4

Why can't I prepare phenylamine by reacting ammonia with bromobenzene?

Accepted Answer

The carbon–halogen bond in bromobenzene is much stronger than in simple alkyl halides because the p-orbital of the halogen is partially delocalised into the benzene ring, giving the C–Br bond some double-bond character. Additionally, the benzene ring repels approaching nucleophiles due to its electron-rich π-cloud. Therefore, direct nucleophilic substitution with ammonia does not occur under normal conditions. Instead, phenylamine is synthesised by nitration of benzene to form nitrobenzene, followed by reduction using tin and concentrated hydrochloric acid, and then treatment with sodium hydroxide to liberate the free amine.

Question 5

What is the difference between an amine and an amide?

Accepted Answer

An amine is a derivative of ammonia where one or more hydrogen atoms have been replaced by alkyl or aryl groups, and the nitrogen is directly bonded to carbon and hydrogen atoms only (e.g., CH₃NH₂, methylamine). An amide, on the other hand, contains a carbonyl group (C=O) directly bonded to a nitrogen atom (R–CO–NR′R″). This carbonyl group significantly alters the chemistry: amides are neutral and much weaker bases than amines because the nitrogen lone pair is delocalised into the carbonyl group. Amides can be formed by the reaction of an amine with an acyl chloride or a carboxylic acid derivative. In naming, amines use the suffix '-amine' or 'amino-', while amides use '-amide'.

Question 6

How do I name amines when there are multiple amino groups or other functional groups?

Accepted Answer

When an organic molecule contains more than one amino group, the prefixes 'di-', 'tri-', etc., are used with '-amine'. For example, H₂NCH₂CH₂NH₂ is named ethane-1,2-diamine. If the amine is a substituent (when a higher priority functional group like a carboxylic acid is present), the prefix 'amino-' is used with a locant. For aromatic compounds, the amino group attached to a benzene ring is named as 'phenylamine' (IUPAC: benzenamine) if it is the principal group, or 'aminobenzene' as a substituent. When numbering, the carbon bearing the –NH₂ group gets the lowest possible number unless another functional group dictates the numbering. Always check the priority of functional groups: –COOH > –CN > –CHO > C=O > –OH > –NH₂ for numbering the parent chain.

Amines (A-level only)

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Related Topics in AQA A-Level Chemistry

Acids and bases (A-level only)

Alcohols

Aldehydes and ketones (A-level only)

Alkanes