How are amines made, why are they basic, and how does aromatic amine basicity differ from aliphatic?
The preparation of amines, their basicity and the comparison of aliphatic, aromatic and ammonia basicity, their reactions as bases and nucleophiles, and the formation of amides and salts.
An Eduqas A-Level Chemistry OA3.1 answer on the preparation of amines, their basicity (comparing aliphatic, aromatic and ammonia), their reactions as bases and nucleophiles, and amide and salt formation.
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What this topic is asking
Eduqas topic OA3.1 covers amines: how they are prepared, why they are basic, the comparison of basicity between aliphatic amines, aromatic amines and ammonia, their reactions as bases and as nucleophiles, and the formation of amides and salts. The explanation of relative basicity in terms of lone-pair availability is the central exam skill.
Why amines are basic
Comparing basicity
Preparation of amines
Aliphatic amines are made by heating a halogenoalkane with excess ammonia in ethanol (nucleophilic substitution); using excess ammonia limits further substitution to secondary and tertiary amines. Amines are also made by reducing nitriles (with hydrogen and a nickel catalyst, or with LiAlH4) and aromatic amines by reducing a nitro compound (tin and concentrated hydrochloric acid reduce nitrobenzene to phenylamine).
Reactions of amines
As bases, amines react with acids to form salts (for example ethylamine plus HCl gives ethylammonium chloride). As nucleophiles, they react with acyl chlorides to form amides, and they can undergo further substitution with halogenoalkanes. These reactions make amines versatile intermediates.
Examples in context
- Example 1. Drug bases and their salts
- Many drugs are amines made water-soluble by converting them to their hydrochloride salts; the amine basicity that allows this comes directly from the nitrogen lone pair.
- Example 2. Dyes from aromatic amines
- Phenylamine is the starting point for azo dyes; its lower basicity and the delocalisation that causes it are central to the diazonium chemistry used to make brightly coloured dyes.
- Example 3. Nylon from a diamine
- A diamine such as 1,6-diaminohexane reacts with a dicarboxylic acid derivative through its nucleophilic nitrogen lone pairs to form the amide (peptide-like) links of nylon; the same nucleophilic-amine reactivity that builds amides in the laboratory underpins this condensation polymer.
Try this
Q1. Write the equation for the reaction of ethylamine with hydrochloric acid. [1 mark]
- Cue. (ethylammonium chloride).
Q2. State the reagents used to reduce nitrobenzene to phenylamine. [1 mark]
- Cue. Tin and concentrated hydrochloric acid (then alkali to liberate the amine).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marks(a) Explain why amines act as bases. (b) Explain why ethylamine is a stronger base than ammonia, but phenylamine (aniline) is a weaker base than ammonia.Show worked answer →
(a) Amines have a lone pair of electrons on the nitrogen atom, which can accept a proton (), so they act as Bronsted-Lowry bases (1).
(b) In ethylamine the alkyl group releases electron density (the positive inductive effect) onto the nitrogen, increasing the availability of the lone pair, so it is a stronger base than ammonia (1). In phenylamine the nitrogen lone pair is delocalised into the benzene ring (1), so it is less available to accept a proton, making phenylamine a weaker base than ammonia (1).
Eduqas 20214 marksEthylamine can be prepared from bromoethane. (a) State the reagent and conditions. (b) Name the type of mechanism and explain why a further product (a secondary amine) can also form.Show worked answer →
(a) Excess ammonia in ethanol, heated in a sealed tube (under pressure) (1).
(b) Nucleophilic substitution (1). The ethylamine formed still has a lone pair and acts as a nucleophile, so it can react with more bromoethane to give a secondary amine (and further substitution gives tertiary amine and quaternary salt) (1); using excess ammonia minimises this (1).
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Sources & how we know this
- WJEC Eduqas GCE A Level Chemistry specification (from 2015) — WJEC Eduqas (2015)