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Preparing Amines

Lajoy Tucker

Teacher

Lajoy Tucker

Contents

What are Amines?

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Preparing Amines Explained

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Methods of Preparing Amines

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Uses of amines

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Practice Question

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What are Amines?

Amines are organic compounds derived from ammonia , where one or more hydrogen atoms are replaced with alkyl or aryl groups.

Types of Amines:

  • Primary:

  • Secondary:

  • Tertiary:

Chemistry revision note showing the structural formulas of ammonia, primary amines, secondary amines and tertiary amines with nitrogen bonded to hydrogen atoms and alkyl groups.

Aliphatic amine: the amino group is directly attached to non-aromatic groups e.g. an alkyl group.

Chemistry revision note showing the displayed structural formula of methylamine with a nitrogen atom bonded to two hydrogen atoms and one methyl group.

Aromatic amine: the amino group is directly attached to an aromatic ring.

Chemistry revision note showing the structural formula of aniline with an amino group attached to a benzene ring.

Preparing Amines Explained

Methods of Preparing Amines

1. From Halogenoalkanes (via Nucleophilic Substitution).

Equation:

  • Reagents: Excess ammonia

  • Conditions: Ethanolic solution, sealed tube, heat

  • Forms primary amines via nucleophilic substitution

Chemistry revision note showing the nucleophilic substitution mechanism where ammonia reacts with bromoethane to form ethylamine and ammonium ions through electron pair movement.

The ammonia must be in excess to reduce the chance of further substitution which leads to secondary and tertiary amines, and quaternary ammonium salts.

If excess halogenoalkane used:

Chemistry revision note showing the stepwise formation of primary, secondary, tertiary and quaternary ammonium compounds from the reaction of halogenoalkanes with ammonia and amines.

2. From Nitriles (Reduction Method).

Step 1: Convert halogenoalkane nitrile

Chemistry revision note showing the nucleophilic substitution reaction of bromoethane with potassium cyanide in ethanol under heat to form propanenitrile and potassium bromide.

(via nucleophilic substitution)


Step 2: Reduce nitrile primary amine

Chemistry revision note showing the reduction of a nitrile using hydrogen to form a primary amine with an extra carbon atom retained in the chain.

Reducing agent : Hydrogen () with a catalyst or in dry ether .


This method prevents further substitution, making it better for producing primary amines with greater purity.

This is also a useful method for extending the length of the carbon chain.


3. Aromatic Amines From Nitrobenzene (Reduction).

Step 1: Nitration of benzene nitrobenzene

Chemistry revision note showing the nitration reaction of benzene with nitric acid to form nitrobenzene and water.

Step 2: Reduce nitrobenzene phenylamine

Chemistry revision note showing the reduction of nitrobenzene using tin and hydrochloric acid followed by sodium hydroxide to produce aniline and water.

  • Reagents: Tin and conc. , reflux.

  • Then add to liberate free amine.

Note – the addition of is not formally assessed by AQA but it is important to acknowledge that under acidic conditions, the amine group would be protonated forming the ammonium salt.

Chemistry revision note showing the structure of the phenylammonium ion with a benzene ring bonded to an NH3+ group.

Uses of amines

Quaternary ammonium salts act as cationic surfactants.

Chemistry revision note showing the general structure of a quaternary ammonium salt with a positively charged nitrogen atom bonded to four alkyl groups and an accompanying anion.

e.g.

Chemistry revision note showing the structure of cetyltrimethylammonium bromide with a long hydrocarbon chain attached to a positively charged quaternary ammonium ion and a bromide ion.

  • The positive charge on the nitrogen atom makes that part of the molecule hydrophilic (attracted to water)

  • The non-polar hydrocarbon chains are hydrophobic and do not mix with water.

  • Cationic surfactants are often used as fabric softeners and hair conditioners as the charged heads are attracted to wet surfaces forming a coating and giving a smoothing effect.

Aromatic amines are used in the manufacture of dyes.

Chemistry revision note showing phenylamine being converted into a diazonium salt and then reacting with phenol to form an azo dye.

Diamines are used as monomers for condensation polymers called polyamides by reacting them with dicarboxylic acids or acyl chlorides.

Chemistry revision note showing the condensation polymerisation reaction between hexanedioic acid and hexane-1,6-diamine to form nylon with amide links and water molecules produced.

Practice Question

Explain why using a nitrile intermediate is better for making a primary amine than direct reaction of a halogenoalkane with ammonia.

Answer

Using ammonia leads to further substitutions forming secondary/tertiary amines (a mixture of products).

The nitrile method produces only one product—primary amine — after reduction therefore giving a higher percentage yield.

Basic Properties of Amines