Preparation (Oxford AQA International A Level Chemistry)

Aliphatic Amines

• Amines can be thought of as derivatives of ammonia, in which one or more of the hydrogens is replaced by an alkyl or aryl group

• The number of substituted hydrogens is the basis of classifying amines

  • Primary amines

  • Secondary amines

  • Tertiary amines

Amines can be classified as primary, secondary and tertiary

Preparing Amines

• Primary amines can be prepared from different reactions including:

  • The reaction of halogenoalkanes with ammonia

  • The reduction of nitriles

Reaction of halogenoalkanes with ammonia

• This is a nucleophilic substitution reaction in which the nitrogen lone pair in ammonia acts as a nucleophile and replaces the halogen in the halogenoalkane

• When a halogenoalkane is reacted with excess, hot ethanolic ammonia under pressure a primary amine is formed

  

Reduction of nitriles

• Nitriles contain a -CN functional group which can be reduced to an -NH₂ group

• The nitrile vapour and hydrogen gas are passed over a nickel catalyst or LiAlH₄ in dry ether can be used to form a primary amine

Reaction of halogenoalkanes with primary amine

• This is also a nucleophilic substitution reaction in which the nitrogen in the primary amine acts as a nucleophile and replaces the halogen in the halogenoalkane

• When a halogenoalkane is reacted with a primary amine in ethanol and heated in a sealed tube, under pressure a secondary amine is formed

  

• Tertiary amines can also be formed by further reaction of the secondary amine

• In this example, further substitution would form triethylamine (CH₃CH₂)N

Aromatic Amines

• Phenylamine is an organic compound consisting of a benzene ring and an amine (NH₂) functional group

• Phenylamine is the starting point for making many other chemicals including dyes

• It is made in industry using benzene produced from crude oil

• Nitrobenzene, C₆H₅NO₂, can be reduced to phenylamine, C₆H₅NH₂ in two steps

Preparation of phenylamine

Stage 1 - Reduction of nitrobenzene

• Nitrobenzene, C₆H₅NO₂, is reacted with tin, Sn, and concentrated hydrochloric acid, HCl

  • Tin and hydrochloric acid act as reducing agents

• The reaction mixture is heated under reflux in a boiling water bath

• The phenylammonium ions, C₆H₅NH₃⁺, are protonated due to the acidic conditions

Stage 2 - Formation of phenylamine

• The phenylammonium ions, C₆H₅NH₃⁺, are deprotonated by the addition of excess sodium hydroxide solution, NaOH (aq)