Preparing Amines (OCR A Level Chemistry A): Revision Note

Preparing Aliphatic Amines

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

  

Formation of primary amine

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

Nitriles can be reduced with LiAlH₄ or H₂ and Ni catalyst

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

  

Formation of secondary amine

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

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

Preparing Aromatic Amines

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

• Nitrobenzene, C₆H₅NO₂, can be reduced to phenylamine, C₆H₅NH₂, according to the following two-stage reaction:

The two-stage reduction reaction of nitrobenzene to 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)