Preparation of Amines (AQA A Level Chemistry)

Aliphatic Amines

Amines

• Amines can be thought of as derivates 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

Classification of amines

• Notice the classification is not the same as in alcohols and haloalkanes, where the designation primary, secondary and tertiary is based on the substituents on the carbon atom rather than the nitrogen atom

• If the R group is an alkyl group (methyl, ethyl, etc) then then it is an aliphatic amine; if it is an aryl group (benzene ring or phenyl) then it is an aromatic amine

• Aliphatic and aromatic amines share similar chemical reactions and the aryl group can strongly influence the chemistry and reactivity of the amine group

Naming Amines

• Amines can be named using common names or IUPAC systematic names

• The common way to name amines is to use the alkyl (or aryl) prefix followed by -amine

• The IUPAC systematic name uses the numbered prefix amino- followed by the alkane (or aromatic) stem

Nomenclature of aliphatic and aromatic amines table

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

Aromatic Amines

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

• It can be produced in a three-step synthesis reaction followed by the separation of phenylamine from the reaction mixture

  • Step 1- Benzene undergoes nitration with concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄) at 25 to 60 ^oC to form nitrobenzene

  • Step 2 - Nitrobenzene is reduced with hot tin (Sn) and concentrated hydrochloric acid (HCl) under reflux to form an acidic mixture that contains the organic product C₆H₅N⁺H₃

  • Step 3 - Sodium hydroxide (NaOH) is added to the acidic reaction mixture to form phenylamine

  • Step 4 - The phenylamine is separated from the reaction mixture by steam distillation

The overall reaction of formation of phenylamine from benzene

Multi-step synthesis of phenylamine from benzene