Amines (OCR A Level Chemistry A): Revision Note

Amine Basicity

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

  • A primary amine has one alkyl or aryl group bonded to the nitrogen (RNH₂)

  • A secondary amine has two alkyl or aryl groups bonded to the nitrogen (R₂NH)

  • A tertiary amine has three alkyl or aryl groups bonded to the nitrogen (R₃N)

  • Ammonia (NH₃) has no groups attached

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 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 are named using the alkyl (or aryl) prefix followed by -amine, e.g. methylamine and phenylamine

• When the molecule also contains a higher-priority group, such as a carboxylic acid, the prefix amino- is used instead, e.g. 2-aminopropanoic acid

Nomenclature of aliphatic and aromatic amines table

Basic properties

• The nitrogen atom in ammonia and amine molecules can accept a proton (H⁺ ion)

• They can therefore act as bases in aqueous solutions by donating their lone pair of electrons to a proton and form a dative bond

  • For example, ammonia undergoes an acid-base reaction with dilute hydrochloric acid (HCl) to form a salt

NH₃ + HCl → NH₄⁺Cl^-

base      acid            salt

• Amines react with HCl to form amine salts, such as ethylammonium chloride

C₂H₅NH₂ + HCl → C₂H₅NH₃⁺Cl^-

base          acid                 salt

• Like ammonium salts, amine salts are ionic and soluble, so when the alkyl group is small they are water soluble but become less so as the carbon chain increases

• We can show the reaction in Brønsted-Lowry terms, where ammonia or the amine acts as a proton acceptor:

The nitrogen atom in ammonia and amines can donate their lone pair of electrons to form a bond with a proton and therefore act as a base