Amines

Definition and Properties of Amines

  • Amines are derivatives of ammonia (NH3), where one or more hydrogen atoms are replaced by an alkyl group.
  • They can be classified as primary, secondary, or tertiary, depending on the number of alkyl groups attached to the nitrogen atom.
  • Amines have a lone pair of electrons on the nitrogen atom, making them basic as they can donate this pair to form a dative covalent bond with hydrogen ions.
  • Because of the presence of nitrogen, amines have higher boiling points than analogous alkanes but lower than analogous alcohols.

Preparation of Amines

  • Amines can be prepared from halogenoalkanes through nucleophilic substitution reactions. The halogen is replaced by an -NH2 group. This reaction is called amination.
  • They can also be produced through reduction of nitriles, which involves reducing the cyano group (-C≡N) to an amine (-NH2) group.

Reactions of Amines

  • Amines can react with acids to form ammonium salts. This is a result of the lone pair of electrons on the nitrogen atom, which can readily form a dative bond with a hydrogen ion.
  • They can also undergo N-substitution reactions, where the alkyl group of the amine is replaced by another group.

Amines in Chemical Analysis

  • Amines play a significant role in qualitative and quantitative chemical analysis. They can act as nucleophiles in many organic reactions.
  • Test for an amine: When an amine reacts with HCl, an ammonium salt is formed. This salt is insoluble and forms white smoke when in contact with ammonia.

Amines as Bases

  • Amines are base compounds. They can accept a proton (H+) due to the lone pair of electrons on the nitrogen atom.
  • Strength of amines as bases depends on the electron-donating ability of the alkyl group(s). Electron-donating groups attached to nitrogen increase the basicity of an amine by increasing the electron density on the nitrogen atom.

Amines and Chirality

  • Amines can be chiral molecules. A chiral molecule is one that cannot be superimposed on its mirror image.
  • If a nitrogen atom within an amine is bonded to three distinct alkyl groups, it can be a centre of chirality. This results in the potential for optical isomerism.