Amines - Structure and Classification

Amines are regarded as derivatives of ammonia ($NH_3$), formed by the replacement of one, two, or all three hydrogen atoms by alkyl and/or aryl groups.

Structure of Amines: The nitrogen atom in amines is $sp^3$ hybridized. Due to the presence of a lone pair of electrons, the geometry is trigonal pyramidal. The $C-N-E$ (where $E$ is $H$ or $C$) bond angle is slightly less than the tetrahedral angle of $109.5^\circ$; for example, it is $108^\circ$ in trimethylamine.

Classification: Amines are classified as primary ($1^\circ$), secondary ($2^\circ$), and tertiary ($3^\circ$) depending upon the number of hydrogen atoms replaced by alkyl or aryl groups in the ammonia molecule.

Primary ($1^\circ$) amines: Only one hydrogen atom of $NH_3$ is replaced by an alkyl/aryl group, resulting in the functional group $-NH_2$ (amino group).

Secondary ($2^\circ$) amines: Two hydrogen atoms of $NH_3$ are replaced, resulting in the functional group $>NH$ (imino group).

Tertiary ($3^\circ$) amines: All three hydrogen atoms of $NH_3$ are replaced, resulting in a tertiary nitrogen atom $>N-$.

Aryl Amines: In these amines, the $-NH_2$ group is directly attached to the benzene ring. The simplest example is Aniline ($C_6H_5NH_2$).