5.6 Introduction
Amines are organic derivatives of ammonia in which the ammonia hydrogens are replaced by alkyl or aryl groups. Amine with one substituent called a primary amine. If it has two or three substituents, it is then called as secondary or tertiary amine (Figure 1).
Figure 1
5.7 Nomenclature
A . Common Name
The suffix –amine is given after the names of the alkyl groups bonded to nitrogen. The prefixes di-, tri-, and tetra - are used to describe two, three, or four identical substituents. Aromatic and heterocyclic amines are known by historical names (Figure 2).
Figure 2
B. IUPAC name
- Find the longest continuous chain of carbon atoms and change the -e ending in the alkane to –amine
- Give the number to show the position of the amino group
- Prefix N - should be given for each substituent on nitrogen.
5.8 Structure
We know that ammonia has sp 3 hybridization and slightly distorted tetrahedral geometry (a pyramidal shape) where a lone pair of nonbonding electrons occupied one of the tetrahedral positions. The lone pair compresses the H-N-H bond angles from the ideal sp 3 bond angle of 109.5° to 107°. But substituted amines show less angle compression because the bulky substituents open the angle slightly. For example, bond angle of ammonia is 107° but it is 108° for trimethylamine (Figure 3).
Figure 3
Amine which has three different substituents is non-superimposable on its mirror image. So it appears to be chiral but its enantiomers cannot be resolved as they interconvert rapidly by nitrogen inversion. Lone pair on the nitrogen moves from one face of the molecule to the other. The nitrogen has sp 2 hybridized orbitals in the transition state, and the lone pair electrons occupy a p -orbital. Interconversion of ( R )- and ( S )-ethyl methylamine is given in Figure 4.
Figure 4