15.1: Amines

Learning Objectives

• Determine the structural feature that classifies amines as primary, secondary, or tertiary.

Amines are classified according to the number of carbon atoms bonded directly to the nitrogen atom. A primary (1°) amine has one alkyl (or aryl) group on the nitrogen atom, a secondary (2°) amine has two, and a tertiary (3°) amine has three (Figure $\PageIndex{1}$).

Figure $\PageIndex{1}$: The Structure of Amines Compared to Water, an Alcohol, and an Ether

To classify alcohols, we look at the number of carbon atoms bonded to the carbon atom bearing the OH group, not the oxygen atom itself. Thus, although isopropylamine looks similar to isopropyl alcohol, the former is a primary amine, while the latter is a secondary alcohol.

The common names for simple aliphatic amines consist of an alphabetic list of alkyl groups attached to the nitrogen atom, followed by the suffix -amine. (Systematic names are often used by some chemists.) The amino group (NH₂) is named as a substituent in more complicated amines, such as those that incorporate other functional groups or in which the alkyl groups cannot be simply named.

Example $\PageIndex{1}$

Name and classify each compound.

1. CH₃CH₂CH₂NH₂
2. 

• CH₃CH₂NHCH₂CH₃
• CH₃CH₂CH₂NHCH₃

Solution

1. There is only one alkyl group attached to the nitrogen atom, so the amine is primary. A group of three carbon atoms (a propyl group) is attached to the NH₂ group through an end carbon atom, so the name is propylamine.
2. There are two methyl groups and one ethyl group on the nitrogen atom. The compound is ethyldimethylamine, a tertiary amine.
3. There are two ethyl groups attached to the nitrogen atom; the amine is secondary, so the compound is diethylamine.
4. The nitrogen atom has a methyl group and a propyl group, so the compound is methylpropylamine, a secondary amine.