Amines are classified according to the number of carbon atoms bonded directly to the nitrogen atom. A primary (1°) amineA compound that has only one alkyl or aryl group on the nitrogen atom. has one alkyl (or aryl) group on the nitrogen atom, a secondary (2°) amineA compound that has two alkyl or aryl groups on the nitrogen atom. has two, and a tertiary (3°) amineA compound that has three alkyl or aryl groups on the nitrogen atom. has three (Figure 15.5 “The Structure of Amines Compared to Water, an Alcohol, and an Ether”).

Figure 15.5 The Structure of Amines Compared to Water, an Alcohol, and an Ether.

Note

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 groupAn NH₂ unit. (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 10

Draw the structure for each compound and classify.

1. isopropyldimethylamine
2. dipropylamine

Show Answer

1. The name indicates that there are an isopropyl group (in red) and two methyl groups (in green) attached to the nitrogen atom; the amine is tertiary.

2. The name indicates that there are two propyl groups attached to the nitrogen atom; the amine is secondary. (The third bond on the nitrogen atom goes to a hydrogen atom.)

CH₃CH₂CH₂NHCH₂CH₂CH₃ 

The primary amine in which the nitrogen atom is attached directly to a benzene ring has a special name—aniline. Aryl amines are named as derivatives of aniline.

Example 11

Name this compound.

Show Answer

The benzene ring with an amino (NH2) group is aniline. The compound is named as a derivative of aniline: 3-bromoaniline or m-bromoaniline.

Example 12

Draw the structure for p-ethylaniline and classify.

Show Answer

The compound is a derivative of aniline. It is a primary amine having an ethyl group located para to the amino (NH2) group.

Example 13

Draw the structure for 2-amino-3-methylpentane.

Show Answer

Always start with the parent compound: draw the pentane chain. Then attach a methyl group at the third carbon atom and an amino group at the second carbon atom.

Ammonium (NH₄⁺) ions, in which one or more hydrogen atoms are replaced with alkyl groups, are named in a manner analogous to that used for simple amines. The alkyl groups are named as substituents, and the parent species is regarded as the NH₄⁺ ion. For example, CH₃NH₃⁺ is the methylammonium ion. The ion formed from aniline (C₆H₅NH₃⁺) is called the anilinium ion.

Concept Review Exercises

1. To what inorganic compound are the amines related?

2. How are amines classified?

Show Answer

1. ammonia
2. by the number of hydrocarbon groups on the nitrogen atom: primary amine, one group; secondary amine, two groups; tertiary amine, three groups 

Key Takeaways

• An amine is a derivative of ammonia in which one, two, or all three hydrogen atoms are replaced by hydrocarbon groups.

• The amine functional group is as follows:

  
• Amines are classified as primary, secondary, or tertiary by the number of hydrocarbon groups attached to the nitrogen atom.
• Amines are named by naming the alkyl groups attached to the nitrogen atom, followed by the suffix –amine.

Exercises

1. Draw the structure for each compound and classify the amine as primary, secondary, or tertiary.

   1. dimethylamine
   2. diethylmethylamine
   3. 2-aminoethanol

2. Draw the structure for each compound and classify the amine as primary, secondary, or tertiary.

   1. 3-aminopentane
   2. 1,6-diaminohexane
   3. ethylphenylamine

3. Draw the structure for each compound.

   1. aniline
   2. m-bromoaniline

4. Draw the structure for each compound.

   1. 2-chloroaniline
   2. 3,5-dichloroaniline

5. Name each compound.

   1. CH₃CH₂CH₂NH₂
   2. 
   3. 

6. Name each compound.

   1. (CH₃CH₂)₃N
   2. (CH₃CH₂)₂NCH₃

7. Draw the structure for each compound.

   1. dimethylammonium chloride
   2. anilinium chloride

8. Draw the structure for each compound.

   1. ethylmethylammonium chloride
   2. anilinium nitrate

9. Name each compound.

   1. [CH₃CH₂NH₂CH₂CH₃]⁺Br⁻
   2. [(CH₃CH₂)₃NH]⁺I⁻

10. Name each compound.

    1. [(CH₃)₃NH]⁺NO₃⁻
    2. [(CH₃CH₂)₂NH₂]⁺Cl⁻

Show Answer

1. a.CH₃NHCH₃; secondary

b. tertiary

c. HOCH₂CH₂NH₂; primary

 3. a.

b. 

5. a.  propylamine

b. isopropylmethylamine

c. 2-aminopentane

7. a. [(CH₃)₂NH₂⁺]Cl^–

b. 

9. a. diethylammonium bromide

b. triethylammonium iodide