INTRODUCTION TO AMINES

Introduction to Amines

Amines are aliphatic and aromatic derivatives of ammonia. Amines, like ammonia, are weak bases (K _b = 10 ⁻⁴ to 10 ⁻⁶). This basicity is due to the unshared electron pair on the nitrogen atom.

Classification and nomenclature of amines

Amines are classified as primary, secondary, or tertiary based upon the number of carbon‐containing groups that are attached to the nitrogen atom. Those amine compounds that have only one group attached to the nitrogen atom are primary, while those with two or three groups attached to the nitrogen atom are secondary and tertiary, respectively.

In the common system, you name amines by naming the group or groups attached to the nitrogen atom and adding the word amine.

In the IUPAC System, apply the following rules to name amines:

1. Pick out the longest continuous chain of carbon atoms. The parent name comes from the alkane of the same number of carbons.

2. Change the ‐e of the alkane to “amine.”

3. Locate and name any substituents, keeping in mind that the chain is numbered away from the amine group. Substituents, which are attached to the nitrogen atom instead of the carbon of the chain, are designated by a capital N.

Aromatic amines belong to specific families, which act as parent molecules. For example, an amino group (—NH ₂) attached to benzene produces the parent compound aniline.

Basicity of amines

Amines are basic because they possess a pair of unshared electrons, which they can share with other atoms. These unshared electrons create an electron density around the nitrogen atom. The greater the electron density, the more basic the molecule. Groups that donate or supply electrons will increase the basicity of amines while groups that decrease the electron density around the nitrogen decrease the basicity of the molecule. For alkyl halides in the gas phase, the order of base strength is given below:

(CH ₃) ₃ N > (CH ₃) ₂NH > CH ₃NH ₂ > NH ₃

most  least

basic  basic

However, in aqueous solutions, the order of basicity changes.

(CH ₃) ₂ NH > CH ₃NH ₂ > (CH ₃) ₃N > NH ₃

most least

basic basic

The differences in the basicity order in the gas phase and aqueous solutions are the result of solvation effects. Amines in water solution exist as ammonium ions.

                             Figure 1

In water, the ammonium salts of primary and secondary amines undergo solvation effects (due to hydrogen bonding) to a much greater degree than ammonium salts of tertiary amines. These solvation effects increase the electron density on the amine nitrogen to a greater degree than the inductive effect of alkyl groups.

Arylamines are weaker bases than cyclohexylamines because of resonance. Aniline, a typical arylamine, exhibits the resonance structures shown in Figure 1.

As structures b through e in Figure show, delocalization of the unshared electron pair occurs throughout the ring, making these electrons less available for reaction. As a result of this electron delocalization, the molecule becomes less basic.