20.4 Basicity of Ammonia and Methylamine

Amines function as bases and nucleophiles because the nitrogen atom possesses a non-bonding pair of electrons (a lone pair). This lone pair can be readily donated to an electron-deficient species, such as a proton ($H^{+}$).

The reaction of an amine with an acid, like hydrochloric acid (HCl), demonstrates this basic character. The amine accepts a proton to form a substituted ammonium salt.

Ammonia with HCl:

${\text{NH}}_3+\text{H-Cl}\to {\text{NH}}_4^{+}+{\text{Cl}}^{-}$

Methylamine with HCl:

${\text{CH}}_3{\text{-NH}}_2+\text{H-Cl}\to {\text{CH}}_3{\text{-NH}}_3^{+}+{\text{Cl}}^{-}$

Measuring Base Strength: pK_b

The strength of a base is quantitatively expressed by its $p{K}_b$ value. A lower $p{K}_b$ value indicates a stronger base.

Since $p{K}_{b,{\text{NH}}_3}>p{K}_{b,{\text{CH}}_3{\text{NH}}_2}$, methylamine is a stronger base than ammonia.

Factors Affecting Basicity

Inductive Effect of Alkyl Groups

The increased basicity of methylamine compared to ammonia can be explained by the positive inductive effect (+I effect) of the alkyl group ($C{H}_3$).

• In ammonia ($N{H}_3$), the lone pair on the nitrogen is influenced only by the hydrogen atoms.
• In methylamine ($C{H}_3-N{H}_2$), the methyl group is an electron-donating group. It pushes electron density towards the nitrogen atom through the C-N sigma bond.
• This increased electron density on the nitrogen makes the lone pair more available for donation, thus increasing the amine's basicity.

Comparison of Primary and Secondary Amines

The presence of more electron-donating alkyl groups further increases the basicity of amines.

• A primary amine (like methylamine) has one alkyl group.
• A secondary amine has two alkyl groups, which donate even more electron density to the nitrogen atom.

Therefore, a secondary amine is generally a stronger base than a primary amine.

The general order of basicity (in the gas phase) is:

$\text{Secondary Amine}>\text{Primary Amine}>\text{Ammonia}$