Basicity of Aliphatic Amines

Aliphatic alkylamines react as Brønsted–Lowry bases—that is, as proton acceptors—to form the conjugate acid, the alkylammonium ions.

The pK_a value of the alkylammonium ion indicates the relative basicity of the amine; a higher pK_a value means a weaker conjugate acid, and a corresponding stronger base. Thus, alkylamines are stronger bases than ammonia.

The higher basicity of alkylamines results from the inductive effects of the alkyl groups and the solvation effects that stabilize the corresponding ammonium ions through hydrogen bonding.

Electron-donating alkyl groups stabilize the ammonium ion by partial delocalization of the positive charge, thereby increasing its pK_a value.

However, the pK_a value does not increase as expected from secondary to tertiary alkylammonium ions due to the reduced solvation effect.

Increased steric hindrance in the tertiary alkylammonium ion decreases the feasibility of solvation.

As inductive and solvation effects equally stabilize the secondary alkylammonium ion, its pK_a value is higher than the corresponding primary and tertiary ions.