Basic character of amines

The lone pair on nitrogen makes amines Lewis bases. They form salts with acids, and NaOH gives the amine back. Amine salts dissolve in water but not in ether, which lets amines be separated from non-basic organic compounds.

Base strength is measured by Kb, or pKb = −log Kb; the larger Kb (smaller pKb), the stronger the base. Ammonia has pKb 4.75.

pKb in water. Methyl series: 1° methanamine 3.38, 2° N-methylmethanamine 3.27, 3° N,N-dimethylmethanamine 4.22. Ethyl series: 1° ethanamine 3.29, 2° N-ethylethanamine 3.00, 3° N,N-diethylethanamine 3.25. Aromatic: benzenamine (aniline) 9.38, N-methylaniline 9.30, N,N-dimethylaniline 8.92; phenylmethanamine (benzylamine) 4.70.

Alkyl groups push electrons (+I) onto nitrogen, making the lone pair more available, and spread the positive charge of the ammonium ion. So aliphatic amines are stronger bases than ammonia; their pKb values run from 3 to 4.22.

In the gas phase basicity follows the +I effect: tertiary > secondary > primary > NH₃.

In water, the ammonium ions are also stabilised by hydrogen bonding to water (solvation). Smaller ions with more N–H hydrogens are solvated best, favouring primary > secondary > tertiary, the reverse of the inductive order. Groups bigger than –CH₃ also hinder that hydrogen bonding sterically.

The balance of inductive effect, solvation and steric hindrance gives the aqueous orders (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃ and (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃.

Aniline is a much weaker base than ammonia: its lone pair is conjugated with the ring. Aniline has five resonance structures, the anilinium ion only two (Kekulé), so protonation costs aniline stability.

On substituted anilines, electron-releasing groups (–OCH₃, –CH₃) raise basic strength; electron-withdrawing groups (–NO₂, –SO₃H, –COOH, –X) lower it.

Worked order, strongest first: (C₂H₅)₂NH > C₂H₅NH₂ > NH₃ > C₆H₅NH₂.