Amines themselves are not usually discussed by “pKa” directly — instead, chemists report the pKa of the conjugate acid (the ammonium ion), often written pKaH. A higher pKaH means a more basic amine, because its conjugate acid holds onto the proton more tightly.

Amine	Conjugate Acid	pKaH	Basicity Note
Aniline	Anilinium ion	4.6	Weakly basic — nitrogen lone pair is delocalized into the aromatic ring, less available to accept a proton
Pyridine	Pyridinium ion	5.2	Weakly basic — lone pair sits in an sp² orbital in the ring plane, not part of the aromatic system, but still less basic than an alkyl amine
Ammonia	Ammonium ion	9.25	Reference value
Methylamine	Methylammonium ion	10.6	More basic than ammonia — the alkyl group donates electron density to nitrogen

Why aromatic amines are weaker bases than alkyl amines: in aniline, the nitrogen lone pair is conjugated into the ring (the same resonance that makes aniline nitrogen a weaker nucleophile — see Appendix A’s note on amide resonance for the analogous effect on amides, which is far more extreme). That lone pair is less available to accept a proton, so aniline is a much weaker base than cyclohexylamine, its non-aromatic analog.

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