Why is aniline a much weaker base than methylamine?
Basicity depends on how available the nitrogen's lone pair is to grab a proton.
Basicity depends on how available the nitrogen's lone pair is to grab a proton. In methylamine the alkyl group pushes electron density toward N, so the lone pair is readily available — more basic than ammonia. In aniline the lone pair is delocalised into the benzene ring by resonance, so it's much less available to bond a proton, making aniline a weaker base. Availability of the lone pair is the deciding factor.
Key point
A delocalised (aniline) lone pair is less available than an alkylamine's, so aniline is the weaker base.
Common mistake
ranking basicity by molecule size instead of lone-pair availability.
Memory tip
lone pair free (alkylamine) → stronger base; tied into a ring (aniline) → weaker.
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