Question

Why are tertiary amines less basic than secondary amines?

Answer 1

Because alkyl groups donate electrons to the more electronegative nitrogen. The inductive effect makes the electron density on the alkylamine's nitrogen greater than the nitrogen of ammonium. Correspondingly, primary, secondary, and tertiary alkyl amines are more basic than ammonia

Answer 2

In gas phase tertiary amines are more basic than secindry amines due to +I effect of alkyl group.But in aqueous phase solvation become the prime factor for stability and basic strength and lower the size higher will be the solvation so secondry amines are more basic than ammonia.