Question
Explain why alkyl amines are more basic than arylamines.
Answer
Simple alkylamines are generally more basic than arylamines because their nitrogen lone pair is more available to accept a proton.
Compare the lone pairs
In an alkylamine, the lone pair is largely localized on nitrogen, and alkyl groups can donate electron density. In an arylamine such as aniline, nitrogen is bonded directly to the aromatic ring. Its lone pair can delocalize into that ring, stabilizing the unprotonated amine.
Protonating nitrogen uses the lone pair to make an N–H bond. The arylamine then loses that particular lone-pair conjugation benefit. The benzene ring itself remains aromatic. Consequently, protonation is less favorable than for a comparable simple alkylamine.
Check with conjugate-acid pKa
In water, anilinium has a pKa of about 4.6, whereas methylammonium has a pKa of about 10.6. The higher conjugate-acid pKa corresponds to the stronger base, so methylamine is substantially more basic than aniline.
This is a general comparison, not an absolute rule for every substituted amine: ring substituents, solvent, and solvation can change basicity.
Evidence boundary
The comparison is between ordinary alkylamines and arylamines with nitrogen directly attached to an aromatic ring, exemplified by methylamine and aniline in water. It is not a universal ranking of all substituted amines or all solvents.
Sources
These references support the concepts and methods used in the explanation above.