Basicity depends on how available the nitrogen lone pair is

Amines are Lewis bases because nitrogen can donate its lone pair to an electron-pair acceptor such as H⁺. In the gaseous phase, increasing alkyl substitution generally strengthens the basicity of comparable primary, secondary and tertiary alkylamines: tertiary > secondary > primary > ammonia. Alkyl electron donation increases nitrogen electron density and stabilises the protonated ion without competing hydration effects.

In aqueous medium, the specified order is ethylamine > ammonia > phenylamine. The ethyl group donates electron density, increasing lone-pair availability relative to ammonia. In phenylamine, the nitrogen lone pair is delocalised into the aromatic ring and is less available to accept a proton. Solvation of the base and its conjugate acid also contributes to an aqueous basicity, so do not transfer a gas-phase primary/secondary/tertiary order unchanged into water.

The same lone-pair donation that weakens phenylamine's basicity activates its aromatic ring towards electrophilic substitution. Aqueous bromine gives a white precipitate of 2,4,6-tribromophenylamine and loses its colour, without requiring an AlBr₃ catalyst. The NH₂ group directs substitution to its two ortho positions and para position.