amine nucleophilicity

The same lone pair that makes an amine basic also makes it a good nucleophile — an electron-rich species hungry to attack an electron-poor carbon. Basicity is about grabbing a proton (H+); nucleophilicity is about attacking carbon. They are related but not identical, and amines happen to be strong at both, which is why nitrogen ends up bonded all over organic and biological molecules.

Picture an amine meeting an alkyl halide like CH3-I. The nitrogen lone pair reaches in and attacks the carbon from the back side, pushing the iodide out: this is an SN2 substitution, and a new C-N bond forms. The same lone pair will add to the electrophilic carbon of a carbonyl (C=O) to start forming an imine, an enamine, or an amide. Neutral amines are already decent nucleophiles; if you deprotonate or otherwise enrich the nitrogen, it becomes even more reactive. Their nucleophilicity is why amines are the attacking partner in so many bond-forming reactions.

There is a catch when you use amines as nucleophiles in alkylation: the product of attacking an alkyl halide is itself a more substituted, more nucleophilic amine, so it keeps reacting (overalkylation), giving a messy mix of 1°, 2°, 3°, and even quaternary products. Chemists dodge this with tricks like the Gabriel synthesis or reductive amination. Also note that nucleophilicity and basicity can diverge: a bulky tertiary amine can be a strong base yet a poor nucleophile because its bulk blocks approach to carbon, which is exactly why hindered amine bases are chosen to do E2 elimination rather than substitution.