Alkylation

Because amines are good nucleophiles, they can attack an alkyl halide directly in a nucleophilic substitution reaction, displacing the halide and installing a new alkyl group on nitrogen:

The product of this first substitution is itself still a nucleophilic amine (now secondary), so it does not stop there — it can go on to react with a further equivalent of the alkyl halide, and the resulting tertiary amine can react again to give a quaternary ammonium salt. In other words, alkylating a primary amine with an alkyl halide tends to march the nitrogen straight through primary $\rightarrow$ secondary $\rightarrow$ tertiary $\rightarrow$ quaternary ammonium salt, all under essentially the same reaction conditions, since each successive amine remains a competent nucleophile towards the alkyl halide still present. …