By ammonolysis of alkyl halides

When an alkyl halide is heated with an excess of alcoholic ammonia solution, it undergoes nucleophilic substitution in which the halogen atom is displaced by an amino (-NH2) group, giving a primary amine; because ammonia is the nucleophile doing the substituting, this process is called ammonolysis of the alkyl halide, and is also described as alkylation of ammonia. The reaction is carried out in a sealed tube at 373 K (since ammonia is a gas at room temperature and pressure and the reaction needs it in solution under some pressure to proceed at a useful rate). A key complication must be noted: the primary amine formed in this first substitution step is actually a STRONGER nucleophile than ammonia itself was, so if ammonia is not present in large excess, this newly-formed primary amine reacts further with more alkyl halide, giving secondary amine, then tertiary amine, and eventually quaternary ammonium salt as successive substitutions occur -- overall: R-X + NH3 (alcoholic, excess, heat) gives R-NH2 (a primary amine) as the desired product only when ammonia is genuinely in large excess, suppressing this over-alkylation. The relative REACTIVITY of different alkyl halides toward ammonia in this substitution follows the order R-I > R-Br > R-Cl, reflecting the decreasing strength (and so increasing leaving-group ability) of the carbon-halogen bond as the halogen gets larger down the group -- iodide is the best leaving group of the three, so an alkyl iodide reacts fastest. One further caution applies specifically to TERTIARY alkyl halides: because a tertiary carbocation is so stable, a tertiary substrate like tert-butyl br …