Alkylation of amines: Hofmann's exhaustive alkylation

Hofmann's exhaustive alkylation describes what happens when a primary amine is heated with an EXCESS of a primary alkyl halide: rather than stopping cleanly at a simple substitution product, the reaction proceeds repeatedly, giving eventually a mixture containing secondary amine, tertiary amine, AND tetraalkylammonium halide, since (as already noted in section 13.3.1) each newly-formed amine is itself nucleophilic enough to react further with more of the alkyl halide present. If the alkyl halide is genuinely used in excess, the reaction can in fact be driven all the way through to completion, giving the tetraalkylammonium halide as the major product -- this specific, driven-to-completion version of the reaction is what is meant by 'exhaustive alkylation of amines'. Tetraalkylammonium halides -- also called quaternary ammonium salts -- are crystalline solids, correctly understood as derivatives of the simple ammonium salt NH4(+)X(-) in which ALL FOUR of the hydrogen atoms originally attached to nitrogen have been replaced by four alkyl groups (which may be all the same, or different from one another). Counting how many successive alkylation steps are needed to reach this quaternary salt starting from each amine class: a PRIMARY amine needs to consume THREE moles of alkyl halide, a SECONDARY amine needs TWO moles, and a TERTIARY amine needs only ONE mole, to be fully converted through to the quaternary ammonium salt. This exhaustive-alkylation reaction is deliberately carried out in the presence of the mild base sodium bicarbonate (NaHCO3), whose role is simply to neutralise the substantial quantity of HX (hydrogen halide) generated as a byproduct across these repeated substitution steps, preventing that acid from building up and interfering with the reaction. When the specific alkylating a …