I was wondering whether the link you added about why tertiary amines are less basic than secondary amines paints a full picture regarding the formation of a quaternary ammonium salt. Whereas one could rightly make the argument that triethylamine is slightly less basic than diethylamine in an aqueous medium, the SN2 reaction could be performed using a polar solvent that isn’t water, right? So couldn’t the comparitively more polar nature of the pentacoordinate transition state and ionic nature of the subsequent salt be stabilised in a sufficiently polar solvent that isn’t water? I ask this because I was taught in my second year of undergrad that the rate of reaction of n-Pr3N + MeI (SN2) could be increased by increasing the polarity of the solvent used due to the reason the increased polarity of the TS compared to the reactants.

Kind regards,
Vignesh

Werner,  J. Chem. Soc. 1918, p. 899 tried a 16:1 ratio of NH3 to ethylamine and got 34% yield of the amine. Maybe if you used a 50:1 or 100:1 ratio of NH3 to alkyl halide, then it would have a higher yield. But that’s starting to get to be a fairly impractical process.