Haloalkyltrialkyammonium salts serve as precursors to alpha ammonio distonic radical cations (i.e. species with separate radical and charged sites) which have been exhibiting some unusual reactivity in solution.  Trimethylammoniomethylene radical is known to readily perform hydrogen abstraction reactions but reluctantly perform bimolecular alkene additions.  In contrast, analogous 4-pentenyl, 5-hexenyl, and 6-heptenyl systems readily undergo unimolecular cyclization.  These results have prompted a fundamental, structural study of haloalkyltrialkylammonium salts.  Halomethyltrimethylammonium salts were synthesized by nucleophilic bimolecular substitution reactions (S_N2) between trimethylamine gas and dihalomethane (Scheme 1). 

Scheme 1

1. X: Br 

2. X: I

The halomethyltrimethylammonium tetrafluoroborates and hexafluorophosphates were synthesized through a counter-ion exchange reaction, using halomethyltrimethylammonium halide and silver tetrafluoroborate or silver hexafluorophosphate, respectively (Scheme 2). 

Scheme 2

3. X: Br, Y: BF₄

4. X: Br, Y: PF₆

5. X: I   , Y: BF₄

The yields of the five synthesized salts were as follows: bromomethyltrimethylammonium bromide in 69.77% yield, bromomethyltrimethylammonium tetrafluoroborate in 12.40% yield, bromomethyltrimethylammonium hexafluorophosphate (new salt) in 31.43% yield, iodomethyltrimethylammonium iodide in 86.56% yield, and iodomethyltrimethylammonium tetrafluoroborate in 80.96% yield.  X-ray crystallography analysis of the bromomethyl salts showed that the crystals were isomorphous.  Both cations and anions lie on mirror planes such that the bromo substitutent is exactly anti to a methyl group.