Triethyloxonium tetrafluoroborate is prepared from boron trifluoride, diethyl ether and epichlorohydrin:
where the Et stands for ethyl. The trimethyloxonium salt is available from dimethyl ether via an analogous route. These salts do not have long shelf-lives at room temperature. They degrade by hydrolysis:
The propensity of trialkyloxonium salts for alkyl-exchange can be advantageous. For example, trimethyloxonium tetrafluoroborate, which reacts sluggishly due to its low solubility in most compatible solvents, may be converted in situ to higher alkyl/more soluble oxoniums, thereby accelerating alkylation reactions.
This reagent is useful for esterification of carboxylic acids under conditions where acid-catalyzed reactions are infeasible: