Electrophilic substitution at a saturated carbon atom. Part XIV. Consecutive unimolecular mercurydeprotonation and mercurydecarboxylation of pyridioacetic acid ions in aqueous solution
In aqueous acidic solutions containing mercury(II) nitrate, the 2- and 4-pyridioacetic acid ions undergo a unimolecular substitution of an α-proton by mercury. This is followed by a slower reaction in which the carboxyl group is also replaced by mercury. The latter reaction involves a slow second unimolecular substitution of an α-proton by mercury, followed by a rapid protodecarboxylation, rather than a direct unimolecular mercurydecarboxylation. The kinetics show that the mercury(II) ion is very much more reactive than the acid medium towards the several intermediate pyridiomethylide ions. Mercury(II) chloride and higher chloromercurates are less reactive and hence the initial mercurydeprotonation is rapidly reversed in the presence of an excess of chloride ion. The 3-pyridioacetic acid ion is unreactive under the conditions used. Alkyl 2- and 4-pyridioacetates undergo the first mercurydeprotonation readily and the second with reluctance; both of these reactions are also reversed on the addition of an excess of chloride ion. The reactions have been developed for the preparation of dimercurated methylpyridinium salts, including 1-pyridiomethylbis[mercury(II) chloride] salts.