Investigation of the Stability of Quaternary Ammonium Methyl Carbonates

Quaternary ammonium compounds are used commercially for a variety of applications and some are of interest as ionic liquids. For many years dimethyl carbonate has been touted as a green reagent, including its use for methylation (quaternization) of tertiary amines. In addition, substitution of the methyl carbonate by other anions can be efficiently and cleanly accomplished by reaction with the corresponding acid. How stable are these methyl carbonate quaternary compounds? High field 13 C NMR shows that in the presence of water, the methyl carbonate is converted to bicarbonate. Headspace GCMS indicates that the alkylammonium methyl carbonate salts are stable below 170–180 °C while the bicarbonate salts are stable to only about 140 °C. Thermal decomposition occurs by decarboxylation and by dealkylation.