Probing the formation of monoalkyl carbonates and pyrocarbonates in water with electrospray ionization mass spectrometry

Abstract

The anionic hemiesters of carbonic acid with aliphatic alcohols were demonstrated to be formed in aqueous medium containing HCO₃⁻ or CO₃²⁻ by using electrospray ionization mass spectrometry (ESI-MS). Some monoalkyl carbonates (MACs) have been previously detected by capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C⁴D), but ESI-MS is structurally diagnostic and more sensitive allowing unequivocal characterization, which allowed, for the first time, detection of hydrogen pyrocarbonate and monoalkyl pyrocarbonates (MAPs). Methanol, ethanol, and different isomers of alcohols from C3 to C6 were studied. The relatively slow kinetics of formation of the MACs allowed us to confirm their formation in solution before the electrospray process takes place. The loss of CO₂ to form the corresponding alkoxide is the most important process in gas phase dissociation, but dehydro-alkoxides were also observed. Monocyclohexyl and monocyclopentyl carbonates were also characterized by ESI-MS. These species, formed from cyclohexanol and cyclopentanol in diluted aqueous solution of HCO₃⁻, are important because these alcohols are prototypes for the hydroxyl-containing portion of sterols. No biological role has been yet attributed to either MACs or MAPs, but their spontaneous formation in aqueous solution at room temperature indicates that these species are present in biological media.