Study of the kinetics and equilibria of the oligomerization reactions of 2-methylglyceric acid

Abstract

The presence of a variety of chemical species related to the gaseous precursor isoprene in ambient secondary organic aerosol (SOA) has stimulated investigations of the nature of SOA-phase chemical processing. Recent work has demonstrated that 2-methylglyceric acid (2-MG) is an important isoprene-derived ambient SOA component and atmospheric chamber experiments have suggested that 2-MG may exist in oligomeric form (as oligoesters) under conditions of low SOA water content. In order to better understand the thermodynamic and kinetic parameters of such oligomerization reactions, nuclear magnetic resonance techniques were used to study the bulk phase acid-catalyzed aqueous reactions (Fischer esterification) of 2-MG. While the present results indicate that 2-MG oligoesters are formed in the bulk phase with similar water content equilibrium dependences as observed in atmospheric chamber SOA experiments, the acid-catalyzed rate of the Fischer esterification mechanism may be too slow to rationalize the 2-MG oligoester production timescales observed in the atmospheric chamber experiments. Furthermore, it appears that unrealistically high ambient SOA acidities would also be required for significant 2-MG oligoester content to arise via Fischer esterification. Therefore, the present results suggest that other, more kinetically facile, esterification mechanisms may be necessary to rationalize the existence of 2-MG oligomers in atmospheric chamber-generated and ambient SOA

Mechanistic study of secondary organic aerosol components formed from nucleophilic addition reactions of methacrylic acid epoxide

“…As discussed in the Supplement to Birdsall et al (2013), sets of peaks postulated to arise from diastereomers were observed in the 1 H and 13 C spectra in a manner that is consistent with the observations of Espartero et al (1996) for similar lactic acid-derived species. The complete 1 H and 13 C chemical shift assignments for MAE and 2-MG are given in Figs.…”

“…In particular, a Fischer esterification mechanism would allow for oligomeric chains of 2-MG to be formed or for nitrate and sulfate ester formation in the presence of those inorganic ions. However, previous kinetics measurements suggest that, under the typical range of aerosol conditions, Fischer esterification of 2-MG proceeds too slowly to account for the extent of oligomer formation observed in atmospheric chamber experiments (Birdsall et al, 2013).…”

“…2-MG was formed from the other fraction of dilute, crude MAE by simultaneous aqueous extraction and acid-catalyzed hydrolysis, as described previously (Birdsall et al, 2013 Birdsall et al (2013), the self-catalyzed conversion of 2-MG via Fischer esterification to oligomer products was observed to appear with an initial concentration of 2 % and roughly 7 % after 6 months of storage at room temperature (295 K). To avoid even this very slow process, 2-MG was also stored at −80 • C when not in use.…”

“…The kinetics of MAE hydrolysis were determined using an NMR-based technique previously developed in our lab Birdsall et al, 2013). For these experiments, 1 H spectra were collected with eight scans (30 s) which gave large enough signal-to-noise ratios to be able to follow MAE reactant loss over more than an order of magnitude of relative concentration.…”

“…Both MAE and 2-MG were synthesized following a procedure previously developed to synthesize 2-MG (Birdsall et al, 2013). All precursor compounds were obtained from Sigma-Aldrich and used as obtained, with given purities, unless otherwise noted.…”