Thermodynamics and Kinetics of Atmospherically Relevant Acetalization Reactions

Accretion reactions have been suggested as an important mechanism in the formation of low volatility secondary organic aerosol (SOA). Acetals are potential accretion products formed through the acid-catalyzed reactions of aldehydes and alcohols (both of which are ubiquitous in the atmosphere) via nucleophilic addition of alcohols to the aldehyde C═O bond. The thermodynamics and kinetics of the acetalization reaction mechanism for model aldehydes and polyols were examined with computational electronic structure methods and experimental bulk kinetics experiments using nuclear magnetic resonance (NMR) spectroscopy. The formation of all possible nucleophilic addition reaction products (hydrates, hemiacetals, acyclic acetals, and cyclic acetals) was found to be thermodynamically feasible under aqueous acidic conditions, a result at odds with an earlier computational study. Except for the formation of the thermodynamically most favored cyclic acetal products, the reactions occurred on a time scale shorter than the NMR experiment (lifetimes <5 min). Polyols that could form both a 5- or 6-membered ring cyclic acetal product formed the 5-membered ring faster but exhibited interconversion to the 6-membered ring on the multihour lifetime scale. Reactions involving structurally different aldehydes and polyols were also examined, but both the thermodynamics and kinetics results depended only weakly on the carbon backbone of reactant aldehydes and polyols. Even the relatively slow cyclic acetal-forming reactions were found to have lifetimes <1 day for pH 2.2 SOA acidities which are well within atmospherically relevant time scales. These thermodynamic and kinetic results indicate that acetalization reactions are a plausible accretion mechanism in SOA.