Effects of a Carboxyl Group on the Products, Mechanism, and Kinetics of the OH Radical-Initiated Oxidation of 3-Butenoic Acid Under Low NO_<i>x</i> Conditions

Concentrations of nitrogen oxides (NO_<i>x</i>) in the U.S. have decreased so much in the last few decades that the oxidation of volatile organic compounds (VOCs), which plays a critical role in the formation of ozone and fine particles, now often occurs in urban areas under conditions that are historically associated with remote rural locations. As a result, alkylperoxy radicals (RO₂^•), which are key intermediates in VOC oxidation, can react with NO, HO₂, and RO₂^• radicals, and isomerize. In this study, we primarily investigated the products, mechanism, and kinetics of the OH radical-initiated oxidation of 3-butenoic acid under conditions where the dominant products of a similar reaction of 1-pentene were hydroxy-hydroperoxides formed through RO₂^• + HO₂ reactions. 3-Butenoic acid has some structural properties that made it ideally suited for this study, which was conducted in an environmental chamber using iodide chemical ionization mass spectrometry and authentic standards. The major reaction products were oxo-propanoic acid, hydroxyoxo-butanoic acid, dihydroxy-butanoic acid, and dihydroxy-dicarboxybutyl-peroxide (a ROOR dimer) formed with measured molar yields of 0.74, 0.09, 0.08, and 0.03 for a total molar yield of 0.94 that effectively achieved a mole balance. Unlike in the 1-pentene reaction, reaction of the dominant RO₂^• radical with HO₂ led almost solely to formation of oxo-propanoic acid + formaldehyde + OH + HO₂, apparently due to hydrogen bonding involving the carboxyl group in the ROO-OOH intermediate complex. Similar hydrogen bonding in the ROO-OOR complex was likely responsible for the formation of the peroxide dimer and the exceptional speed of RO₂^• + RO₂^• reactions, which were competitive with RO₂^• + HO₂ reactions and were calculated from measurements and kinetics modeling to occur with a rate constant ≥3 × 10^-11 cm³ molecule^-1 s^-1 that may approach the collision limit value of about 5 × 10^-10 cm³ molecule^-1 s^-1. The results of the study demonstrate that functional groups can have dramatic effects on the atmospheric chemistry of VOCs.