Kinetics and mechanism of reactions between aromatic olefins and hydroxyl radicals

Abstract

Abstract The rates of the reactions of hydroxyl radicals (OH) with styrene, α‐methylstyrene, and β‐methylstyrene have been measured by irradiating mixtures of these aromatic olefins and NO in an environmental chamber at 298 K. Experimental conditions were used whereby the competition of ozone with OH in oxidizing the hydrocarbons could be considered negligible. The rate constant values, obtained by a relative method using isooctane as reference hydrocarbon, are: styrene (5.3 ± 0.5) × 10 −11 cm 3 /molec·s, α‐methylstyrene (5.3 ± 0.6) × 10 −11 cm 3 /molec·s, and β‐methylstyrene (6.0 ± 0.6) × 10 −11 cm 3 /molec·s. A simplified kinetic treatment of the experimental data shows that styrene and β‐methylstyrene are stoichiometrically converted to benzaldehyde, suggesting that OH attack occurs only on the aliphatic moiety of the aromatic olefins. Benzaldehyde was observed to undergo consecutive oxidation by OH, and its maximum formation yield was about 60%. A reaction mechanism is proposed where the primary rate‐determining OH attack leads to the formation of 1‐hydroxy‐2‐phenyl‐2‐ethenyl radicals, from which benzaldehyde is formed through fast intermediate reactions.