Mechanism and Kinetics of the Reaction of Nitrate Radicals with Carboxylic Acids

Rate constants for the reaction of nitrate radical (NO₃ ⋅) with several carboxylic acids (RCO₂ H) were measured in acetonitrile using laser flash photolysis, and found to be on the order of 10⁵ -10⁶  M^-1 s^-1 . No observable H/D kinetic isotope effect was observed at the carboxyl O-H group, α-C-H bond and (possibly) in the case of formic acid, the formylic C-H bond. This suggests that NO₃ ⋅ does not abstract hydrogen from any of these positions despite the fact that all these processes are thermodynamically favorable. Reactivity increases with increased length and/or branching of the alkyl side chain (R), and approaches, but does not quite reach, that of an alkane towards NO₃ ⋅. The relative inertness of carboxylic acids towards NO₃ ⋅ can be explained by the polar effect.