Base-Induced Decomposition of Alkyl Hydroperoxides in the Gas Phase. Part 3. Kinetics and Dynamics in HO⁻ + CH₃OOH, C₂H₅OOH, and tert-C₄H₉OOH Reactions

The E_CO2 elimination reactions of alkyl hydroperoxides proceed via abstraction of an α-hydrogen by a base: X⁻ + R¹R²HCOOH → HX + R¹R²CO + HO⁻. Efficiencies and product distributions for the reactions of the hydroxide anion with methyl, ethyl, and tert-butyl hydroperoxides are studied in the gas phase. On the basis of experiments using three isotopic analogues, HO⁻ + CH₃OOH, HO⁻ + CD₃OOH, and H¹⁸O⁻ + CH₃OOH, the overall intrinsic reaction efficiency is determined to be 80% or greater. The E_CO2 decomposition is facile for these methylperoxide reactions, and predominates over competing proton transfer at the hydroperoxide moiety. The CH₃CH₂OOH reaction displays a similar E_CO2 reactivity, whereas proton transfer and the formation of HOO⁻ are the exclusive pathways observed for (CH₃)₃COOH, which has no α-hydrogen. All results are consistent with the E_CO2 mechanism, transition state structure, and reaction energy diagrams calculated using the hybrid density functional B3LYP approach. Isotope labeling for HO⁻ + CH₃OOH also reveals some interaction between H₂O and HO⁻ within the E_CO2 product complex [H₂O···CH₂O···HO⁻]. There is little evidence, however, for the formation of the most exothermic products H₂O + CH₂(OH)O⁻, which would arise from nucleophilic condensation of CH₂O and HO⁻. The results suggest that the product dynamics are not totally statistical but are rather direct after the E_CO2 transition state. The larger HO⁻ + CH₃CH₂OOH system displays more statistical behavior during complex dissociation.