Abstract
The pyrolysis of peracetic, perpropionic acids (RCO3−H) and t-butyl hydroperoxide (ROOH) has been studied in the gas phase. Under the conditions studied both peracetic and perpropionic acids exhibit pressure dependent unimolecular rate constants. This is particularly marked for peracetic acid. An RRKM analysis, using the Gilbert method, provided the best fit with the data with an A factor of 1015.6 s−1 and ΔEdown−200 cm−1. In fact the A factors for all three decompositions are all −1015.6 s−1 identical to that found by dialkyl peroxides (ROOR). The O−O bond strengths for the three types of peroxides are RO−OH, 180±2 kJ mol, RCO2−OH, 170±2 kJ mol−1 and RO−OR, 160±2 kJ mol−1 i.e. there is a subtle difference of −10 kJ mol−1 between each group. This difference in bond dissociation energies explain why acyl hydroperoxides decompose faster than the alkyl hydroperoxides in the “cool” flame region. Under high pressure conditions the ratio of the rate constants varies from 10 to 5 from 500–700 K in favour of the acyl peroxides—a subtle difference! Pressure dependent decomposition modifies this conclusion.
Published Version
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