Absolute rate constants for hydrocarbon autoxidation. 24. Rate constants and Arrhenius parameters for reaction of tert-butylperoxy radicals with 3-methylpentane

Autoxidation of 3-methylpentane in the presence of tert-butyl hydroperoxide gives, after reduction with triphenylphosphine, 3-methyl-3-pentanol and 3-methyl-2-pentanol as the major reaction products. The overall rate constant for reaction of (CH3)3COO· with this substrate at 30 °C is 0.008 ± 0.001 M−1 s−1 which consists of k/tertiary C—H = 0.007 ± 0.001 M−1 s−1 and k/secondary C—H = 0.0002 ± 0.0001 M−1 s−1. The products from cooxidation of cumene and 3-methylpentane in the presence of tert-butyl hydroperoxide are formed in yields which are consistent with these rate constants relative to a value of 0.16 M−1 s−1 for cumene. The Arrhenius equations for reaction of (CH3)3COO• with the t-C—H and s-C—H of 3-methylpentane relative to the Arrhenius equation for reaction of (CH3)3COO• with cumene (log (k10/M−1 s−1) = (8.7 ± 0.3) − (13.2 ± 0.4)/θ) are[Formula: see text]respectively, where θ = 2.303 RT kcal mol−1.

The autoxidation of optically active 1-bromo-2-methylbutane

(+)-1-Bromo-2-methylbutane undergoes autoxidation at 1 atm of oxygen and 30 °C to give, after reduction with triphenylphosphine, 1-bromo-2-methyl-2-butanol with a (+) to (−) enantiomer ratio of 1.9 ± 0.2. The optically active 1-bromo-2-methyl-2-butyl radical is, therefore, partially trapped by oxygen at this pressure before it has time to completely racemize. The half-life for racemization of the preferred conformation is approximately 2 × 10−8 s−1 at 30 °C. At 100 atm of oxygen the yield of (+)-CH3CH2COH(CH3)CH2Br is >90% and the reaction appears to proceed with retention of configuration. A small percentage of (−)-1-bromo-2-methyl-2-butanol is, however, still produced, presumably by inversion of configuration.