A kinetic study of the reactions of tert-butoxyl with alkenes: hydrogen abstraction vs. addition

Abstract

Rate constants and Arrhenius parameters have been measured for the reactions of tert-butoxyl radicals with alkenes by using laser flash photolysis in benzene-di-tert-butyl peroxide (1 :2 v/v) as solvent. With norbornene and norbornadiene, additions to the double bonds were the only modes of reaction, whereas with cyclopentene and cyclopentadiene, hydrogen abstraction was a competitive process. The measured activation energies were quite low; thus for norbornene, norbornadiene, and cyclopentene, E, (kcal/mol) = 2.36 f 0.40, 1.56 f 0.55, and 2.35 f 0.22, respectively, while the corresponding A factors were log (A/(M-' s-I)) = 7.76 f 0.31, 7.65 f 0.43, and 8.56 f 0.18. For comparison, hydrogen abstraction by tert-butoxyl at cyclopentane proceeds with E , (kcal/mol) = 3.47 f 0.59 and log (A/(M-' s-l)) = 8.47 f 0.45. The reactions of the fert-butoxyl radical with hydrocarbon substrates in solution have been the subject of extensive invest i g a t i~n .~-~~ With alkanes the reaction is a simple hydrogen

Reactivities of ω‐Unsaturated Methacrylate Oligomers toward tert‐Butoxy Radicals: Investigation of the Effect of Degree of Polymerization and Ester Alkyl Group

“…Arrhenius parameters were estimated based on all of the values for n ¼ 3-5 (the values of k add /k b for MMA-3 to MMA-5 were similar (Figure 2)) using the Arrhenius parameters for b-fragmentation of t-BuO Á ; E b ¼ 44.7 kJ Á mol À1 and A b ¼ 1.95 Â 10 12 s À1 . [33,34] The resulting approximate values of E add % 31 kJ Á mol À1 and A add % 5.4 Â 10 9 L Á mol À1 Á s À1 fall well within the range of values previously reported for similar addition reactions of Me Á to various alkenes. [36] The value of E add is also of a reasonable magnitude in comparison with the activation energy for propagation of MMEA (E add ¼ 33.6 kJ Á mol À1 ), [2] which is a SHAP-type monomer.…”

“…The absolute values of k H are of the order 10 4 -10 5 at 60 8C based on the value of k b at 60 8C. [33,34] The polymerization of ethyl methacrylate (EMA) in the presence of MMA-n is currently being investigated in our laboratory, and a lower molar fraction of MMA-n in the polymer has been revealed for n ¼ 2 than n ¼ 3 (the majority of MMA-n units were introduced by AFCT). This is consistent with a higher contribution of depropagation of the adduct radical in the case of MMA-2.…”

“…The values of E H and A H were also estimated from all data points for MMA-n, giving E H % 21 kJ Á mol À1 and A H % 2.2 Â 10 8 L Á mol À1 Á s À1 (Figure 3), similar to the reported values for hydrogen abstraction from hydrocarbons. [34] However, those reported values were used for the calculation of k b and are therefore not independent of our estimation of E H and A H .…”

Oxidation of HMG‐CoA Reductase Inhibitors by tert‐Butoxyl and 1,1‐Diphenyl‐2‐picrylhydrazyl Radicals: Model Reactions for Predicting Oxidatively Sensitive Compounds During Preformulation

“…The compounds studied here have allylic hydrogens, which are reactive towards tert-butoxyl radical. 12,13 The rate constant for hydrogen atom abstraction by tert-butoxyl radical from cyclohexene is 5.8 × 10 6 s −1 M −1 . 11,13 Scheme 2 describes the Scheme 1.…”

“…13,17 In our studies, we are measuring the rate of hydrogen atom abstraction and not the rate for the reaction of tertbutoxyl radical with the substrate as one does when laser flash techniques are used. 12 It is worth noting that the absolute rate constant of tert-butoxyl radical with 4-hydroxyanisole (1.8 × 10 9 M −1 s −1 ) 4 is about two orders of magnitude greater than those obtained from these HMG-CoA reductase inhibitors, suggesting that substituted phenols would work well as antioxidants in formulations containing these drugs.…”