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Linear free energy relationships for peroxy radical‐phenol reactions. Influence of the para ‐substituent, the ortho‐di‐tert ‐butyl groups and the peroxy radical
Author(s) -
Héberger Károly,
Lopata Antal,
Müller József
Publication year - 1989
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550211211
Subject(s) - chemistry , substituent , radical , hydrogen atom abstraction , electrophile , phenol , reaction rate constant , medicinal chemistry , phenols , hammett equation , photochemistry , solvent , solvent effects , computational chemistry , organic chemistry , kinetics , catalysis , physics , quantum mechanics
Calculations were carried out on several data sets to study the mechanism of hydrogen abstraction from phenols by peroxy radicals:Rate constants, k values, were collected for the reactions of cumyl‐, 1‐phenylethyl‐ and tert ‐butyl‐peroxy radicals with ortho ‐ para ‐substituted phenol inhibitors. The rate constants were recalculated for the same temperature. Solvent effects were neglected because the solvents used were similar in nature. The phenol ortho substituents were characterized by an indicator variable I tBu accounting for the presence or absence of di ‐ tert ‐butyl groups. The phenol para substituents were characterized by Charton's σ I , σ R , and σ R +substituent constants. The dependence of log k values on I tbu , σ I , σ R , σ R +was investigated using stepwise linear regression analysis. The combined data set of 32 reactions gives:\documentclass{article}\pagestyle{empty}\begin{document}$$\log \,\,k_{(333{\rm K)}} = - 0.801\sigma _I - 2.483\sigma _{\rm R} + 3.766\,\,\,\,\,\,\,\,\,(r = 0.851)$$\end{document} and\documentclass{article}\pagestyle{empty}\begin{document}$$\log \,\,k_{(333{\rm K)}} = - 0.932\sigma _1 - 2.302\sigma _R^ + + 3.802\,\,\,\,\,\,\,\,\,(r = 0.848)$$\end{document}The results suggest that hydrogen abstraction from phenols by peroxy radicals proceeds by an electrophilic mechanism, and that neither the peroxy‐radical nor the ortho ‐ di ‐ tert ‐butyl groups have considerable effect on the rate of reaction (1).