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Kinetics of the gas phase reaction of methyl benzoate with bromine and iodine: The methoxyl CH bond strength of methyl esters
Author(s) -
Solly Richard K.,
Benson Sidney W.
Publication year - 1971
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.550030605
Subject(s) - chemistry , bromine , reaction rate constant , halogen , mole , torr , decomposition , medicinal chemistry , kinetics , organic chemistry , alkyl , physics , quantum mechanics , thermodynamics
The gas phase reactions of PhCOOCH 3 with I 2 and Br 2 were studied spectrophotometrically in a static system over the temperature ranges 344–359° and 246–303°, respectively. For each system the initial rate was first order in PhCOOCH 3 and half order in halogen as the concentration of PhCOOCH 3 was varied from 1.4 to 15.2 torr, that of I 2 from 6.2 to 26.4 torr, and that of Br 2 from 3.0 to 13.6 torr. The rate‐determining step is the extraction of a methoxyl hydrogen atom:Empirical assignment of A ‐factors for k 1 lead to\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm log(k}_{\rm 1} {\rm /M}^{- {\rm 1}} {\rm sec}^{- {\rm 1}} {\rm) = (11}{\rm .5} \pm {\rm 0}{\rm .3)} - (29.9 \pm 0.8)/\theta $$\end{document}for the I 2 system, and to\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm log(k}_{\rm 1} {\rm /M}^{- {\rm 1}} {\rm sec}^{- {\rm 1}} {\rm)} = {\rm (11}{\rm .3} \pm {\rm 0}{\rm .3)} - (14.6 \pm 0.8)/\theta $$\end{document} for the Br 2 system, where ϕ = 2.303 RT in kcal/mole. Combined with the assumption that E –1 = 1 ± 1 kcal/mole and 2 ± 1 kcal/mole for HI and HBr, respectively, DH 298 0(PhCOOCH 2 H) calculated from the two systems shows excellent agreement at 100.2 ± 1.3 kcal/mole and 100.1 ± 1.3 kcal/mole. Using a value of δ H f,298 0(PhCOOMe) = –65.6 ± 1.5 kcal/mole obtained from group additivity estimates, δ H f,298 0 (PhCOOCH 2 ) is calculated to be –16.7 ± 2.0 kcal/mole. Unimolecular decomposition of the Ph(CO)O°CH 2 radical was also observed:\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm Ph}\left( {{\rm CO}} \right){\rm O\dot CH}_{\rm 2} \mathop {\hbox to 20pt{\rightarrowfill}}\limits^3 {\rm Ph\dot CO} + {\rm H}_{\rm 2} {\rm CO} $$\end{document}with a rate constant equal to\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm log(}k_{\rm 3} {\rm /sec}^{- {\rm 1}} {\rm)} = {\rm (15} \pm {\rm 0}{\rm .5)} - (26 \pm 3)/\theta $$\end{document}The abnormally high methoxyl CH bond strength is discussed in relation to the bonding in ethers, alkanes, and esters.