Kinetics of the gas phase reaction of methyl benzoate with bromine and iodine: The methoxyl CH bond strength of methyl esters

Abstract

AbstractThe gas phase reactions of PhCOOCH3 with I2 and Br2 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 PhCOOCH3 and half order in halogen as the concentration of PhCOOCH3 was varied from 1.4 to 15.2 torr, that of I2 from 6.2 to 26.4 torr, and that of Br2 from 3.0 to 13.6 torr. The rate‐determining step is the extraction of a methoxyl hydrogen atom: magnified image Empirical assignment of A‐factors for k1 lead to for the I2 system, and to for the Br2 system, where ϕ = 2.303RT 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 (PhCOOCH2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 δH (PhCOOMe) = –65.6 ± 1.5 kcal/mole obtained from group additivity estimates, δHf,2980 (PhCOOCH2) is calculated to be –16.7 ± 2.0 kcal/mole. Unimolecular decomposition of the Ph(CO)O°CH2 radical was also observed: with a rate constant equal to The abnormally high methoxyl CH bond strength is discussed in relation to the bonding in ethers, alkanes, and esters.