International Journal of Chemical Kinetics p. 599 - 604 (2005)
Update date:2022-08-17
Topics:
Espenson, James H.
Yiu, Douglas Tak-Yeung
The title reaction gives rise to a benzylic cation that is rapidly transformed to its bromide in competition with the reaction of the radical with carbon tetrachloride. Experiments were carried out over 17-69°C in acetic acid containing cobalt(II) acetate, para-xylene, hydrobromic acid, carbon tetrachloride, and meta-chloroperoxybenzoic acid. The product ratio, ArCH 2Cl/ArCH2Br, in combination with other pertinent rate constants, was used to determine the rate constant for the step of interest: log script k sign (L mol-1 s-1) = 19.9 - (75 ± 11 kJ mol-1/2.303 RT). The large pre-exponential factor, which gives ΔS? = 128 J K-1 mol-1, signals an unusual transition state, because a negative value of ΔS? would be expected for a simple bimolecular reaction. The production of the ion pair ArCH 2+||OAc- in HOAc, which has the same dielectric constant as benzene, may be responsible, at least in part. Furthermore, inner sphere reorganization of cobalt may also contribute.
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