Kinetics and activation energy of the oxidation of para-tolyl radical by cobalt(III) in acetic acid: Competition kinetics

✍ Scribed by James H. Espenson; Douglas Tak-Yeung Yiu

• Publisher: John Wiley and Sons
• Year: 2005
• Tongue: English
• Weight: 100 KB
• Volume: 37
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.20116

✦ Synopsis

Abstract

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~2~Cl/ArCH~2~Br, in combination with other pertinent rate constants, was used to determine the rate constant for the step of interest: log k (L mol^−1^ s^−1^ = 19.9 – (75 ± 11 kJ mol^−1^/2.303 RT). The large pre‐exponential factor, which gives Δ__S__‡ = 128 J K^−1^ mol^−1^, signals an unusual transition state, because a negative value of Δ__S__‡ would be expected for a simple bimolecular reaction. The production of the ion pair ArCH‖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. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 599–604, 2005