Oxidations of organic compounds by cobalt salts. Part XI. Kinetic and product studies of oxidations of some ω-phenylalkanecarboxylic acids

Abstract

The oxidation by cobaltic perchlorate of the acids Ph[CH₂]_n·CO₂H (n= 2–5) have been studied in aqueous methyl cyanide. Both kinetic measurements and product analyses substantiate the mechanism of Part VI viz. RCO₂H +{Co(H₂O)₆}³⁺⇌{RCO·O·Co(H₂O)₅}²⁺+ H₃O⁺(1), RCO·O·Co^III(H₂O)₅→ R·+ CO₂+ Co^II(H₂O)₅(2) The organic radicals from equation (2) can be trapped by the addition of bromoform and estimated as the corresponding alkyl bromides. Further oxidation of these radicals leads to stepwise degradation to lower alcohols and aldehydes; the latter are oxidised sufficiently slowly for their isolation to be possible.

Benzaldehyde is always a major oxidation product and cannot, in the main, result from this stepwise degradation, but must arise from attack on the benzylic CH₂ groups of the acids and subsequent C–C fission. Reactions such as those shown in equation (3) play a significant part in this oxidation since hydrocarbons Ph[CH₂]_mMe can be isolated from the higher acids. Ph[CH₂]_n·+ Ph[CH₂]_n·CO₂H → PhĊH[CH₂]_n–1·CO₂H + Ph[CH₂]_n–1Me (3)