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Title: Some one electron transfer reactions of organic compounds
Author: Cooper, T. A.
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 1966
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The oxidation of a series of aromatic hydrocarbons and ethers by cobaltic perchlorate has been investigated. This was undertaken a. to extend the range of known one-electron oxidations of organic compounds to include much less reactive molecules than those previously investigated; and b. to investigate the possibility of oxidation by direct electron transfer, rather than by the homolytic fission of a C---H bond. The oxidations studied were (generally much slower than any previously investigated cobaltic oxidations, so that the self-decomposition of the cobaltic ions in solution was often of comparable rate. The latter reaction shows no simple order with respect to cobaltic ions, and is not accurately reproducible. The oxidations of a series of arossatic hydrocarbons approximately follow the rate law: (i.)

- d[CoIII]/dt = k2[CoIII][ArH]/[H+] In many cases the kinetic form is complicated by variations in the extent of secondary oxidation. However, there is no exact comparison between the deviations from a first order dependence on substrate and oxidant, and the variation of stoichiometry with substrate concentration. The approximate reactivity order is:


PhCO + PhCH3 ⟶ PhCHO + PhCH2 The production of PhCO radicals is supported by the detection of methyl benzophenones in the oxidation products. A similar conclusion is reached for the oxidation of dibenzyl ether. C---C fission is a major process in the oxidation of bibenzyl, and occurs to lesser extents in the oxidations of diphenylmethane and ethyl benzene. Thus the oxidation of bibenxyl yields benzaldehyde, and not benzyl phenyl ketone. The oxidation of p-nitrotoluene is exceptional since it appears to be inhibited by the substrate. Explanations for this involving the conplexing of cobaltic ions by the nitro- group, or the nitro- group acting as a trap for intermediate radicals, are tentatively suggested. The results obtained from the direct oxidation of toluene are used to analyse mathematically detailed kinetic and product studies of the oxidation of systems containing toluene and a cerboxylic acid. The oxidation of toluene is promoted by several aliphatic acids, and this is ascribed to the reactions: (iii.)

RCOOH + CoIII ⟶ R + CO2 + CoII + H+ (iv.)

R + PhMe ⟶ PhCH2 + RH The significance of such processes with regard to the catalysis of autoxidation reactions by carboxylate salts of transition metals is discussed. The oxidations of a series of benzyl ethers are faster than the oxidations of similar hydrocarbons, but much slower than the oxidation of benzyl alcohol. The overall reactivities, which are in the sequence:


PhCH2OR + CoIII ⟶ PhĊHOR + COII + H+ (vi.)

PhĊ ⟶ PhCHO + R


PhĊHOR + 3CoIII +H2O ⟶ PhCOOR + 3COII + 3H+ (viii.)

R + CoIII.OH2 ⟶ ROH + COII + H+

The formation of an ester can be correlated with the ability of the R group to be eliminated as R. The detailed kinetic behaviour is similar to that found for the oxidation of the aralkyl hydrocarbons.,/p>

The oxidation of di-isopropyl ether is slower than the oxidations of the bensyl ethers. Larger yields of acetone are obtained than can be explained by the stoichiometry of a reaction scheme analogous to that shown in equations v. to viii. This indicates that there is a short regenerative chain:


Me2ĊOPri ⟶ Me2CO + Pri• (vi.)

Pri• + Me2CHOPri ⟶ Me2ĊOPri + C3H8

which ran produce acetone without consumption of cobaltic ions. The oxidation of 2,2'- dichlorodiethyl ether is autocatalytic, and liberates chloride ions. The autocatalysis is ascribed to the rearrangement of an intermediate radical to yield chloride ions which are then oxidised to Cl, or else to elimination of Cl from the primary radical. The Cl radicals can then attack the substrate ether. The substrates which have been investigated are all rather insoluble, and so solutions containing high concentrations of methyl cyanide co-solvent were necessary. The increase in the oxidation rates of the hydrocarbons and ethers with increasing methyl cyanide concentration is far too large to fit the expression: (xi.)

k = ko eMX (where X is % methyl cyanide v/v) which has been previously found satisfactory for solutions of up to 50% methyl cyanide. Spectral changes indicate that a cobaltic-methyl cyanide complex, which is an active oxidant, is formed. Its enhanced reactivity is due to a decrease in the energy of activation for the oxidations, which outweighs the effect of a less favourable activation entropy. In general, the thesis shows that reaction mechanisms cannot be elucidated solely from a knowledge of reaction kinetics. Accurate measurements of reaction products are also necessary before any interpretation of mechanism can be attempted. Thus, similar kinetic behaviour is found a. when the same substrate is oxidised to give different products under different conditions; and b. when similar substrates are oxidised by quite different routes.

Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available