Some one electron transfer reactions of organic compounds

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:
p-NO2C6H4Me<C6H12<PhCH2CH2Ph~PhMe~PhEt<Ph2CH2<p-ButC6H4Me<Ph2<naphthalene<phenanthrene<anthracene.
This order can be correlated with both the ionisation potentials of the compounds, and their relative rates of attack by several free radicals. No firm conclusion can be reached concerning the point of initial attack on the aralkyl hydrocarbons, but it is suggested that both the aromatic ring and the side chain can be involved. The slow oxidation of cyclohexane demonstrates that cobaltic ions can oxidise a paraffin chain. The reactivity order shows little steric influence, which supports an outer sphere mechanism for the oxidations. It contrasts with the reactivity order of related alcohols, which are thought to be oxidised by an inner sphere mechanism, and which do show marked steric retardation. Polycyclic hydrocarbons such as biphenyl and naphthalene, which possess no side chain, are oxidised rapidly. It is suggested that in these cases there is direct electron transfer from a π-orbital.
The side chain oxidation products from toluene, (benzyl alcohol, benzaldehyde, and benzoie acid), were accurately estimated by a gas chromatographic method developed during the course of the work. In 50% methyl cyanide solution, these products account for all the oxidant consumed. At higher co-solvant concentrations there is less side chain oxidation, and yellow products are formed. It is suggested that there is oxidation of the aromatic ring under these conditions, stationary state calculations for the side chain oxidation show that the measured yields of benzyl alcohol are much greater than the theoretical maxima. This suggests the occurrence of some additional reaction, possibly:
(ii.)
PhCO + PhCH3 &longrightarrow; 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 &longrightarrow; R&bullet; + CO2 + CoII + H+
(iv.)
R&bullet; + PhMe &longrightarrow; PhCH&bullet;2 + 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:
PhCH2OMe<PhCH2OCH2Ph<PhCH2OC6H11<PhOph
are not governed by steric effects. Oxidation occurs by direct attack on the ether rather than by its prior hydrolysis, and an outer sphere mechanism similar to that for the aralkyl hydrocarbons is suggested. Quantitive measurements, by gas chromatography, of the oxidation products indicate a reaction scheme such as:
(v.)
PhCH2OR + CoIII &longrightarrow; Ph&Cdot;HOR + COII + H+
(vi.)
Ph&Cdot; &longrightarrow; PhCHO + R&bullet;
(vii.)
Ph&Cdot;HOR + 3CoIII +H2O &longrightarrow; PhCOOR + 3COII + 3H+
(viii.)
R&bullet; + CoIII.OH2 &longrightarrow; ROH + COII + H+
The formation of an ester can be correlated with the ability of the R group to be eliminated as R&bullet;. 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:
(v.)
Me2&Cdot;OPri &longrightarrow; Me2CO + Pri&bullet;
(vi.)
Pri&bullet; + Me2CHOPri &longrightarrow; Me2&Cdot;OPri + C3
H8
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&bullet;, or else to elimination of Cl&bullet; from the primary radical. The Cl&bullet; 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.