Gareth H. Williams

Radical addition to alkynes: electron spin resonance studies of the formation and reactions of vinyl radicals

The e.s.r. spectra are described of a series of vinyl radicals XRCCR·(R = CO2H, CO2–) formed by the addition of a variety of alkyl, hydroxyalkyl, and aryl radicals (X) to butynedioic acid and the corresponding dianion. For the reaction between Pri˙ and HO2CCCCO2H, k is estimated as ca. 2 × 106 dm3 mol–1 S–1. In acid solution the first formed vinyl radicals react with more alkyne to give further vinyl radicals XRCCR–CRCR· which, if structurally suited, undergo a 1,5-shift to give pentadienyl radicals. The spectra of the latter [e.g. for CH2C(CO2H)–C(CO2H)C(CO2H)–ĊHCO2H from ·CH3 and butynedioic acid] indicate there to be considerable twisting around the C(3)–C(4) bonds so that conjugation is reduced compared to that in the fully conjugated unsubstituted pentadienyl radical.

Aroyl peroxides. Part VI. Reactions of aroyl peroxides with nitrogen dioxide

The products of the decomposition of benzoyl peroxide in carbon tetrachloride in the presence of nitrogen dioxide have been investigated quantitatively, and the mechanism previously suggested has been confirmed. The reaction of benzoyl peroxide with nitrogen dioxide in the absence of solvent has been found to be similar, but less complex. The formation of benzoyl nitrate was indicated by the isolation of a nitration product from reactions in the presence of p-xylene. The decomposition of a series of aroyl peroxides in the presence of nitrogen dioxide gave the corresponding nitro-compounds.

Homolytic reactions of perfluoroaromatic compounds. Part I. Arylation of hexafluorobenzene

The reactions between hexafluorobenzene and a series of diaroyl peroxides have been shown to give 2,3,4,5,6-pentafluorobiphenyl and appropriately substituted derivatives. The significance of the yields of the biaryls obtained and the nature of the other reaction products are discussed, and a mechanism for the reaction is suggested tentatively.

Aroyl peroxides. Part IV. The decomposition of benzoyl peroxide in nitrobenzene. The effect of added nitrobenzene on the decomposition of benzoyl peroxide in benzene

The kinetics and products of the reactions of benzoyl peroxide with nitrobenzene and with benzene containing 1% of nitrobenzene have been studied over a range of initial peroxide concentrations [P]0. The rate equation in each case was dependent on the value of [P]0, and had the two forms. (i)–d[P]/dt=k1[P]+kI[P]+k3/2[P]3/2([P]0 0·06–0·08M). The catalytic effect of nitrobenzene on the rate of disappearance of the peroxide in benzene, which is accompanied by a marked change in the distribution of the products, is discussed in terms of the formation of complexes of the radical intermediates with nitrobenzene, leading to increased concentrations of the complexed radicals, and consequent modification of their reactions.

Electron spin resonance studies of radical addition to alkynes and intramolecular reactions of vinyl radicals

E.s.r. spectra are described of radicals formed by the reactions of some alkynes both with hydroxyl and also with radicals formed from a variety of ethers in the presence of hydroxyl. Vinyl radicals, though not normally detected, are apparently involved, and evidence is presented for their intermolecular addition and intramolecular abstraction reactions. For example, the vinyl radical produced by addition of ·CHMeOEt to HO2CCCCO2H reacts to form the allyl radical ·CHMeC(CO2H)CHCO2H, evidently via a 1,5 carbon-to-carbon hydrogen shift and fragmentation. Other instances of ready 1,5-shifts (including an oxygen-to-carbon example) are described.

Reactions of alkyl radicals. Part III. Relative rates of methylation and hydrogen abstraction by methyl radicals

Relative rates of reaction of several aromatic substrates towards attack by methyl radical formed by photolysis of methylmercuric iodide (‘methyl affinities’, benzene = 1) are reported.

Homolytic reactions of perfluoroaromatic compounds. Part IV. Formation of pentafluorophenyl radicals from pentafluoroaniline, and their reactions with aromatic compounds

The pentafluorophenylation of benzene, hexafluorobenzene, chloro-, bromo- and nitro-benzene, and toluene, and the competitive pentafluorophenylation of an equimolar mixture of benzene and chlorobenzene with pentafluorophenyl radicals prepared by reaction of pentafluoroaniline with pentyl nitrite in the refluxing solvents are described. The reactions with the benzene derivatives gave mixtures of the products of pentafluorophenylation at the 2′-, 3′-, and 4′-positions, and the compositions of these mixtures were measured. The relative rate of pentafluorophenylation of chlorobenzene, and hence partial rate factors for this reaction were obtained. The results establish the electrophilic character of the pentafluorophenyl radical. Phenylation of hexafluorobenzene with radicals obtained analogously from aniline and pentyl nitrite gives 2,3,4,5,6-pentafluorobiphenyl.

Homolytic reactions of perfluoroaromatic compounds. Part II. Reactions of perfluorobenzoyl peroxide with aromatic compounds

Perfluorobenzoyl peroxide decomposes in benzene and in nitrobenzene at 80° to give mainly pentafluorobiaryls and pentafluorobenzoic acid. In hexafluorobenzene the main product is a high-boiling residue, and in chloro- and bromo-benzene pentafluorobenzoic acid and phenyl pentafluorobenzoate are formed. The latter product must arise by benzoyloxylation of the halogenobenzenes at the 1-position. The results are discussed.

Homolytic reactions of perfluoroaromatic compounds. Part III. Reactions of benzoyl peroxide with hexafluorobenzene, chloro-, bromo-, and nitro-pentafluorobenzene, and pentafluoropyridine

The reaction of benzoyl peroxide with hexafluorobenzene has been shown to give 2,3,4,5,6-pentafluorobiphenyl and a residue whose relative yields depend mainly on the conditions used for the distillation of the biaryl from the reaction mixtures. This supports the suggestion made previously that the biaryl is formed partly by defluorination of σ-complexes during reaction, and partly by thermal breakdown of dimers of the σ-complexes during the distillation. Analogous phenylation of chloro-, bromo-, and nitro-pentafluorobenzene and pentafluoropyridine gives mixtures of isomeric products. The ratios in which these isomers are formed with chloro- and bromo-pentafluorobenzene and pentafluoropyridine have been measured and are discussed.

Aroyl peroxides. Part V. Decomposition of benzoyl peroxide in fluorobenzene

The kinetics and products of the reaction of benzoyl peroxide with fluorobenzene have been studied over a range of initial peroxide concentrations, [P]0. The rate of disappearance of peroxide is governed by simultaneous reactions of order 1·0 and 1·5 corresponding to primary and induced decomposition of the peroxide respectively. A mechanism is proposed which is consistent with the kinetics and products of the reaction and which is analogous to the reaction in benzene rather than to that in chlorobenzene or bromobenzene.