Hydrocarbon oxidative side-chain substitution

According to this hypothesis, the results are modified from what would be expected from classical radical reactions. The interest in this hypothesis is that, with the sole exception of saturated hydrocarbons, it could apply to all the compounds that can be coordinated at the Tiiv center, such as alkenes, aromatics, alcohols, and sulfides. According to this hypothesis, the weak Lewis acidity of Tilv would help to bring the reactant into its coordination sphere. The initial coordination of the reactant would explain the oxidation of methyl-substituted aromatics in the aromatic ring and not in the side chain, even with a radical-type mechanism. 

In the case of halogen-substituted hydrocarbons the principal method used is the mercuration of the sulphinic acid, and not the hydrocarbon itself, whilst the method acts equally well with nitrobenzene. The nitrotoluenes when mercurated by mercuric oxide in the presence of sodium hydroxide yield compounds containing mercury in the side chain, but with mercuric acetate at 140° C., in the absence of a solvent, the mercury enters the ring. If nitrotoluene sulphinic acids are used as starting-points, the acid group is replaced b mercury by prolonged boiling with 50 per cent, aqueous alcoholic mercuric chloride. 

Gasoline hydrocarbons volatilized to the atmosphere quickly undergo photochemical oxidation. The hydrocarbons are oxidized by reaction with molecular oxygen (which attacks the ring structure of aromatics), ozone (which reacts rapidly with alkenes but slowly with aromatics), and hydroxyl and nitrate radicals (which initiate side-chain oxidation reactions) (Stephens 1973). Alkanes, isoalkanes, and cycloalkanes have half-lives on the order of 1-10 days, whereas alkenes, cycloalkenes, and substituted benzenes have half- lives of less than 1 day (EPA 1979a). Photochemical oxidation products include aldehydes, hydroxy compounds, nitro compounds, and peroxyacyl nitrates (Cupitt 1980 EPA 1979a Stephens 1973). 

Chang-Chien, G., Terminally carboxyl oligo(ethylene oxide) monomethyl ether-substituted side chain liquid crystalline polysiloxane polymer as stationary phase in capillary gas chromatography for the separation of polynuclear aromatic hydrocarbons, J. Chromatogr. A, 808, 201-209, 1998. 

Effect of Oxidants, Kharasch has shown that, in the presence of benzoyl or other organic peroxides, sulfuryl chloride selectively chlorinates the side chain and not the nucleus of alkyl-substituted benzene. The reaction, which is carried out in the dark, is also useful for the chlorination of aliphatic compounds such as acyclic hydrocarbons, cycloparaflSns, carboxylic acids, and their acid halides. ... 

Toluene. The oxidation of hydrocarbons having an aromatic nucleus and one or more side chains may be effected in the side chain without marked rupture of the ring itself, since each component behaves more or less as it would if it alone constituted the major part of the molecule. Thus, the ring component exhibits the characteristic stability of the aromatic compounds, and the aliphatic substituent shows the relatively greater ease of oxidation of the aliphatic hydrocarbons. Under specific conditions, oxidation of such substituted aromatic hydrocarbons may be controlled to give satisfactory yields of the side-chain products viz., toluene may be oxidized to benzaldehyde or benzoic acid o-xylene, to phthalic anhydride ethylbenzene, to benzoic acid etc. 

In solution of perchlorates and tosylates in acetic acid, the primary products of electro-oxidation of aromatic hydrocarbons are shown to be benzylic acetates, i.e., substitution occurs preferentially in the side chain. A large, primary isotope effect, ku/kjy = 2.6, provided strong evidence that in this type of substitution reaction the rate-determining step involved the loss of a proton.Further refinement of the mechanism correlated isomeric product ratios to the distribution of positive charge density in the intermediate cation radical. 

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