Direct Introduction of Functional Groups into Hydrocarbons by Using Low-Temperature Plasma

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Recent advance in synthetic chemistry has enabled the tailormade preparation of a diversity of organic compounds. However, direct introduction of functional groups into the carbon skeletons still remains a problem to be solved. In this study, the application of low-temperature plasma was investigated to introduce oxygen-containing groups such as hydroxyl or carbonyl group into hydrocarbons. The plasma was generated by means of radiofrequency discharge of oxygen gas flowing through a Pyrex reactor tube. Organic substrates were distilled together with oxygen through the reactor into cold traps on liquid nitrogen under reduced pressure.Some aromatic hydrocarbons were effectively oxidized in a gaseous oxygen plasma. Benzene and naphthalene were partially oxidized to give phenol and naphthol as a single volatile organic product, respectively. When alkylbenzene were reacted, the oxidation of alkyl groups competed with ring hydroxylation. Thus, toluene gave benzaldehyde and benzyl alcohol as well as phenol and three isomeric cresols. With increase in size of alkyl groups, alkyl oxidation became preferential. Based on the result that the oxidation favorably took place on the alpha-carbons, the reaction scheme involving autoxidation of benzyl radicals formed through electron collision onto starting material molecules. From the substrate having olefinic double bond, e. g. styrene, the epoxide or its derivatives, mainly ketones, were obtained in fairly good yields. There, seemingly, atomic oxygen played a significant role.The oxidation of saturated hydrocarbons was also examined. Cycloalkanes (C_5-C_8) gave the corresponding cycloalkanol and cycloalkanone in almost equimolar amounts, although their yields were unsatisfactory.