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

合成化学的最新进展使各种有机化合物的定制制备成为可能。然而，将官能团直接引入碳骨架仍是一个有待解决的问题。本研究探讨了低温等离子体在碳氢化合物中引入羟基或羰基等含氧基团的应用。等离子体是通过流经PYREX反应堆管的氧气的射频放电产生的。在减压下，有机底物与氧气一起通过反应器被蒸馏到液氮上的冷凝器中，一些芳香烃在气态氧等离子体中被有效地氧化。苯和萘部分氧化生成苯酚和萘酚，分别作为单一的挥发性有机产物。当烷基苯发生反应时，烷基的氧化与环羟化反应竞争。因此，甲苯生成苯甲醛和苯甲醇以及苯酚和三种同分异构体甲酚。随着烷基大小的增加，烷基氧化变得更有利。基于氧化有利地发生在α-碳上的结果，通过电子碰撞在原料分子上形成了包括苯基自由基自氧化在内的反应方案。从具有烯烃双键的底物，例如苯乙烯，以相当好的产率获得了环氧化物或其衍生物，主要是酮。在那里，原子氧似乎扮演了重要的角色。饱和碳氢化合物的氧化也被研究了。环烷烃(C_5-C_8)得到几乎等摩尔量的环烷醇和环烷酮，但它们的产率并不令人满意。

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.