Methylene. IV. Gas phase reactions with 2-chloropropane

Abstract

As in earlier studies in this series, the reaction mechanism has been investigated by identi­fication and estimation of the products formed by the interaction of methylene, prepared by the photolysis of ketene, and the chloroalkane. The reaction was examined over a range of initial pressures, with different wavelengths of photolysing light, and in the presence of oxygen and carbon monoxide. Both insertion and abstraction processes are important, but insertion into C—Cl bonds is negligible under our conditions. Singlet methylene, which is responsible for insertion, is again found to be highly selective in its abstraction reactions, the ratio of the relative rates of abstraction of chlorine and hydrogen exceeding 12. The results are consistent with the mechanism suggested earlier (part II), according to which singlet methylene behaves as an electrophilic reagent, and forms a bond with a chlorine atom involving the vacant p-orbital of CH 2 and a filled p-orbital of Cl. Stereo­chemical considerations suggest that the transition state of this reaction is such that no bond­ing between the C atom of CH 2 and that of C—Cl, and hence no insertion into C—Cl, can occur. We believe that insertion into a C—H bond involves interaction of singlet CH 2 with the electrons of the bond, with a triangular transition state. The effect of a chlorine atom in generally reducing the probability of insertion into neighbouring C—H bonds is thought to be the result of the rapid competing reaction between singlet methylene and the halogen atom. A five-centre transition state in which singlet CH 2 is bonded to Cl through its vacant p-orbital and to a hydrogen atom on C 2 through its filled sp 2 -orbital may be partly respon­sible for chloromethane formation. Triplet methylene has been shown to resemble alkyl radicals in its abstraction reactions.