A study of the reduction of organic halogen compounds by chromium(II) perchlorate.

Abstract

Phenacyl chloride was found to react with chromium(II) perchlorate, in the presence of methanol, to giver both acetophenone and o-methoxyacetophenone. In the presence of a large excess of methanol, the yield of o-methoxyacetophenone increased, and that of acetophenone decreased. If, on the other hand, the acid concentration of the methanolic solution was high, acetophenone was formed in over 80% yield, with trace amounts of o-methoxyacetophenone. In acetone, ethanol and t-butanol, reduction of phenacyl chloride gave mainly acetophenone. Chloroacetone was also found to yield o-methoxyacetophenone, when allowed to react with chromous ion in a large excess of methanol. The catalyzed solvolysis of alpha-haloketones, by chromium(II) perchlorate in methanolic solution, appears to have no known precedent. The reduction of methylene iodide with chromous ion did not take place in aqueous solution, but when methanol was used as solvent, iodomethylpentaaquochromium(III) perchlorate was formed. In the reduction of methylene bromide, bromomethylpentaaquochromium(III) perchlorate was obtained. Reduction of bromoform in methanol gave dibromomethylpentaaquochromium(III) perchlorate. It was found that when the halomethane contained both bromine and chlorine, attack took place preferentially at the bromine atom. In the reduction of bromodichloromethane, dichloromethylpentaaquochromium(III) perchlorate was formed, and the dibromochloromethane yielded bromochloromethylpentaaquochromium(III) perchlorate. The reduction of chloroform in 50% t-butanol-water was found to follow second order kinetics. Styrene dibromide on reduction was found to yield styrene as the sole organic product and Cr(H2O)5Br++ as the only inorganic product. The reduction of meso- and racemic 2,3-dibromobutane was found to be non-stereospecific, mixtures of cis and trans 2-butene being obtained when either isomer was reduced. The non-stereospecificity of this reduction suggests the possible presence of a radical intermediate, most probably a bromine atom, since such radicals are known to cause isomerization. This postulate is supported by the fact that cis-2-butene undergoes isomerization when present during the reduction of 1,2-dibromocyclohexane by chromous ion. The reduction of meso-2,3-dibromobutane in 50% t-butane-water and 75% t-butanol-water, and of racemic 2,3-dibromobutane in 50% t-butanol-water was found to follow second order kinetics. The reduction of 1,2-dibromo-2-methylpropane in 50% t-butanol-water was also found to be second order.