Mechanism of the photochemical reduction of CCl bond in fluorine compounds

Abstract

The photochemically initiated substitution of a halogen for hydrogen atom represents a preparative reaction, that can be expressed by the general scheme: ▪ The reaction is characterized by a high selectivity in polyhalogeno compounds as demonstrate examples of successfully performed preparations: CF 3CBr 2Cl → CF 3CHBrCl, CCl 2FCClF 2 → CHClFCClF 2, CF 3CClFCCl 3 → CF 3CClFCHCl 2. A marked directive effect on the regioselectivity of the reduction is exhibited by the ester group in fluorinated propanoates: CFXYCClZCOOR → CFXYCHZCOOR (X,Y = F,Cl). The quantum yields of this reduction in 2-propanol are as high as several hundreds of units, which is indicative of the chain mechanism. The photo-reduction is auto-sensitized by the resulting acetone. The kinetic model of the reduction indicates, under a definite ratio of the terminal reaction rates, a rate limit independent of the concentration of the starting substance. The kinetic model of the reduction initiated by α-phenyl benzoin and benzoin isopropyl ether was verified also using the Stern-Volmer dependence of a total quantum yield of HCl.