Formation and structure of 1,1-dialkoxyalkene radical cations in aqueous solution. An in situ electron spin resonance and pulse conductivity study
Abstract
The radical cations (1), (10), (11a and b), and (12)–(15) have been produced in aqueous solution and have been identified by e.s.r. spectroscopy and conductivity investigation. The open chain radical cations exist in Z,E-configurations. They exhibit two sets of aδH couplings. The larger couplings were assigned to the protons in groups with the Z-configuration. In all cases the radical spin is located mainly at the carbon atoms. Under our conditions radical (13) disappeared pseudo-monomolecularly upon reaction with water (k= 7 × 103 s–1) whereas the other radical cations are longer lived and decayed bimolecularly with diffusion controlled rates. The radical cations were generated from open chain or cyclic acetals bearing Br, Cl, or CH3CO2 groups β to the acetal CH group, e.g.(CH3O)2CH–CH2Cl or (CH2O)2CH–CH2Cl. These substrates were subjected to hydrogen abstraction by OH˙ or SO4˙– radicals or triplet acetone in aqueous solution. Hydrogen abstraction from the acetal CH group led to radicals which undergo fast heterolytic dissociation into radical cations and leaving group anions, e.g.(CH3O)2Ċ–CH2Cl →(CH3O)2C–ĊH2+ Cl–.