화학공학소재연구정보센터
Journal of Physical Chemistry A, Vol.106, No.46, 11098-11106, 2002
Reduction of substituted benzenediazonium salts by solvated electrons in aqueous neutral solution studied by pulse radiolysis
A pulse radiolysis study of six parasubstituted benzenediazonium salts, X-C6H4N2+ [X = COOC2H5, F, H, CH3, OCK3, and N(CH3)(2)], has been carried out in neutral aqueous solutions, where the principal reductant is the solvated electron. In the first step, the solvated electron reduces the diazonium salts to the corresponding diazenyl radicals, X-C6H4N2., in a diffusion-controlled process. On a short time scale, X-C6H4N2. is either involved in an equilibrium reaction with the parent diazonium. salt (K > 100 M-1) in which a (X-C6H4)(2)N-4(.+) adduct is formed or decays to the aryl radical, X-C6H4., through the expulsion of dinitrogen. Simulation of this kinetic scheme allows us to make a rough determination of the rate constants involved. The rate constants for the formation of the adducts are in the range (0.7-1.9) x 10(7) M-1 s(-1), whereas the fragmentation rate constants are in the range (0.4-4.0) x 10(5) s(-1). The aryl radicals subsequently attack the diazonium. salts mainly at the terminal nitrogen atom with rate constants of (0.13-5.9) x 107 M-1 s-1, thereby forming the radical cations of the corresponding azobenzenes, (X-C6H4)(2)N-2(.+), or eventually the corresponding OR adducts, (X-C6H4)(2)N2OH., upon further reaction with water. These intermediates were identified in pulse radiolysis by generating the very same species through an alternative pathway involving oxidation of the parent azobenzenes. The reaction between the aryl radical and the salt exhibits a clear substituent effect in the sense that the reactivity increases as the electron-donating power of the substituent is enhanced. This is attributed to the stabilizing effect exerted by electron-donating groups on radical cations. Thus, the radical cations can be detected only for the methoxy and dimethylamino groups, whereas for the other substituents the transformation of (X-C6H4)(2)N-2(.+) to (X-C6H4)(2)N2OH. takes place instantaneously.