Reduction of Substituted Benzenediazonium Salts by Hydrogen Atoms in Aqueous Acidic Solution Studied by Pulse Radiolysis

Abstract

A pulse radiolysis study of six parasubstituted benzenediazonium salts, X−C6H4N2+ [X = COOC2H5, F, H, CH3, OCH3, and N(CH3)2], has been carried out in strongly acidic aqueous solution (1 M HClO4), where the principal reductant is the hydrogen atom. Initially, H adducts of the diazonium salts are formed with rate constants of (2.3−9.0) × 108 M-1 s-1, followed by their decay to protonated aryl radicals without the intermediacy of the diazenyl radicals that are observed when the reductant is the solvated electron. After a deprotonation reaction, the thus formed aryl radicals attack the diazonium salt mainly at the terminal nitrogen atom to afford the radical cations of the corresponding azobenzenes, (X−C6H4)2N2•+, or eventually the corresponding OH adducts, (X−C6H4)2N2OH•, upon further reaction with water. For the dimethylamino substituent, the basicity is so high that the protonated radical cation of 4,4‘-bis(dimethylamino)azobenzene is formed. Studies carried out at different pH values for this system lead...