Kinetics and mechanism of the decomposition of thionitrites derived from thioureas

Abstract

The decomposition of [(NMe₂)₂CSNO]⁺ to [(NMe₂)₂CSSC(NMe₂)₂]²⁺+ 2NO˙ has been investigated spectrophotometrically using the method of initial rates. The thionitrite was generated by mixing deoxygenated solutions of tetramethylthiourea and nitrous acid in a stopped-flow apparatus. For reaction with a large excess of tetramethylthiourea over nitrous acid the observed rate law (below) was similar to that reported for the analogous thiourea system. In solutions initially saturated with NO˙ν₀=k₁[(NMe₂)₂CSNO⁺][(NMe₂)₂CS]+k₂[(NMe₂)₂CSNO⁺]² the first term is largely suppressed. For systems in which [(NMe₂)₂CSNO]⁺ or [(NH₂)₂CSNO]⁺ was generated quantitatively from (NMe₂)₂CS or (NH₂)₂CS by reaction with a large excess of HNO₂ the first term is again suppressed, but a new term in the rate law involving catalysis by nitrous acid occurs. The mechanisms of the decomposition reactions are discussed.