Kinetic study of the intermediate in the reaction between picryl chloride and sodium sulphite in aqueous solution

The reaction between picryl chloride and sulphite ion in aqueous solution takes place rapidly to give an intense color which is attributed to a 1, 3-σ-complex formed by the reversible attack of a sulphite ion on the C_3-position of picryl chloride. The color slowly fades because picryl chloride present at equilibrium is attacked by sulphite ion to give 1, 1-σ-complex which rapidly loses chloride ion to form picryl sulphonate ion finally. The 1, 3-σ-complex dissociates to maintain equilibrium. The rates of appearance and disappearance of the transient colored species were measured in the range of 15-35℃ by the stopped-flow method. The rate constants at 25℃, activation enthalpies and activation entropies an each process are shown in Table 2 in the text. The 1, 3-σ-complex was verified by NMR spectrum in DMSO-H_2O solution. The formation of the 1, 1-σ-complex was inferred from the nature of the final products. It may be concluded that the sulphite attacks on C_1-position and C_3-position take place concurrently and the former may be accompanied with higher energy barrier