Kinetic and Thermodynamic Studies of the Reactions of Sulfite Ion with Picramide, N-Methylpicramide, and N,N-Dimethylpicramide in Aqueous Solution

Abstract

The kinetics of the reactions between sulfite ion and picramide, N-methylpicramide, and N,N-dimethylpicramide, to form 1:1 σ-complexes in aqueous solutions of constant ionic strength 0.14 M, have been investigated using a stopped-flow spectrophotometer. Specific rate constants for the formation and decomposition of these three complexes at 25.0 °C are 5.7 ± 0.2 × 104M−1 s−1 and 7 ± 1 s−1, 1.4 ± 0.1 × 104M−1 s−1 and 0.20 ± 0.02 s−1, and 4.1 ± 0.2 × 103M−1 s−1 and 0.14 ± 0.04 s−1, respectively. Enthalpies and entropies of activation characterizing the formation of the 1:1 σ-complexes have been determined. Equilibrium constants, determined spectrophotometrically at a number of temperatures, have been used to calculate the enthalpy (ΔH0) and entropy (ΔS0) changes associated with 1:1 and 2:1 σ-complex formation. These values are compared with corresponding ones for complex formation of sulfite ion with 1,3,5-trinitrobenzene and 2,4,6-trinitrobenzaldehyde. The extent of solvation of the σ-complexes is considered to play a primary role in determining the observed trends in the enthalpies and entropies of reaction.