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 a-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 a t 25.0 "C are 5.7 + 0.2 x lo4 M-' s-' and 7 f 1 s-', 1.4 t 0.1 x lo4 M-' s-' and 0.20 + 0.02 s-', and 4.1 t 0.2 x lo3 M -' s -' and 0.14 t 0.04 s-', respectively. Enthalpies and entropies of activation characterizing the formation of the 1 : 1 a-complexes have been determined. Equilibrium constants, determined spectrophotometrically at a number of temperatures, have been used to calculate the enthalpy ( A H 0 ) and entropy (AS0) changes associated with 1 : 1 and 2: 1 a-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 o-complexes is considered to play a primary role in determining the observed trends in the enthalpies and entropies of reaction.On a Ctudie a I'aide d'un spectrophotomttre a debit interrompu la cinitique des reactions entre I'ion sulfite et la picramide, la N-methylpicramide et la N,N-dimithylpicramide; ces reactions conduisent a des complexes-o 1 :1 dans des solutions aqueuses de force ionique constante 0.14 M . Les constantes spicifiques de vitesse pour la formation et la dCcomposition de ces troix complexes a 25.0 "C sont respectivement egales a 5.7 + 0.2 x

Anionic σ‐complexes from sulfite ion attack on n‐methyl‐N‐β‐hydroxyethyl picramide and its spiro complex. Search for geometric isomerism in 1:2 complexes. An instructive example of complex kinetics

“…Figure 2 shows spectra of various equilibrium mixtures of PH, PHS2-, and PHSi-. It is apparent that A,,, (420-425 nm) and t at A,,, (19,000-19,500) for P H S -and PHSi-are virtually identical; this is in agreement with findings for the two corresponding sulfite complexes of N,N-dimethyl-picramide [7,8].…”

“…(b) Our k2 and K2 values are somewhat higher than those reported for the 1:2 complex derived from N,N-dimethylpicramide [8]. This is undoubtedly due to the higher ionic strength used in the present work.…”

“…One of these factors, already identified by Crampton and Willison [ll] while comparing K1 and K2 values for several 1-X-2,4,6-trinitrobenzene derivatives with varying bulkiness of X, is undoubtedly steric strain in the ground state (PH) which is being relieved when the 1:l and even more so when the 1:2 complex is formed, because the requirement for ring planarity diminishes in going from PH to PHS2-to PHS4-. This is probably the most important factor, although solvation effects as discussed by Buncel and co-workers [8], may also play a role.…”

“…(2) by the following procedure. The substrate solution which is placed in one of the syringes of the stopped-flow apparatus is kept at pH -6 so that the equilibrium concentration of M-becomes negligible, whereas the pH of the sulfite solution in the other syringe is slightly higher than 8 In general it is difficult to evaluate relaxation times with high precision when they are so close, unless conditions can be found where the amplitude of one of them is very small or zero 19, chap. 91.…”

Kinetic and equilibrium studies of ?-adduct formation and substitution in the reactions of sulphite ions with some alkyl and aryl ethers and thioethers

“…It has been argued that in related systems solvation is of prime importance in determining adduct stability. 5 In the 1:2 adducts 3 which carry four negative charges, solvation by the largely aqueous medium will be very important. The presence of hydrophobic groups will reduce such solvation.…”