Solvent Effect on the α-Effect:  Ground-State versus Transition-State Effects; a Combined Calorimetric and Kinetic Investigation

Abstract

In a study of the solvent effect on the alpha-effect, second-order rate constants (kNu-) have been determined spectrophotometrically for reactions of a series of substituted phenyl acetates with butan-2,3-dione monoximate (Ox-, alpha-nucleophile) and p-chlorophenoxide (p-ClPhO-, reference nucleophile) in DMSO-H2O (DMSO = dimethyl sulfoxide) mixtures of varying compositions at 25.0 +/- 0.1 degrees C. The magnitude of the alpha-effect, kOx-/kp-ClPhO-, increases as the DMSO content in the medium increases up to 40-50 mol %, reaching 500, one of the largest alpha-effect values, and then decreases on further addition of DMSO, resulting in a bell-shaped alpha-effect profile regardless of the nature of the substrates. The magnitude of the alpha-effect is found to be significantly dependent on the substrates (or, more quantitatively, on beta(nuc)). Thus, beta(nuc) is an important predictor of the magnitude of the alpha-effect. The bell-shaped alpha-effect profile found in the present system is attributed to the differential change in the sensitivity of the medium effect on the Ox- and p-ClPhO- systems but not due to a change in the reaction mechanism or to a drastic change in the basicity of the two nucleophiles on addition of DMSO to the medium. Through application of calorimetric measurements of ground-state solvation combined with the diagnostic beta(nuc) values, it is shown that the transition-state effect is more dominant than the ground-state effect as the origin of the alpha-effect in the present system.

Aminolysis of Y- Substituted Phenyl Benzenesulfonates in MeCN: Effect of Medium on Reactivity and Reaction Mechanism

“…[1][2][3][4][5][6][7][8][9][10] It is well known that rate of solvolysis increases with increasing solvent polarity and/or solvent nucleophilicity. [4][5][6] Besides, the effect of medium on rates of nucleophilic substitution (S N ) reactions is known to be highly dependent on the nature of reactants, e.g., significant rate acceleration for reactions involving anionic nucleophiles while a decrease in rate for reactions between neutral molecules upon changing reaction medium from H 2 O to dipolar aprotic solvents (e.g., DMSO and MeCN).…”

Origin of the α-Effect in Nucleophilic Substitution Reactions of Y-Substituted Phenyl Benzoates with Butane-2,3-dione Monoximate and Z-Substituted Phenoxides: Ground-State Destabilization vs. Transition-State Stabilization

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Thus, the enhanced reactivity shown by these nucleophiles was termed the α-effect. 1 Numerous studies have been performed to investigate the cause of the α-effect.…”

“…[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Many theories have been advanced to explain the α-effect, e.g., destabilization of the ground state (GS) due to electronic repulsion between the nonbonding electron pairs, transition state (TS) stabilization including general acid/base catalysis, thermodynamic stability of products, solvent effects. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] However, none of these theories is conclusive. Particularly, solvent effect on the α-effect remains controversial.…”

“…Particularly, solvent effect on the α-effect remains controversial. [8][9][10][11][12][13][14][15][16][17][18][19] Solvent effect was suggested to be unimportant since the magnitude of the α-effects was found to be similar for reactions performed in H 2 O and in organic solvents such as MeCN and toluene. 8 Besides, it has been reported that α-nucleophiles are intrinsically more reactive than normal nucleophiles of similar basicity in gas-phase reactions.…”

“…[15][16][17][18][19] Our calorimetric study has revealed that Ox -is ca. 4 kcal/mol less solvated than 4-ClPhO -in H2O.…”