Origin of the Bell-Shaped .alpha.-Effect-Solvent Composition Plots. pKa-Solvent Dependence of the .alpha.-Effect at a Phosphorus Center

Medium effect on the α-effect for nucleophilic substitution reactions of p-nitrophenyl acetate with benzohydroxamates and m-chlorophenoxide in DMSO–H2O mixtures as contrasts with MeCN–H2O mixtures: comparing two very different polar aprotic solvent components

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Thus, the enhanced nucleophilic reactivity shown by these nucleophiles was termed the α-effect by Edward and Pearson. 1 Since this definition only refers to the structure of the α-nucleophiles, an alternative operational definition was given to the α-effect, i.e., a positive deviation exhibited by an α-nucleophile from a Brønsted-type nucleophilicity plot.…”

“…2 Numerous studies have been carried out to investigate the origin of the α-effect and many theories have been proposed to account for the cause of the α-effect, e.g., ground-state (GS) destabilization, transition-state (TS) stabilization through aromatic or radicaloid character, thermodynamic stability of the reaction products, and solvent effects. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] However, the effect of medium on the α-effect remains controversial although it has been studied most widely. [9][10][11][12][13][14][15][16][17][18] DePuy et al carried out the first gas-phase reaction of methyl formate with HOO -and HO -and found that HOO -does not exhibit higher reactivity than HO -9 and, on this basis, concluded that the α-effect shown by HOO -in aqueous reactions is due to solvent effect because HOO -was previously reported to be 12 kcal/mol less strongly solvated than HO -in H 2 O.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] However, the effect of medium on the α-effect remains controversial although it has been studied most widely. [9][10][11][12][13][14][15][16][17][18] DePuy et al carried out the first gas-phase reaction of methyl formate with HOO -and HO -and found that HOO -does not exhibit higher reactivity than HO -9 and, on this basis, concluded that the α-effect shown by HOO -in aqueous reactions is due to solvent effect because HOO -was previously reported to be 12 kcal/mol less strongly solvated than HO -in H 2 O. 10 Bierbaum and coworkers have drawn a similar conclusion (i.e., the α-effect is not due to an intrinsic property but instead due to a solvent effect) from various gas-phase reactions 17a We found that medium effect on the α-effect is significant, e.g., the magnitude of the α-effect (i.e., the k Ox -/k p-ClPhO -ratio) increases as the DMSO content in the medium increases up to 50 mol % DMSO and then decreases upon further addition of DMSO (a bell-shaped α-effect profile Combination of the calorimetric data with activation parameters has led us to conclude that desolvation of Ox -(i.e., GS effect) upon addition of DMSO to the medium is mainly responsible for the increasing α-effect up to 50 mol % DMSO and that the decreasing α-effect in the DMSO-rich medium is caused by differential TS stabilization.…”

Solvatochromism and preferential solvation of Brooker's merocyanine in water–methanol mixtures

“…Preferential solvation, a phenomenon whereby a solute is solvated more preferentially by one solvent than by others, usually occurs in a mixed solvent, and the local mole fractions of a solvent around the solute differ from the bulk mole fractions . Preferential solvation has attracted the attention of researchers because many chemical reactions and biological processes occur in a mixed solvent, and the environment around a solute has a large influence on the reaction and physical properties …”

On the mechanism of the reaction between aryl acetates and hydroxylamine

“…[1][2][3][4][5][6][7][8][9] Besides describing the structural relationships which lead to the reactivity of carboxylic acid derivatives, the search for nucleophiles able to cleave selectively peptide and ester bonds is still a challenge in many areas of chemistry and biochemistry. [10][11][12] The development of such stable and highly reactive nucleophiles has a wide range of applications in chemical detoxification, e.g., where quantitative phosphate bond cleavage is the target. 13 Some researchers in this field have noted that α-nucleophiles, such as hydroxylamine, represent a new perspective, but a better understanding of the attacking nucleophilic atom is still to be gained and this has prompted extensive mechanistic studies of the reactions of α-nucleophiles with activated acyl derivatives [14][15][16][17][18][19][20] , phosphate esters [21][22][23][24][25][26][27][28][29] and amides [30][31][32][33] .…”