In Sun Koo

Role of hydroxyl concentrations in solvatochromic measures of solvent polarity of alcohols and alcohol-water mixtures-evidence that preferential solvation effects may be overestimated.

For single solvents, primary alcohols and water, there is a good linear correlation (r = 0.994) between the solvent polarity index ET(30) and the molar concentration of OH groups (or 1000/Vm, where Vm is the solvent molar volume). The corresponding correlations for alcohol-water mixtures are plots vs. the sum of molar concentrations of alcohol and water, alternatively expressed as plots of ET(30)vs. volume fraction. Our quantitative treatment is an extension of recent theoretical and experimental results. In contrast, previous studies of alcohol-water mixtures have relied on plots of ET(30)vs. mole fraction, and have overestimated the effect of preferential solvation of solvatochromic dyes by the more hydrophobic alcohols.

Ab initio and DFT studies on hydrolyses of phosphorus halides

Hydrolyses of phosphorus halides, (RO)2POX where R = H or Me and X = F or Cl, in the gas phase and in the reaction field have been investigated theoretically with ab initio and the density functional theory (DFT). The free energy of activation in the reaction field was also estimated using the Onsager method with a correction of entropy change and basis set superposition error (BSSE). The reaction of (MeO)2POF proceeds through a path with bifunctional catalysis regardless of the medium, but the reaction of (MeO)2POCl proceeds through bifunctional and general base catalysis in the gas phase and in water, respectively. The estimated free energy barrier of 23 kcal/mol for the hydrolysis of (MeO)2POF is in good agreement with the experimental values of 24 kcal/mol, and relative barrier of 3 kcal/mol to the (MeO)2POCl is also in good agreement with the experimental values of 5 kcal/mol of diisopropyl phosphorus halides ((PriO)2POX, X = F and Cl).

Rapid solvolyses of 2,6-dimethyl-and 2,4,6-trimethyl-benzoyl chlorides: model systems for solvent effects on the reactivity of acid chlorides

Rates of solvolyses of 2,6-dimethyl- and 2,4,6-trimethyl-benzoyl (mesitoyl) chlorides are reported for aqueous binary mixtures with acetone, ethanol, and methanol. These data are compared with those obtained for solvolyses of p-methoxybenzoyl (anisoyl) chloride. A consistent definition of selectivity is proposed to remove ambiguities in published values. Selectivities (S) for formation of ester relative to acid in alcohol–water mixtures are almost independent of solvent composition and are inverse for ethanol mixtures; methanol mixtures show higher S values. Acid catalysis does not appear to be significant. The results show that, for solvolyses of acid chlorides, a solvation effect and possibly a mass law effect increase the reaction rates in methanol–water mixtures as compared with those in ethanol–water mixtures having the same Y value. Differences between the mechanisms of solvolyses of mesitoyl and anisoyl chlorides are revealed by rate enhancements with added m-nitroaniline, and by a comparison of m values. Experience of recent conductimetric studies of a wide range of relatively repid solvolytic reactions is summarised.

Trends in selectivity. Evidence from rates and products for simultaneous reaction channels in solvolyses of benzoyl chloride and substituted derivatives

Rates of solvolyses of para-Z-substituted benzoyl chlorides (Z = OMe, Me, or Cl) are reported for highly aqueous binary mixtures (with acetone, ethanol, and methanol cosolvents). Product data (ester and acid) are reported for a wide range of compositions of the aqueous binary alcohol mixtures (for Z = Me, H, or Cl). As water is added to alcohol, the selectivity S([ester][water]/[acid][alcohol] initially increases but then reaches a plateau and/or decreases. This reversal of the trend in S occurs at different solvent compositions for each substrate (Z = Me, H, or Cl) and is very close to the region of mechanistic change predicted previously from rate–rate profiles; these predictions have now been refined by replacing solvolyses of 1-adamantyl chloride by p-methoxybenzoyl chloride as the model SN1 process. The results provide new and sharply defined evidence for mechanistic change in nucleophilic substitutions at acyl carbon, supporting the co-existence of two different mechanisms or reaction channels, each of which could show variable structures of transition states. A quantitative dissection of observed rates into contributions from two reaction channels is described, and an attempt is made—with partial success—to calculate variations in S values occurring as the mechanism changes. Trends in S values are calculated successfully.

Limitations of the transition-state variation model. Part 3. Solvolyses of electron-rich benzenesulphonyl chlorides

Rate constants for solvolyses of 4-methoxy-2,6-dimethylbenzenesulphonyl chloride 3(Z = OMe) and of 4-methyl- and 4-methoxybenzenesulphonyl chlorides 4(Z = Me and OMe) are reported for aqueous binary mixtures with acetone, ethanol and methanol. Some additional rate constants are reported for aqueous acetonitrile and dioxane mixtures, as well as product selectivities (S) in alcohol–water mixtures. For each binary mixture, rates of solvolyses of 3(Z = OMe)vs. YCl or Y are approximately bilinear. As water is added to alcohol, S values for solvolyses of 3(Z = OMe) pass through a maximum and for solvolyses of 4-methoxybenzenesulphonyl chloride 4(Z = OMe) the position of the maximum shifts to more aqueous media. For solvolyses of 4-methylbenzenesulphonyl chloride 4(Z = Me), S values are shifted such that they reach a plateau rather than a maximum, and the rate–rate profiles with YCl are approximately linear rather than bilinear. All of the rate–rate profiles show ‘dispersion’ into separate correlations for the various binary mixtures. These substituent effects follow the same trends as corresponding solvolyses of benzoyl chloride and strengthen recent proposals that solvolyses of 3(Z = Me) proceed via competing (dual) reaction channels.

Precise dissection of kinetic data for solvolyses of benzoyl chloride into contributions from two simultaneous reaction channels

Rate–rate profiles for solvolyses of benzoyl–p-methoxybenzoyl chlorides in ethanol–water and in methanol–water permit a precise dissection of the kinetic data into contributions from two simultaneous reaction channels.