Bronsted Coefficient Research Articles

Nitroalkane anomaly: computational study with cluster and continuum modeling

The origin of anomalous relation between rates and equilibria for the proton-transfer reactions of nitroalkanes, known as nitroalkane anomaly, was investigated by theoretical calculations with a cluster model, in which three water molecules are explicitly considered in the system, as well as the PCM solvent continuum model. For the CH3NO2/CH3CH2NO2 system, B3LYP and MP2 computations reproduced the anomaly, and the imbalance in the charge distribution at the transition state (TS) was observed. In contrast, although the TS imbalance was detected for the substituted phenylnitromethane system, the Bronsted plots did not show any anomaly. The experimentally observed abnormally large Bronsted coefficient (α ~ 1.4) for this system does not arise from the charge imbalance at the saddle point structure, but is likely due to the effect of the reaction dynamics.

A semiclassical approach to concerted proton transfer in condensed media

Rate expressions are provided for proton transfer reactions in which a proton is transferred from a donor site to a catalyst molecule “in concert with” proton transfer from a different site on the catalyst to a proton acceptor (bifunctional catalysis). The basis of the description is a semiclassical approach according to which the process is represented by three-well potential energy surfaces spanned by low-frequency nuclear co-ordinates. The minima correspond to localization of a proton on the donor, the catalyst and the acceptor. Each vibrational state of the proton causes a vertical shift of the appropriate surface, and within the semiclassical approach both weak and strong intermolecular interactions are covered.For strong catalysts, corresponding to low energies of the intermediate state, the two protons are transferred separately. In such cases Bronsted relation and kinetic isotope effect variation with pKa of the catalyst do not differ fundamentally from those of step-wise individual proton transfers. For weak catalysts the two protons are transferred in a “synchronous” motion. The effect of the catalyst is then reflected in the pre-exponential factor of the rate expressions and gives a small Bronsted coefficient and slow variation of the kinetic isotope effect with pKa of the catalyst.

The Brønsted relationship, hydroxide solvent isotope effects, and isotope exchange in E1cB elimination of 9-(dimethylaminomethyll)fluorene

A comparison is reported between Bronsted coefficients for catalysis by amine bases (β) and from kinetic and equilibrium measurements of hydroxide solvent isotope effects: kOD/kOH=(KOD/KOH)βi, for E1cB elimination of 9-(dimethylaminomethyl)fluorene and its conjugate acid to form dibenzofulvene. As in related reactions, agreement between values is poor (βi 0.31; β > 0.65), and better agreement is sought by separating the isotope effects into contributions from the hydrogens of the hydroxy-group and its solvation shell, making use of kinetic measurements in 1 :1 H2O–D2O mixtures. Following Gold, β values are defined from fractionation factors for the transition state (ϕ1 and ϕ2) and hydroxide ion (ϕa and ϕb) as ϕ1=ϕa1 –βa and ϕ2=ϕb1 –βb for the hydroxy and solvating hydrogens, respectively. It is found that the measured β values are very sensitive to the values of ϕa and ϕb, and since these themselves are poorly defined experimentally it appears that evaluation of a β value providing a practical measure of transition state structure is at present not possible. On the other hand, the measurements in 1 : 1 H2O–D2O provide a constraint on ϕa and ϕb in that solutions for the transition state fractionation factors ϕ1 and ϕ2 exist only outside the ranges ϕa 0.45–1.43 and ϕb 0.99–0.67. This result is sensitive to experimental error but not to the fractionation model chosen for the transition state. An attempt to compare hydroxide with methoxide solvent isotope effects in MeOH and MeOD was prevented by the occurrence of isotope exchange between solvent and substrate in MeOD. Kinetic analysis of the competing reactions of exchange and elimination in MeOD, however, offers confirmation of the mochanism of reaction and of the involvement of carbanion and zwitterion intermediates in the elimination.

Base-catalysed mutarotation of glucose in dimethylformamide and dimethyl sulphoxide

The rate of mutarotation of glucose has been measured in pure dimethylformamide and dimethyl sulphoxide with substituted benzoate ions as catalysts, and in 80 vol % dimethyl sulphoxide-water with acetate and chloroacetate as catalysts. Unlike many other base–catalysed reactions in these solvents, the rates show no subtantial increase over those observed in water. This is interpreted in terms of a strong involvement of water with the transition state in aqueous media, in accordance with earlier suggestions concerning the mechanism of related reactions such as the addition of water and alcohols to carbonyl compounds. In combination with previously reported pK values in the appropriate solvents, the results show that there is a good correlation of the catalytic effect of the anion bases with their basic strengths. The observed Bronsted coefficients are slightly higher (β= 0.6 in dimethylformamide and dimethyl sulphoxide) than in water (β= 0.4).

Equilibrium and kinetic acidities of carbon acids

Bnomalous Bronsted coefficients observed for nitroalkanes are rationalized in terms of a mechanism involving an intermediate anion. The results of measurements of equilibrium acidities of carbon acids in dimethyl sulphoxide (DMSO) solution are presented and compared with similar measurements made in solvents of low dielectric constant and in water. A linear correlation between gas-phase acidities and DMSO solution acidities for a series of ketones and for a series of nitriles is presented. Equilibrium measurements indicate the presence of a high degree of strain in the anion derived from trifluoromethylsulphonylcyclopropane. This result shows that the sulphur atom in the trifluoromethylsulphonyl group is entering into strong conjugation with the carbanion.

Unusual slopes of free energy plots in kinetics

Theoretical equations are used to comment on the relation between the slope of activation free energy-standard free energy of reaction plots and the concept of the position of the transition state along the reaction coordinate. The equations are then used to consider some recent experimental findings of unusual Bronsted coefficients.