A thermodynamic approach to analyzing relative permittivity and solvent mole fraction models, and application to SN1 reactions

The standard methods for analyzing solvent effects on chemical reactions largely include linear free energy relations that relate kinetic and spectroscopic terms to solvent interactive parameters. The number of these parameters has grown over the years in order to make linear free energy techniques more accurate and cover a wider range of reaction systems. However, even with the myriad of parameters, the details of specific reaction systems make the application of these techniques sometimes unreliable. On the other hand, a thermodynamic approach provides a more precise analysis, and has proven particularly useful for reactions in multi-component solvent systems. In this article we present the mathematical formalism for relating the activation free energy to the bulk thermodynamic properties for a binary (cosolvent) system. We then use this thermodynamic approach, coupled with selected solvent models, to analyze the hydrolysis rates of tert-butyl chloride in the acetonitrile/water solvent system under iso-mole fraction, isodielectric, and isothermal conditions. These analyses allow us to differentiate and quantify bulk electrostatic effects and the effects of close-range solute-solvent interactions.

Properties of the tert-butyl halide solvolysis transition states

We have obtained properties (or descriptors) of the transition states in the solvolysis of tert-butyl chloride, bromide and iodide. We show that all three transition states, in both protic and in aprotic solvents, are highly dipolar and are strong hydrogen bond acids and strong hydrogen bond bases, except for the tert-butyl iodide transition state in aprotic solvents, which has a rather low hydrogen bond acidity. Thus, the transition states are stabilized by solvents that are hydrogen bond bases (nucleophiles) and are hydrogen bond acids (electrophiles). We show also that the partition of the transition states between water and solvents is aided by both nucleophilic and electrophilic solvents and conclude that the rate of solvolysis of the three halides is increased by both nucleophilic and electrophilic solvents.

Systems Designed with an Ionic Liquid and Molecular Solvents to Investigate the Kinetics of an SNAr Reaction

The microscopic features of binary solvent systems formed by molecular solvents (methanol, ethanol, 2-methylpropan-2-ol, water, pyridine, acetonitrile, 1-propanol, 2-propanol, dimethylformamide) and the ionic liquid ([bmim]BF4) as cosolvent were used to select mixed solvents with particular characteristics. Molecular-microscopic solvent parameters corresponding to these binary mixtures were utilised to study the kinetics of the nucleophilic substitution reaction between 2-chloro-3,5-dinitropyridine and aniline to investigate and compare the effects of the solvents on a model chemical reaction. For these mixtures, the solvation behaviour is dominated by both the dipolarity/polarisability and hydrogen bonding interactions of the media. In these solutions, the rate constants of the reaction decrease with the mole fraction of ionic liquid.