John Marshall William Scott

Kinetics of reactions in microheterogeneous aqueous systems

Kinetics of reaction involving two low-spin iron(II) di-imine complexes, Fe(phen)2+3 and Fe(hxsbH)2+, are reported where the solvent media include three microemulsions and a range of aqueous 2-butoxyethanol mixtures. In certain systems containing Fe(hxsbH)2+ the rate of reaction follows zero-order kinetics for a considerable part of the reaction. The kinetics are accounted for using a reaction scheme in which the complex metal ion is adsorbed at an interphase between organic-rich and water-rich domains. The same model is used to account for the marked acceleration in the rate of reaction between Fe(phen)2+3 and hydroxide ions in these systems. The kinetics of a scheme in which reaction takes place in both domains are examined, the distribution of the substrate being at all times in equilibrium between the two microphases. In such cases a plot of In k(obs.) against 1/T can be S-shaped, where k(obs.) is the overall first-order rate constant. Effects of changes in acid, acid concentration and added surfactant on the rate of reaction in the microemulsions are reported.

Heat capacities of solutions: coupled chemical equilibria

The isobaric equilibrium heat capacity of a solution is related to the composition and thermodynamic parameters characterising two coupled chemical equilibria between solutes in solution. The resulting equation is compared with the corresponding equations for the isobaric equilibrium heat capacities of a solution in which a single chemical equilibrium between solutes is present. The presence of coupling between equilibria means that the isobaric equilibrium heat capacity of the solution is not simply the sum of two contributions linked with each equilibrium. The procedures described form the basis of treatments for more complicated systems.

Kinetic deuterium isotope effects and kinetic solvent isotope effects for the solvolyses in water and in water + acetonitrile mixtures related to a series of substituted benzyl nitrates

In aqueous solution, the α-deuterium isotope effects in the solvolyses of benzyl nitrates derivatives depend on the nature of the substituent in the benzene ring. In addition, the isotope effect for some derivatives depends on mole fraction of added acetonitrile while for others the isotope effect is insensitive to solvent composition. However, the kinetic solvent isotope effects for para-methyl and meta-trifluoromethyl derivatives remain unchanged when acetonitrile is added. These observations are accounted for in terms of a model which describes the solvolytic reaction as a two-stage process and contrasts the relative importance of bond-making and bond-breaking.

Kinetics of the reaction between cyanide ions and tris(4-methyl-1,10-phenanthroline)iron(II) cations in aqueous solutions. Analysis of kinetic data for this reaction and for solvolysis of benzyl chloride in water in terms of isothermal, isobaric and related isochoric activation parameters

Kinetic data for the title reaction and for the solvolysis of benzyl chloride in water are analysed to obtain isobaric and isothermal activation parameters. Isochoric parameters are defined and the rate constants described as functions of temperature, pressure and molar volume of the pure solvent, V*1. A quantity Δ‡ψ(V*1) characterises the dependence of rate constant on temperature at constant V*1. We counter claims that isochoric activation parameters necessarily provide an insight into the process of activation in chemical reactions. Isobaric–isothermal functions, within the limitations of transition-state theory, have a sounder basis in terms of thermodynamic analysis of chemical reactions.

Chemical equilibria and related isochoric functions

The meaning is examined of the term ‘isochoric’ when used in examination of the dependence of equilibrium parameters on temperature and pressure. A number of terms which identify the extrinsic nature of this term are defined. The question of a definition of the standard and reference states is considered.

Kinetic solvent isotope effects [k(H2O)/k(D2O)] for benzyl nitrate, 4-chlorobutan-l-ol and piperidylsulphamoyl chloride. The significance of activation parameters to the mechanism of the reaction

Kinetic solvent isotope effects (KSIE =k[H2O]/k[D2O]) are reported for benzyl nitrate, 4-chlorobutan-l-ol and piperidylsulphamoyl chloride. The dependences of first-order rate constants on temperature are analysed to obtain related standard activation parameters. The significance of these parameters is discussed in relation to the mechanism of reaction. Where the solvolysis involves direct solvent participation on going from initial to transition state (e.g. an SN2 reaction), an implicit extrathermodynamic assumption in the analysis dispenses with the information which would otherwise lead to details of the reaction mechanism.

Application of a solvent-coupled model to a calculation of partial molar isobaric heat capacities of neutral and ionic solutes in aqueous solution. Estimates of isobaric heat capacities of activation

Partial molar isobaric heat capacities and volumes of apolar and ionic solutes in water have been examined using the Lumry–Frank two-state model for water in conjunction with the Grunwald extrathermodynamic assumption describing the impact of a solvent equilibrium on the properties of a solute. The analysis has been extended to a consideration of isobaric heat capacities of activation. The predicted dependence of the latter parameter on temperature for a first-order unimolecular reaction resembles that calculated using the Albery–Robinson mechanism for solvolytic reactions.

Kinetics of solvolytic reactions. Dependence of composition on time and of rate parameters on temperature

Kinetic parameters describing the solvolysis of alkyl halides in water at fixed temperature and pressure are considered in terms of both one-stage and two-stage processes. In the latter case the consequences are explored of using both the steady-state hypothesis and the fully integrated form of the rate expression. The first-order rate constant calculated from experimental data has not discriminated between these alternative descriptions. The integrated form based on a two-stage reaction is considered the correct approach. Calculations for model systems throw some doubt on the validity of the steady-state approximation for these systems. Further, the steady-state assumption leads to an underestimate of the apparent heat capacity of activation when compared with the values calculated using the fully integrated form of the rate expression.