ANALYSIS OF SOLVENT EFFECTS ON THE KINETICS OF REACTIONS IN SOLUTION USING KIRKWOOD-BUFF INTEGRAL-FUNCTIONS - ALKALINE-HYDROLYSIS OF THE DIAZOBUTADIENE COMPLEX TRIS(GLYOXAL BIS(METHYLIMINE))IRON(II) COMPLEX AND SPONTANEOUS HYDROLYSIS OF 4-METHOXYPHENYL 2,2-DICHLOROPROPIONATE IN WATER + 2-METHYL-2-PROPANOL MIXTURES AT 298.2 K

Rate constants and volumes of activation for the spontaneous hydrolysis of 4-methoxyphenyl 2,2-dichloropropionate and for the alkaline hydrolysis of the low-spin iron(II) complex [Fe(MeN=CHCH=NMe)3]2+ in water + 2-methyl-2-propanol (tert-butyl alcohol) mixtures are analyzed to yield Kirkwood-Buff integral functions describing preferential solvation of solutes in this mixed aqueous solvent system. The derived integral functions characterize the affinities of initial and transition states for water and for alcohol in the mixtures. The integral functions are calculated by using kinetic data and the thermodynamic properties of the water + alcohol mixture. The dependence of rate constants on alcohol mole fraction is analyzed, yielding the change in relative affinities of the initial state for the two solvent components on proceeding to the transition state. Incorporation of data describing the dependence of volumes of activation on mixture composition offers an insight into the separate affinities of initial and transition states for the components of the solvent mixture. For the hydrolysis of 4-methoxyphenyl 2,2-dichloropropionate, the dominant feature is preferential solvation (hydrophobic bonding) of the initial state by added alcohol. This trend is more important in determining the kinetic medium effect than solvation of the hydrophilic transition state by water. The reverse trend is established in the case of the iron(II) complex undergoing alkaline hydrolysis. With increase in mole fraction preferential solvation of the initial state by water is the dominant feature, increasing until the alcohol mole fraction reaches 0.2 before falling sharply away. As the composition of the solvent mixture becomes alcohol-rich, there is a dramatic decrease in the preferential solvation by either water or alcohol. © 1990, American Chemical Society. All rights reserved.