Solvent effects and activation parameters for the isotopic exchange reaction between lithium bromide and butyl bromide in acetone-water mixtures

The effect of minute amounts of water (<= 1.5 wt-%) in acetone on the kinetics of the isotopic exchange between lithium bromide and butyl bromide has been investigated over a range of temperatures (15-35degC) using 82Br as a radioactive indicator. The observed rapid decrease in the specific rate of exchange with increasing concentration of water in the solvent mixture is interpreted in terms of preferential solvation of the bromide ion. Activation parameters ΔG0, ΔH0, and ΔS0 have been calculated. The inhibitory effect of the protic constituent on the exchange reaction is reflected in a decreasing entropy of activation with increasing water concentration rather than in an increasing enthalpy of activation. Transfer activity coefficients for the transfer of bromide ions from pure to aqueous acetone have been estimated from the kinetic data and are used together with ion-pair association for lithium bromide to obtain information about solvation of the ion-pairs. (author)