Kinetics of S_N1 Reactions in Binary Liquid Mixtures near the Critical Point of Solution

Abstract

We have measured the rates of the SN1 hydrolysis reactions of 2-chloro-2-methylbutane in isobutyric acid + water and 2-bromo-2-methylpropane in trietylamine + water near their respective consolute points. In the former mixture, two phases coexist below the critical solution temperature, while in the latter, two phases coexist above the critical solution temperature. Because each hydrolysis reaction produced a strong electrolyte among its products, we could follow the progress of the reaction by making measurements of the conductance. To interpret these measurements properly, however, we had to take into account the following phenomena: (1) the effect of the composition on the critical temperature, (2) the effect of the liquid−liquid-phase transition when it occurred during the course of the run, and (3) the competition between the effects of temperature and the effects of concentration on the response of the conductance probe. With these phenomena accounted for, however, we found in the case of both reactions that the specific rate of hydrolysis slowed at temperatures above the critical solution temperature and accelerated at temperatures below it. From the point of view of thermodynamics, the causative factor in slowing down is the derivative of the reaction Gibbs free energy with respect to the extent of reaction, which goes to zero at the critical point. In contrast, speeding up has no ready theoretical explanation. The experimental results imply, nevertheless, that speeding up is not confined to the two-phase side of the phase diagram.

Chemical Dynamics and Critical Phenomena: Electrical Conductivity and Reactivity of Benzyl Bromide in Triethylamine+Water Near its Consolute Point

“…To explore the effect of this principle on the net rate of a chemical reaction, one can use as the solvent for the reaction the homogeneous liquid phase that exists near the consolute point of a binary mixture [8][9][10][11][12][13][14][15]. Consider the general chemical reaction…”

“…where ξ(t) is the extent of reaction, and the upper sign applies to products while the lower sign applies to reactants [11][12][13][14][15][16][17]. At time t = 0, the extent of reaction has the value ξ(0) = 0, while at chemical equilibrium, it has the value ξ = ξ e .…”

“…where 1/τ is the specific relaxation rate [13,[15][16][17][18]. The Gibbs free energy difference separating reactants and products is…”

“…where R is the universal gas constant, T is the absolute temperature, r (ξ e ) is the rate of reaction in the forward direction, and the subscript, e, denotes evaluation at chemical equilibrium [13,[15][16][17][18].…”

“…In search of kinetic critical phenomena, we have measured the rates of nine different chemical reactions in three different binary liquid mixtures [10][11][12][13][14][15]. The solvent pairs were isobutryric acid + water (UCST), 2-butoxyethanol + water (LCST), and triethylamine + water (LCST).…”