Effect of the consolute point of isobutyric acid plus water on the rate of an SN1 hydrolysis reaction

The rate constant for the S-N 1 hydrolysis of 2-chloro-2-methylbutane was measured near the consolute point of the liquid mixture isobutyric acid + water. At temperatures far from the upper critical solution temperature, T-c, of this mixture, the rate constant obeyed the Arrhenius equation. In the one-phase region just above T-c, however, the rate constant decreased below the Arrhenius background, while in the two-phase region below T-c, it increased. This combination of slowing-down with speeding-up appears to be beyond the scope of explanation of the current theories of dynamic critical slowing-down.