Concentrated Salt Effects on the Solvolysis Reaction Rates in Methanol–Water Solution

Abstract

Abstract The solvolysis rates of aliphatic halides and related compounds (RX) were determined in an 80 vol% MeOH–20 vol% H2O solvent in the presence of very highly concentrated salts (0.25—5.0 mol dm−3) at 25—75 °C. For typical SN1 substrates, such as 1-adamantyl bromide and t-butyl chloride, the pseudo-first-order reaction rates (k/s−1) increased exponentially with increasing concentration of LiClO4, NaClO4, Mg(ClO4)2, and Ba(ClO4)2. The cation effects increased as Na+ < Li+ < Ba2+ ≤ Mg2+. A nonmetallic salt, Et4NBr, caused the k/s−1 value for 1-adamantly bromide to be slightly increased at lower concentrations ( < 1.0 mol dm−3), but to be greatly decreased at higher salt concentrations ( ≥ 2.0 mol dm−3). The large positive effects of metal perchlorates were explained by a change in the solvent structure and the formation of “stable” carbocations through a “chemical” interaction between X− (Cl−, Br−, or I−) and M+ (Li+, Na+) or M2+ (Mg2+, Ba2+) in the “modified” solvent. On the other hand, for SN1–SN2 intermediates, the solvolysis reaction rates were decreased by a decrease in the activity of the solvent containing a large amount of salts. The apparent rate constant (“k/s−1”) for isopropyl bromide at 75 °C remained an almost constant value in the presence of 0—5.0 mol dm−3 LiClO4. The rate constant for ethyl bromide was decreased substantially by the addition of the alkali metal and alkaline-earth metal perchlorates. A good linearity was observed between log (k1/k0) and the m values for RX by Grunwald and Winstein, where k1 and k0 are the solvolysis rates in the presence of 1.0 mol dm−3 LiClO4 and in the absence of the salt, respectively. Upon the solvolysis of neophyl chloride (1-chloro-2-methyl-2-phenylpropane), the change in the reaction scheme (e.g., methyl shift) was suspected during the increase in the LiClO4 or Ba(ClO4)2 concentration.