Concentrated salt effects on the solvolysis reaction rates in methanol-water solution

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 degrees C. For typical S(N)1 substrates, such as l-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+ < Ba(2+)less than or equal to 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 (greater than or equal to 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+ (M2+, Ba2+) in the "modified" solvent. On the other hand, for S(N)1-S(N)2 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 degrees 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(k(1)/k(0)) and the m values for RX by Grunwald and Winstein, where k(1) and k(0) 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.