Robert Eliason and Maurice M. Kreevoy
2658
~
o ~ of General-Acid-Catalyzed ~ a ~ ~ ~ Ethyl o Vinyl ~ Ether Hydrolysis in 80% Dimethyl ulfoxicle with that in Waterla chert Eliason*lb and Maurice M. Kreevoy Chemical Dynamics Laboratory, University of Minnesota, Minneapolis, Minnesota 55455 (Received June 26, 1974) Publication costs assisted by the National Science Foundation
The acid-catalyzed hydrolysis of ethyl vinyl ether in 80% DMSO-20% water, by weight, has been investigatied. The Br4nsted catalysis law for a series of five monobasic carboxylic acids is compared with the Brqhsted relation previously obtained in water. The present Br4nsted plot appears to show some curvature; however, the paucity of points and the limited PKHArange do not allow this to be uniquely established. The results strongly suggest there is no real change in mechanism upon changing the solvent from pure water to aqueous DMSO.
In many mfechanisticstudies it is convenient to compare results obtained in water with those from mixed aqueousorganic so1vei:its. ‘This study was undertaken to see how valid such comparisons are by studying a system, already carefully studied in water, in 80% dimethyl sulfoxide (DMSO)-20% water, by weight. The hydrolysis of ethyl vinyl ether in aqueous solutions has been shown to be general-acid-catalyzed and a Br4nsted 01 i s knovm2 The reaction has also been studied in DMSO-water mixtures using HCl as the c a t a l y ~ t .The ~ present paper reports the results of the hydrolysis of ethyl vinyl ether in 80%’DMSO using a series of carboxylic acids as catalysts.
phenol buffer covering 13 pH units. The ~ K H A ’ofS the organic acids have been determined independently by Baughman and K r e e ~ o y . ~ Kinetic measurements were made by the usual spectrophotometric techniques in a thermostated cell, 25.0 f 0.2O. For very slow reactions the reaction mixtures were kept in a thermostated water bath. Aliquots were periodically withdrawn, and optical density measurements made. It was inconvenient to follow the slow reactions to completion; thus, an “artificial” infinity point for each experiment was prepared by adding a known amount of 1 M HC104 solution to an equivalent amount of reaction mixture. The infinity points were corrected for the dilutions.
Experimental Section DMSO (Aldrich Chemical Co.) and the water used for all solutions were puriified as previously d e ~ c r i b e d .All ~ the carboxylic acids were purified by distillation under vacuum. Ethyl vinyl ether was purified as described before.2 Potassium trifluorome thane sulfonate was prepared by the method of Gramstad and Hazeldine5 (mp 250-252°;6 lit. rnp 230’)Aqueous buffer solutions of the carboxylic acids were prepared in the usual way.7 A weighed amount of the aqueous buffer solution was added to a known weight of DMSO giving solutions with a buffer ratio 1:1, an ionic strength, p , of 0.1 M and a solvent composition of 80% DMSO-20% water, by weight. Differing buffer concentrations were prepared by dilution of the parent buffer solution, maintaining the ionic strength a t 0.1 M by addition of 0.1 M potassium trifluorornethaaie sulfonate in 80% DMSO-20% water. Perchloric acid solutions in the solvent mixture were prepared by adding the acid first to water, then adding this solution to the DMSO. Adding perchloric acid directly to the DMSO-water solution resulted un a violent reaction producing a blue flash. Acid dissociation constants for trifluoroacetic, dichloroacetic, and cyanoacetic acids were measured in 80% DMSO20% water by t h e methods of Kolthoff and Chantooni.8 All emf rneasuremlents were made on a Radiometer (Coperihagen) pH meter using a Radiometer G202 B standard glass electrode. The cell was maintained at ambient temperature (25 i 2’) by air conditioning. In 80% DMSO a response of 56 mV/pH unit was found using HC104 solutions, acetic acid buffers, p - chlorophenol buffer, and 2,4,6-trimethyl-
Results The pK HA values for trifluoroacetic acid, dichloroacetic acid, and cyanoacetic acid a t p = 0.1 M in 80%DMSO were determined to be 1.16 f 0.13, 3.48 f 0.02, and 4.92 f 0.01, respectively. These values were obtained from measurements on five buffer concentrations varied from 0.02 to 0.1 M. The errors cited are average deviations from the mean. For dichloroacetic acid Baughman and Kreevoy4 obtained an infinite dilution value of 3.87, and Ballash, et al.,9 obtained 3.12 at p = 0.2 M. A plot of ~ K H (80% A DMSO) against PKHA(water) for dichloroacetic, chloroacetic, and acetic acids from the data of Baughman and Kreevoy4 was linear.loa The present dichloroacetic and cyanoacetic acid ~ K H A ’ (80% S DMSO) were both displaced from the Baughman-Kreevoy line by about 0.4 unit. It was assumed that chloroacetic and formic acid would have ~ K H A ’for S p = 0.1 M similarly displaced. The calculated pK HA (80% DMSO) for chloroaeetic acid is 5.38 and for formic acid 6.42. General-acid catalytic coefficients, k HA, were obtained ) [HA] in from the slopes of plots of (kl - h ~ ( H + lagainst buffered solutions of constant ionic strength and nearly constant [H+]. The method has previously been described.’ l A least-squares criterion was used to evaluate the slopes, HA, in each case forcing the line thru the origin. These plots for chloroacetic and formic acid were quite scattered, and the slopes are near the maximum one would reasonably draw. The scatter was probably due to the small percentage of reaction (
