The hydrolysis of thioacetic, thiobenzoic, and three substituted thiobenzoic acids in perchloric and sulfuric acids

Abstract

SIN CHEONG WONG. Can. J. Chem. 56,935 (1978). The rates of hydrolysis of thioacetic, thiobenzoic, and three substituted thiobenzoic acids increase with concentration of solvent sulfuric or perchloric acid to a maximum in 30-40% acid and then decrease. Yates-McClelland r, Bunnett-Olsen 6, and Hammett p parameters, and entropies of activation indicate an AA,2 mechanism over this range of acid concentrations. In acid concentrations above 50-60% the rates increase sharply and the same mechanistic criteria now indicate an AA,l mechanism. The difference between the rate-acidity profile of thiobenzoic acid and that of ethyl thiolbenzoate can be explained by the different response of the activity coefficients of their transition states to increase in sulfuric acid concentration. JOHN T. EDWARD, GRAEME WELCH et SIN CHEONG WONG. Can. J. Chem. 56,935 (1978). The hydrolysis of thiocarboxylic acids is catalyzed 1 by acid (I). 1Hipkin and Satchel1 (2) investigated the rates of hydrolysis of thioacetic and thiobenzoic acids in varying concentrations of hydrochloric acid, up to 4.80 M. The pseudo-first-order rate constants kb followed [H,O+] rather than the Hammett acidity function h,. This was taken to indicate an A-2 rather than A-1 mechanism for the acid-catalyzed hydrolysis. However, we have recently shown that although thiol esters are hydrolyzed by an AAc2 mechanism in 30-60% sulfuric acid, this mechanism is superceded by a very fast AAcl reaction in higher acid concentrations (3, 4). Consequently, it became of interest to investigate the acid-catalyzed hydrolysis of the thio acids over a wider range of acid concentrations. In this paper we report kinetic studies of the hydrolysis of thioacetic acid, thiobenzoic acid, and three substituted thiobenzoic acids in concentrations of sulfuric acid up to 80% and of perchloric acid up to 67%. Experimental MaterialsThioacetic and thiobenzoic acids were conlmercial products. p-Methoxy-, p-methyl-, and p-chlorothiobenzoic acids were obtained by a standard procedure (5) from the substituted benzoyl chlorides and potassium hydrosulfide. The thioacids were freshly distilled before each kinetic run; titration against standard alkali then indicated them to be approximately 99.9% pure. (On standing several days the titre dropped, presumably because of oxidation to the disulfide.)Mineral acid solutions were prepared by dilution of reagent grade perchloric acid (70%) or sulfuric acid (96%) with distilled water and their concentrations determined by titration against standard alkali to phenolphthalein end point.

Mechanistic studies in strong acids. VIII. Hydrolysis mechanisms for some thiobenzoic acids and esters in aqueous sulfuric acid, determined using the excess acidity method

“…In this paper the method is applied to the hydrolysis reactions undergone by four thiobenzoic acids (I), by thioacetic acid (2), by eight ethyl thiolbenzoates (3), and by eight ethyl thionbenzoates (4), in aqueous sulfuric acid. All of these compounds give the parent carboxylic acid as product, and previous studies (6)(7)(8), using the r-parameter (9) and transition-state activity coefficient (10) methods, have suggested that, following a pre-equilibrium protonation step, they described previously (3). log I data could not be obtained for l a , m * X , where X is the excess acidity for a standard i d , 30, and 3b (7,8); for these pKSH+ was obtained from po plots base B * ( 1 1, 15), eq.…”

“…[I] log I -log CH+ = m*X + pKsH+ values of X and log CH+ were available ( l l ) , and these were corrected to the measurement temperature, 5°C or IO°C (7,8), as…”

“…at the ester Z rather than the carbonyl Z, represented by KtsH+ and m*' rather than KsH+ and m * ; however, the latter is still the most favoured protonation site (1,3,6,7 ) , and the amount of substrate "locked up" in the carbonyl-or thiocarbonyl-protonated form is accounted for by the protonation correction term, log [CsI(Cs + CsH+)] …”

“…[S], [7], [lo], and [13]; pKsH+ was corrected to the reaction temperature, if necessary, by assuming that AGO = AH0 for the protonation process, as before (3,4). The 25°C values given in Tables 2 and 3 have been corrected in this way.…”

“…Observed rate constants as a function of sulfuric acid concentration at 25"C, k*, were available for 3 and 4 (6,7), as were data at 50°C and two other temperatures for 1 and 2 (8). Temperaturecorrected log water activities and log bisulfate ion concentrations in rnolarity units were those used before (3,4); log sulfuric acid activitiesat 25°C were obtained from Giauque el al.…”