No abstract

A method is suggested for determining the standard state entropies (s$) of the transition state for the neutral hydrolysis of esters in water. This has required the development of methods for approximating initial state parameters where experimental data are lacking.Characteristic linear correlations between the entropy and enthalpy of activation are observed for hydrolysis in water, as well as for the bimolecular halide exchange reaction in acetone and for acid-base equilibria. Expla~~ations are advanced to explain the observed trends.Fro111 the derived standard state entropies, a method for estimating the charge development in the transition state for the methyl and isopropyl halides is proposed. With this further detailed knowledge of the transition state in the methyl halide series, reasonable values of the activation enthalpy can be calculated from available thermochemical data.

No abstract

The substituent effects of o-and P-methyl and -trifluoromethyl groups in aromatic nucleophilic substitution have been measured and compared. The methyl group is slightly deactivating; the trifluoromethyl group has ail activating power approaching that of methoxycarbonyl and trimethylammonium. The modes of action are discussed.

The aim of this \vorlc \\.as to explore the effect of a-methylation on the reactivity and detailed mechanism of hydrolysis. Accordingly, rate d a t a and derived parameters are given for the hydrolysis of a series of halides and sulfonates In water. From the differences in 1ti1:etic parameters, an attempt is made to infer further information relating structure and rnechan~sm. This paper sumnlarizes the results of a study of the effect of 0-methylation on the solvolytic displacement of halides and sulfonates in ~vater, thus extending our earlier study on the corresponding methyl compounds (1).As a illedium for solvolysis, water is k n o~~~n to have excellent ionizing properties and to be a fair nucleophile. Its position in these respects, relative to other solvents, is well li110\\-11 (2) and is not altered by our rate studies.Relative rates, however, provide little evidence on jvhich to base conclusions concerning the detailed mechanism, particularly with respect to solvent reorganization. This point llas been repeatedly illustrated in the studies of Tommila (3) and of \Tiinstein and Fainberg (4) and may be attributed to the compensating effect of solvent reorganization on AH+ and AS*. Hence it was froin the latter terius and particularly from the values of AC,,* that inore detailed inforillation about the reactions was expected. Even though the relation bet\\-een changes in these paranleters and mechanism is not as yet defined, these values should provide both an indication and a guide to the developnlent of an acceptable li~.pothesis.The basic aim here \\-as to explore the pattern of values for a series of compounds rather than to investigate the temperature dependence of the rates for a few substrates \\-ith highest precision. T o achieve this aini certain approximations were accepted, but tllere is no evidence that these approximations introduce significant differences in the derived parameters (see Experimental section). ESPERIi\~IENrl'XL I-lnlidesl'he halides \\.ere reagent grade samples, p~lrified by distillation to give physical constants in agreemer~t \vith the literature and passed through alumina for adsorption. These purified samples were protected fro111 light and refrigerated during the Itinetic s t~~d y .'l'he final criteria for Itinetic purity \vas adherence to first order linearity. Because of the lo\v solubility, particularly for the chlorides and iodides, the experitncntal diflicultics here were much greater than for the correspo~iding sulfonates or even the rneth1.l halides (1) and the esperime~ltal errors were correspondingly increased. Szllfn?7atcsEthyl p-toluenesulfonate w s obtained from distillation products and recrystallized several times fro111 the melt; m.p. 32