The large kinetic solvent isotope effects for the neutral hydrolysis of acetic and propionic anhydride show unusual temperature dependency; the former passing through a maximum at about ISo, the latter showing a minimum at 30". This unusual temperature dependency is the consequence of widely different values of the apparent ACp* in H 2 0 and D 2 0 : the value for acetic anhydride in H,O being -74 f 2 cal deg-' mol-' but -32 + 4 in D 2 0 . The corresponding values for propionic anhydride being -31 + 2 in H 2 0 but -94 f 10 in D20. The implications of these differences are discussed.L'importance des effets isotopiques cinktiques dans l'hydrolyse en milieu neutre d'anhydride acetique et propionique depend Ctrangement de la temperature; le premier passe par un maximum a environ 15" tandis que le dernier presente un minimum a 30". Cette dependance inhabituelle par rapport a la temperature est la conskquence des valeurs tres differentes du ACp* apparent dans H 2 0 et D 2 0 : la valeur pour l'anhydride acetique dans H 2 0 etant de -74 t 2 cal deg-' mol-' mais de -32 + 2 dans D20.Les valeurs correspondantes pour I'anhydride propionique sont de -31 k 2 dans H 2 0 mais -94 f 10 dans D 2 0 . Les consequences de ces differences sont discutees. Canadian Journal of Chemistry, 49, 3665 (1971) IntroductionThe spontaneous hydrolysis of carboxylic acid anhydrides have been widely studied (1-10). The reaction is characterized by large kinetic solvent isotope effects (k.s.i.e. = kHZo/kD2,) and large negative entropies of activation (AS*). Such results were interpreted as being consistent with the requirements of a general base catalyzed mechanism, BAc2, and were taken to imply (1 1) that more than one water molecule was tightly constrained in the transition state. Our interest in the effect of temperature on solvolytic rates led initially to an investigation of the temperature dependency of the large kinetic solvent isotope effect for acetic anhydride.The experimental problem simply required that we extend the careful investigation of Gold (5) over a wide temperature range and obtain a complementary set of rate data in D,O. The results were so surprising that we were led to extend our investigation to study the hydrolysis of propionic anhydride and to other anhydrides. In this paper we report our findings for the hydrolysis of acetic and propionic anhydride.

Rates of solvolysis of methanesulfonyl chloride and benzenesulfonyl chloride have been determined in H 2 0 and D,O. The free energy, enthalpy, entropy, and heat capacity of activation were calculated. The exceptional accuracy of the data permitted an estimation of dAC,*/dTfrom a four parameter temperature dependence of the kinetic rates.From these data we conclude that both sulfonyl chlorides hydrolyse by the same mechanism (SN2) The change in R from CH3 to CsH5 in RS02CI did not alter AC,* but AS* (20') was changed from -8.32 to -13.25 cal deg-' n~o l e -~, resoectivelv. The significance of this difference is attributed to the probability of bond formation rather than to differences-in solvent reorganization.Canadian Journal of Chemistry, 47. 4199 (1969) This paper is the first of a series reporting detailed studies of the hydrolysis of sulfonyl chlorides5. The experimental approach is the same'as that applied in preceding studies on the halides and sulfonates: a careful examination of the effect of temperature and of isotopic substitution in the solvent on the temperature dependence of the rate of hydrolysis.In the halide series we found a remarkable similarity in the values of the heat capacity of activation, AC,,' (lo), and in the differences in AG* for hydrolysis in H,O and D,O over a range of structures for which it was reasonable to expect different degrees of bond-making in the activation process (11). These observations led to the conclusion that the SN2 transition state was reached when the charge on the halide being displaced created interaction with the solvent equal to water-water interaction; i.e. the transition state corresponded to that degree of charge development where exothermic solvation of the quasi-halide was about to begin. Consequently, it followed that for a given temperature, the charge level (ti-) on the chlorine atom, for example, is the same whether the ion is being displaced from allyl, ethyl, or methyl chloride. This in turn implies that the charge level on the anion at the transition state of an SN2 displacement is independent of the degree of nucleophilic interaction. The evidence in favor of this con-'NRCC No. 10 915. ZPresent address: Chemistry Department, The University of Calgary, Calgary, Alberta. 3NRCC Postdoctoral Fellow, 1968-1969. 4NRCC Postdoctoral Fellow, 1965. 5For references to previous work, see refs. 1-9. clusion derived from the similarity in AC,* values and from the limited range of 6 4 H * = AHD2o* -AHHzo* for SN2 displacement for the hydrolysis of alkyl halides, is weakened by the possibility that the difference in the degree of bond-malung at the transition state for the hydrolysis of the available halides is small, a possibility suggested by the uniformity of our kinetic solvent isotope effect (k.s.i.e.) values (10).To fill this deficiency we were led to examine the hydrolysis of the sulfonyl chlorides, since in this series there are good indications that a greater degree of nucleophilic overlap is required at the transition state (10, 12). We assumed ...

The temperature dependence of the rate of hydrolysis of benzoic, phthalic, and succinic anhydrides have been determined in H2O and D2O under "neutral" conditions. Corresponding data have been obtained for methyl trifluoroacetate. While both series supposedly react by the same BAc2 mechanism, remarkable differences are made obvious by this investigation. Possible sources of such differences are proposed.

The enthalpies and entropies of ionization in methanol of 4-t-butylphenol, phenol, 4-bromophenol , 4-formylphenol, 4-nitrophenol and their 2,6-di-t-butyl substituted analogues have been measured. 3-Nitrophenol, 2-t-butylphenol and 2,4-and 3,5-di-t-butylphenol have also been studied. The influence of substituents on the acidity of the unhindered phenols is primarily governed by the changes in the phenol OH bond dissociation energy. However, the effect of solute-solvent interactions on acidity is considered to be more iniportant in methanol than in water. Solute-solvent interactions are probably the predominant factor which decides the increment in pK, produced on substituting ortho t-butyl groups in a phenol. may be fitted to the Hammett relationship with p = 2.229. However, the ionization constants of 2-substituted phenols often do not conform with this result because of the steric requirements of the substituents near the acid site in the rn01ecule.~-~ The pronounced acid weakening effect of bulky 2-substitutents has been primarily explained in terms of steric inhibition to solvation of the phenol a n i o n ~. ~-l ~ Knowledge of this entropies of ionization of sterically hindered phenols would be useful to test this interpretation.Enthalpies and entropies of ionization of several phenols in water are considered by Hepler et aZ.l3-l8 in terms of equilibrium (1) in ' I

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