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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.

The a-deuterium isotope effect has been examined for the solvolysis of a series of esters containing a fully deuterated methyl group. The possible sources of the effect have been divided into "thermodynamic" effects which appear to favor more rapid reaction of the protium compound and "zero point" effects where stiffening of out-of-plane vibrations may account for the direction of the observed isotope effects. It appears that the inverse isotope effect may be a measure of the spatial restrictions placed on the hydrogen atoms on the carbon atom in the activated complex.The substitution of deuterium for a-hydrogen in alkyl halides and substituted sulphonates has been shown to produce varying effects on reaction rates. Thus the solvolysis of methyl p-bromobenzenesulphonate in ~nethanol and the reaction with ~nethoxide ion are accelerated by a-deuteration ( 1 ) . Isopropyl bromide reacting with ethoxide ion is apparently unaffected by this substitution (2), and phenyl ethyl toluenesulphonate solvolyzes in formic acid a t a reduced rate when isotopically substituted a t the a-hydrogen (3). Johnson and Lewis systematized these changes, suggesting that they were the consequence of the sensitivity of the C-H bending vibrations to the presence of entering and leaving groups and noted a decrease in the isotope effect, kH/kD, with increasing necessity for nucleophilic attack from the rear ( 1 ) . Fro111 information available a t the time the); were unable to choose between several possible sources for the inverse isotope effect observed for nlethyl esters.Streitwieser and co-workers (4) have observed a reduction in the acetolysis rate for cyclopentyl p-toluenesulphonate followi~lg a-deuteration. They have attributed this to the change in vibrational frequencies involving the a-hydrogen, produced by the difference in hybridization between initial and transition states, together with the effect of the leaving group which was postulated to stiffen the out-of-plane vibrations of the hydrogen.Thus there appear to be three factors for consideration in assessing the zero-point contribution to the change in rate for nucleophilic displace~nents caused by a-deuterium substitution. These factors are: ( a ) the effect of the incoming group, ( b ) the effect of the leaving group, and (c) the change in hybridization.Factors ( a ) and (b) will be related, inasmuch as bulky entering or leaving groups will be expected to stiffen the out-of-plane C-H bending modes. This effect will be modified by distance, but clearly, stiffening will increase the disparity in the energy levels of deuterium and hydrogen compounds in the transition state, and will produce changes tending to reduce the free energy of activation for the deuteriu~ll compound, conlpared with that of the protiu~n compound. Reactions having the highest degree of nucleophilic interaction with large entering groups will presumably have the most stiffening.The change in hybridization will also affect the characteristic frequencies but presents a problem in that even a qualita...

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 ...

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