Kinetic Solvent Isotope Effect for the Spontaneous Solvolysis Acetic and Propionic Anhydrides

Abstract

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.

Bond Scission in Sulfur Compounds. V. A Kinetic Study of Sulfur–Oxygen and Sulfur–Chlorine Bond Scission in the Methanolysis of p-Nitrophenyl Chlorosulfate

“…In the case of S-Cl scission, the entropy of activation is negative, as reported for the hydrolysis of sulfonyl chlorides (15)(16)(17). Though the greater solvation of the incipient chloride ion, compared with the aryloxide ion, in the transition state will tend to lead to a more negative AS* in the former case, it appears unlikely that such differential solvation would account for a difference in AS* of about 20 e.u.…”

Neutral Solvolysis of Acyl Carbon and the Temperature Dependence of the Kinetic Solvent Isotope Effect

“…If K, was estimated to within + lo%, then the error introduced into k0 was less than 0.1%. For the cyclic anhydrides which produced dicarboxylic acids the effective ionic concentration was estimated by using the K, (1) values, hereto, tests indicated this approximation produced no significant error in kO.…”

“…Water and D 2 0 were purified by methods re~orted previously (1). In all experiments the % D exceeded 99.7Z D. ~i r ; t order rate constants for the reaction of each substrate in H 2 0 and D 2 0 were determined over the widest conveniently attainable temperature range by the conductance method used in the previous study ( I ) and treatment of the temperature dependence of these rates and derivation of pseudo-thermodynamic parameters followed previous formalism.…”

Thermodynamics of basic ionization of some aminoethers in water

“…Some time ago Rossall and Robertson (1) reported that the substitution of a y -M e 0 group in benzenesulfonyl chloride resulted in a more positive temperature coefficient of the enthalpy of activation (AC,') of the solvolysis reaction in water than would be expected from other data (2). This positive shift in AC,* was attributed to a temperature dependent solvation effect, a rationalization supported by reference to corresponding effects found for ACPao for the aqueous ionization of anisic acid (3,4).…”