The hydrolysis of acetic anhydride

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.

Section: Introductionmentioning

confidence: 99%

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

Robertson¹,

Rossall²,

Redmond³

1971

“…Rloreover, values reported for the neutral hydrolyses of esters (16), anhydrides (17), and acid chlorides (18), reactions which have been shown, a t least in some cases (19), to involve reversible addition of water to the carbonyl group. Even stronger support is provided by the solvent deuterium isotope effects (Table VI) …”

mentioning

confidence: 99%

Kinetics of the hydrolysis of acyl chlorides in pure water

Queen¹

1967

The activation parameters AH', AS*, and AC,' for the hydrolyses of a series of alkyl chloroformates and dimethylcarbamyl chloride in water have been determined. The results indicate that, with illcreasing electron donation to the chlorocarbonyl group, the mechanism changes from bimolecular to unimolecular (SN~) displacement a t this position. For isopropyl chloroformate, some co~lci~rrent alkyl-oxygen bond fission is also indicated. The bimolecular mechanism involves reversible addition of water t o the carbonyl group followed by ionizatiotl of the carbon-chlorine bond.Canadian Journal of Chemistry. Volume 45. 1619 (1967) As a conseauence of increased initialstate conjugation, chloroformate esters undergo solvolysis much more slowly than other acyl chlorides. Only a fen. quantitative studies of these reactions have been reported (1)(2)(3)(4)(5)(6). T h e rate sequence for methanolysis, which leads to mixed carbonate esters ( 2 ) , is ClC2H4-0-CO-Cland is consistent with bilnolecular displacement involving acylhalogen bond fission. T h e opposite order for the corresponding reactions with formic acid reflects the operation of ionization processes invoiving either acyl-halogen or alkyl-oxygen bond breaking (4). Yet a third sequence, Ale-0-CO-CIfor hydrolysis in aqueous acetone (4) has been interpreted as indicating a change from bimolecular to unimolecular hydrolysis across the series. T h e details of the two mechanisms have not been firmly established, although experiments with cycloalkyl chloroformates ( 5 ) have indicated that. in the unimolecular reactions. anv , a alkyl-oxygen fission probably occurs after rate-determining ionization of the acylhalogen bond.T o obtain further information about the hydrolytic reactions, a careful investigation of the solvolyses of several chloroformate esters and dimethvlcarbamvl chloride in pure water has been carried out over wide ranges of temperature. There were 'Present address: Department of Chemistry, University of Manitoba, \I7innipeg, Manitoba. several reasons for choosing this solvent, not the least important being the great accuracy with which rates may be determined (7) and elinlination of the possibility of nucleophilic intervention by the organic components of mixed solvents (8). RESULTS AND DISCUSSIOYThe rate data for the hydrolyses of phenyl, methyl, ethyl, propyl, and isopropyl chlorofortnates and dimethylcarbamyl chloride in xater a t a number of teinperatures are given in Tables I and 11 and represent average values of several independent experiments a t each temperature. By follo~ving the practice of Robertson and his co-workers (9), the rate data over the whole temperature range were fitted by the method of least squares to the empirical three-constant equation\vhere T is the ternperature on the absolute scale. For ethyl and propyl chloroformates, this procedure was not justified (10) because the heat capacities of activation (A&+) varied in a complicated manner with temperature (Fig. 1). Holvever, in both cases the changes in this parameter were sma...

“…The data for ethyl p-toluene sulphonate in water were determined by a conductance method essentially as described by Gold (10).…”

Section: Titrationmentioning

confidence: 99%

“…Elimination in solvolysis of these esters was considered t o be negligible for the purpose of this conlparison since i t was shown in preliminary runs both by a titration method (23) previously calibrated against a known, and by a fractionation of products and estimation of the propylene with a mass spectrograph,** that the amount of elimination in the presence of 0.025 A f NaOEt was For personal use only. 10-1270 for isopropyl p-~netllyl benzenesulphonate in dry ethanol a t 50°C.…”

mentioning

confidence: 99%

Reactions of Sulphonic Esters: I. A Kinetic Study of the Effect of Substituents for a Series of Sulphonic Esters

Robertson¹

1953

ICi~ietic data for the solvolysis of parasubstituted ~nethyl, ethyl, and isopropyl benzenesulphonates in ethanol, methanol, methanol-water, and dioxane-water mixtures have bee11 determined. I t has been showr~ that the effect of the ionizing properties of the solvent of a-methylation, and of the addition of a strong base show the sarlie general characteristics which for the halides have been consideretl diagnostic of a change i l l mechanism from a bimolecular solvolysis to a reaction in which ionization is rate controlling. The changes in the relative effect of para-substitnents which accornpariy the apparent change of mechanism are examined. The greater effect of the para-NO? substituent over the meta-NOr substituent for the isopropyl esters under ionizing conditions is attributed to the contribution of resonance stabilization in forming ion. This is the first paper of a series in which we propose to examine in sonle detail certain characteristics of reactions involving the displacement of the benzenesulphonate group in the generalized equationwhere Y is a nucleophilic agent which may or may not be charged, R is an alkyl group, and X is a para-substituent.In this paper we shall report the results of an investigation of the changes in the relative substituent effects for a series of methyl, ethyl, and isopropyl esters in a series of solvents of different ionizing properties, where other evidence has suggested that a possible change in mechanism may occur. Significant changes would be expected in the relative effect of substituents a t X if structural changes in R were such as to change the mechanism of the reaction from one having the characteristics of a bimolecular substitution (SN2) to a reaction in which the rate-controlling step is an ionization (SNl) (1, 18).The evidence that the alkyl benzenesulphonates react by an SN2-SN1 type of mechanism rather than by some alternative mechanism (29), the problems involved in the shift from one mechanism to the other (34j, ancl an investigation of salt effects properly belong to another publication. In this paper we shall limit our discussion of mechanism to the establishment of the correspondence between the kinetic effects of changes in structure, reagent, solvent, and substituents in the benzenesulphonate series with those observed for the organic halides for which the SN2-S,l classification was first established (15), and for the alkyl nitrates recently discussed by Baker and Easty (2). The latter show the same evidence for a change of mechanism 'as the alkyl halides, and like the sulphonates, usually involve the breaking ol a C-O bond in the transition state.Swain and Langsdorf (31), in a recent study of relative effects of para-1 Manirscript