The Temperature Dependence of the Solvent Isotope Effect 1

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 values of the constants are given in Table 111 together with the derived parameters AH*, AS+, and AC,+ a t 25 OC (see ref. 9).…”

Section: Results and Discussioymentioning

confidence: 99%

“…The kinetic data were obtained using a conductance method (9) and employing the relation log (l/Rz -1/R1) = kt -C.…”

Section: E X P E R I M E N T a L Mute~ialsmentioning

confidence: 99%

Kinetics of the hydrolysis of acyl chlorides in pure water

Queen¹

1967

“…The conductance methods used in these studies have been previously described (2,12) as have the methods of calculating the rate constants. For the reactions in aqueous acetone, the resistance readings were converted to concentrations by means of large scale calibration curves.…”

Section: Measurernet~l Of Ra/esmentioning

confidence: 99%

Kinetics and Mechanism for the Hydrolysis of Chlorothionoformate and Chlorodithioformate Esters in Water and Aqueous Acetone

McKinnon¹,

Queen²

1972

The effects of structural changes on the rates of hydrolysis of a series of chlorothionoformate esters and the analogous chlorodithioformate esters have been studied. For both classes of compound, the reactivity is enhanced by increased electron donation by the hydrocarbon group. These results, the activation parameters for the hydrolyses of the methyl compounds, and the solvent isotope effect are shown to be consistent with the operation of the SN1 mechanism.Les elTets de modifications structurales sur les vitesses d'hydrolyse d'une serie d'esters chlorothionoformates et d'esters chlorodithioformates analogues, ont ete etudits. Pour les deux classes de composes, la rtactivite est accrue si l'on augmente le pouvoir donneur d'tlectrons du groupe hydrocarbure. Ces resultats concernant les parametres d'activation pour les hydrolyses des composes methylts et I'elTet isotopique du solvant, sont consistants avec l'intervention d'un mecanisme SN1.Canadian Journal of Chemistry, 50, 1401Chemistry, 50, (1972 Introduction Previous studies (1-3) have established that chloroformate esters (1 ; X = Y = 0 ) and chlorothiolformate esters (1 ; X = S, Y = 0 ) hydrolyze much more slowly than other acid chlorides, a feature that is most reasonably attributed to initial state conjugation, 2. This effect is enhanced by structural changes that increase electron donation by the hydrocarbon group (R) (3,4) and is diminished in compounds where the hetero-atom (X) has only a limited tendency to use its unshared electrons for n-bond formation. Thus, chloroformates are much less reactive than their thio analogues (3).

“…Rates were determined by the conductance method described previously (6). Details of temperat~lre control, determillati011 of time, and calculations were identical with the earlier worlc, save xvhere noted specifically.…”

Section: Rate Deternzinationsmentioning

confidence: 99%

EFFECT OF Α-Methylation ON THE PARAMETERS CHARACTERIZING HYDROLYSIS IN WATER FOR a SERIES OF HALIDES AND SULFONATES

Heppolette¹,

Robertson²

1966

Self Cite

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