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 effects of structural changes on the rates of hydrolysis of a series of thiochloroformate esters in water have been investigated. The reactivity is enhanced by increased electron donation by the hydrocarbon group. These results, the activation parameters for the hydrolysis of methyl thiochloroformate and the solvent deuterium isotope effect, are shown to be consistent with the operation of the SN1 mechanism.Canadian Journal of Chemistry, 48, 522 (1970) Introduction It is well known that alkyl chloroformates (1; X = 0) undergo solvolytic reactions much more slowly than other acid chlorides (1, 2) and also seem to find the S,1 mechanism unfavorable. Two factors are responsible for these characteristics.( i ) The initial states are stabilized by conjugation (2; X = 0) and the effect is enhanced by structural changes that increase electron donation by the alkyl group. This is shown by the rate sequence

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).

Alkylated pyrimidine nucleosides are of interest from the viewpoint of mutagenesis and carcinogenesis. 1H and 13C nmr data are presented for a series of 2′-deoxynucleosides methylated at the O2-, O4-, and N3-positions of the base, and discussed in terms of their physical properties. The pH dependence of the stability of the O2- and O4-methylated 2′-deoxyribosides as well as the corresponding ribosides was examined by 1H nmr and ultraviolet (uv) spectrophotometric methods.

Esters of fluoroformic acid hydrolyse more rapidly than the corresponding chloroformate esters, suggesting that carbon-halogen bond breaking is not greatly advanced in the rate-determining step. The rate of reaction of 4-methoxyphenyl fluoroformate with aqueous dioxan is markedly decreased in the presence of acetate ions whereas the rate of reaction of 4-methoxyphenyl chloroformate is increased. The rate curves for the latter reaction are sigmoid in shape. The results are interpreted in terms of the formation of an intermediate mixed anhydride for both reactions, which then undergoes nucleophilic attack by acetate ions to yield acetic anhydride and the phenol. This step is rate determining in the case of the fluoroformate.