Kinetics of the hydrolysis of acyl chlorides in pure water

Abstract

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...

Correlation of the Rates of Solvolysis of t-Butyl Fluoroformate Using the Extended Grunwald-Winstein Equation

“…12 The value for methanolysis of 1 of k MeOH /k MeOD = 1.26 is similar to a solvent isotope effect value for unimolecular pathway of i-PrOCOCl (footnote Table 3). 10 A powerful test in considering detailed mechanisms of solvolysis is to carry out a correlation analysis using the GrunwaldWinstein equation (eqns. 1 and 2).…”

The kinetics of the reactions of chlorodithioformate esters with azide ions in 70% aqueous acetone

“…However, the results d o not allow conclusions to be drawn concerning the mechanism of the bimolecular process. For solvolysis in 70% aqueous acetone, the entropies of activation for reaction by the S, 1 (AS* cv -2 to (9,10) to enable a choice to be made. Unfortunately, the standard state to which the activation parameters for azide attack refer (Table 4) is not the same as for solvolysis.…”

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

“…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.…”