Kinetics and mechanism of the acetate catalysed hydrolysis of 4-methoxyphenyl chloroformate and 4-methoxyphenyl fluoroformate in aqueous dioxan. Evidence for rate-determining attack of acetate ions on an intermediate mixed anhydride

Abstract

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.

Application of the Extended Grunwald-Winstein Equation to Solvolyses of n-Propyl Chloroformate

“…There is probably general-based catalysis [20,21] to the nucleophilic attack, leading to a relatively high l value of 1.7 for phenyl chloroformate solvolyses. The addition-step is believed to be rate-determining, largely on the basis of the very similar rates for fluoroformates and chloroformates, despite the much stronger carbon-fluorine bond [22].…”

Application of the Extended Grunwald-Winstein Equation to Solvolyses of n-Propyl Fluoroformate and a Consideration of Leaving Group Effects

“…Favoring 4 , suggesting that a value of less than unity would be observed upon incorporation of a bulky secondary alky group. This trend could possibly be governed by increasing steric effects.…”

“…However, replacement of chlorine by fluorine (1-adamantyl fluoroformate 3 ) led to behavior very similar to that previously observed for the solvolyses of n-propyl chloroformate, which have been shown to solvolyze with the addition step of an addition-elimination pathway in all solvents but the most ionizing and weakly nucleophilic solvents. For the bimolecular hydrolyses of a pair of fluoroformate and chloroformate esters, 4 the addition step was believed to be rate determining, largely on the basis of the similar rates for the two halogenoformate esters, despite the stronger carbonfluorine bond. Particularly, the comparison of leaving group effects (k F /k Cl ratios) on the rates of solvolysis of chloroformate and fluoroformate esters has provided useful information about the reaction mechanism.…”

“…[5][6][7][8] For example, k F /k Cl ratios of 1.09 to 7.16 for 70% aqueous acetone at 30.1 o C have been reported. 4 For solvolyses, the two proposed nucleophilic substitution mechanisms can be expressed as an addition-elimination mechanism (Scheme 1) and as an ionization mechanism (Scheme 2).…”

Rate and Product Studies of 1-Adamantylmethyl Haloformates Under Solvolytic Conditions

“…For the ionization pathway, a value of k F /k Cl = 1.3 × 10 24 As mentioned above, the specific rates for solvolysis of fluoroformate are somewhat faster for binary solvents, despite the stronger carbon-fluorine bond. As shown in Table 1, the k F /k Cl ratios for solvolyses of 1 and 2 are similar to those previously reported for all other the primary alkyl substrates.…”