Acylation. Part XXXIV. The kinetics and mechanism of metal ion-promoted hydrolysis of S-esters, -acids, and -anhydrides in aqueous solution

Abstract

We report a comparative study of the mercuric ion promoted hydrolysis of three S-ethyl p-substituted thiobenzoates (p-RC₆H₄·CO·SEt), of thiobenzoic acid, and of dibenzoyl sulphide. The stoicheiometric equations for the last reactions have been established for the first time; the mercuric ion is converted into mercuric sulphide. In an excess of an essentially aqueous solvent the rate equations all have the form –d[Thio-deriv.]/dt=k₁[Hg²⁺][Thio-deriv.]. For the esters the substituent effects (p-MeO pH > p-NO₂), the values of the activation parameters, and the solvent isotope effects, all suggest that when R = NO₂ the hydrolysis mechanism is A_Ac2, but that when R = MeO (and probably when R = H) the mechanism is A_Ac1. For the S-acid and -anhydride the kinetic evidence is more compatible with a uni- than with a bi-molecular mechanism. At 8 °C the values of k₁ for PhCO·SEt, PhCO·SH, and (PhCO)₂S are in the approximate ratio 1 : 600 : 38,000. In each case Hg²⁺ is ca. 10⁶-fold more effective than H₃O⁺ in assisting the hydrolysis.

The assignment of mechanism for the esters is supported by results using promotion by silver ion. The rate equation now takes the form: –d[Ester]/dt={k₁[Ag⁺]+k₂[Ag⁺]²}[Ester]. A significant term in [Ag⁺]² is found only when R = MeO. The effect of substituents on k₁ is much less marked than, and in the opposite sense from, that observed for Hg²⁺ promotion. These facts, and the values of the activation parameters, identify the hydrolysis mechanisms with Ag⁺ as A_Ac2 for all the esters.

The ions Ag⁺ and Hg²⁺ are ineffective with O-analogues of the S-esters and owe their efficacy to their relatively great affinity for sulphur compared with oxygen (i.e., to their class B character). Experiments with other class B ions (Cu²⁺, Ni²⁺, Pb²⁺, and Cd²⁺) show, however, that these ions are apparently entirely without effect, and certainly less effective than H₃O⁺.