Kinetics of the acid-cleavage of some organomercuric compounds in pure and mixed solvents: Part I methanol water mixtures

ABSTRACT

The rates of reaction of a series of trans-2-hyrdoxy and 2-alkoxy. cyclohexylmercuric bromides with hydrochloric acid in methanol-water mixtures have been investigated. It was found by plotting the rate constant K[r] as a function of mole percent methanol, that the plots exhibit minima whose locations are disclosed within the range 30-75 mole percent methanol. The rate constant values were used to calculate the number of water molecules involved in the transition states. The least distance of approach between solvated hydrogen proton and trans-2-ethoxycyclohexylmercuric bromides were calculated from linear plots of logarithm of rate constant versus reciprocal of dielectric constant at 288 degree, 293 degree, and 298 degree K. The rate constants are closely related to Hammett's acidity function of the reaction solution in accord with bimolecular mechanism suggested. From the validity of Taft's and isokinetic relationships in methanol water mixtures, we assume a single deoxymercuration mechanism for all compounds under investigation. Activation energies [E[a]] together with enthalpies [delta H], entropies [delta S], and free energies delta G of activation have been computed for the organomercurials under investigation. The correlation of these values with solvent composition are discussed in terms of changing solvation of the polar transition state. The common mechanism of the deoxymercuration reaction of organomercurials under investigation in all methanol-water matures is shown by the presence of a compensating effect between the enthalpies of activation [delta H] and entropies of activation delta S] The feasibility of Taft's equation is verified and fairly linear plots were obtained at all methanol-water mixtures