The kinetics of the addition of ethanol to p-chlorophenyl isocyanate in diethyl ether solution in the presence of covalent metal halides

Abstract

The effects of the presence of BF₃, ZnCl₂, SbCl₃, PhSnCl₃, (Buⁿ)₂SnCl₂, and (Buⁿ)₃SnCl on the kinetics of ethanolysis of p-chlorophenyl isocyanate in diethyl ether solution have been studied at 25 ° C. The covalent halides rapidly form 1 : 1 adducts with ethanol in ether solution and the formation constants, K, are 350 ± 100,205 ± 30, 10 ± 2,61 ± 5,1.5 ± 0.2, and 0.50 ± 0.10 dm³ mol^–1, respectively, at 25 ° C. Under most of the concentration conditions studied, the 1 : 1 adduct and the isocyanate lead to the urethane product in a reaction (rate constant k_a) that is first order in each reactant. Except for the boron trifluoride adduct, this reaction always leads to catalysis of the alcoholysis and the value of k_a generally increases as that of K decreases; with the butyltin chlorides the spontaneous alcoholysis becomes negligible compared with the catalysed reaction. In solutions containing a sufficient concentration of free covalent halide, departures from the foregoing simple kinetic pattern occur. Our results and the mechanism of the alcoholysis are discussed in the light of previous work.