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 ka) 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 ka 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.