The kinetics and mechanism of the spontaneous alcoholysis of p-chlorophenyl isocyanate in diethyl ether. The association of alcohols in diethyl ether

Abstract

The self-association of four simple aliphatic alcohols in ether solution has been investigated. In the concentration range 0·1–1·2 mol dm–3 the principal polymer is in every case the dimer. At 25 °C the dimerisation constants K2(=[dimer]/[monomer]2) have the values 0·49, 0·71, 0·50, and 0·72 for MeOH, EtOH, PriOH, and BunOH respectively. The kinetics of the spontaneous alcoholysis of p-chlorophenyl isocyanate by the same alcohols in ether solution has also been studied. The observed rate equation is –d[isocyanate]/dt= d[product]/dt={k2[M]2+Kn[M]n}[isocyanate] where [M] represents the alcohol monomer concentration, and n= 3 or 4 depending on the structure of the alcohol involved. The second-order term represents a route via dimeric alcohol. The various dimers have similar reactivities, the bulkier Bun and Pri substituents leading (at 25 °C) to species with about half the reactivity of dimeric methanol and ethanol. The higher-order term probably represents a route via trimeric or tetrameric alcohol. The contributions of the second- and of the higher-order terms to the overall rate are comparable, the latter dominating at high stoicheiometric concentrations. The probable mechanism of urethane formation is a slow, one-step cyclic addition of alcohol polymer to the isocyanate and is analogous to the mechanism of alcoholysis of ketens. The fact that alcoholysis of isocyanates involves, in almost all solvents, a kinetic order >1 in the stoicheiometric alcohol concentration arises from the generally greater reactivity of alcohol polymers than of monomers, and the circumstance that the relative concentration of the former inevitably rises as the stoicheiometric alcohol concentration increases.