Novelties of solid–liquid phase transfer catalyzed synthesis of o-nitrodiphenyl ether

Abstract

o-Nitrodiphenyl ether is an important intermediate in the fine chemical industry and used in a number of drugs. This ether is typically prepared from o-chloronitrobenzene (OCNB) by condensing with alkali metal phenoxides in toluene or xylene in presence of copper or cuprous chloride and the process requires a high temperature to initiate the formation of cuprous salt of phenol. Once initiated the reaction is exothermic and can sometimes become uncontrolled leading to the formation of tarry masses. In the current work synthesis of o-nitrodiphenyl ether was accomplished by reacting o-chloronitrobenzene with solid potassium phenoxide using tetra-n-butylphosphonium bromide as a catalyst under solid–liquid phase transfer catalysis (S–L PTC). The advantages of S–L PTC are that the reaction is conducted at controllable temperatures, the rates of reaction are increased by orders of magnitude and the reaction is 100% selective, in comparison with the liquid–liquid (L–L) PTC which is very slow and produces by-products. The mechanism based on homogeneous solubilization of solid resulting in the formation of an active ion-pair with the nucleophile was found to prevail in the system. A complete theoretical analysis is done to determine both the rate constant and equilibrium constant from the same set of data. The reaction is intrinsically kinetically controlled. The solid–liquid PTC reaction is 100% selective with very high rates of reactions vis-à-vis the liquid–liquid PTC process.