Synthetic and mechanistic studies in polynitroaromatic chemistry.

2,2',4,4',6,6'-Hexanitrostilbene (HNS) is a heat resistant explosive which is also important as a crystal growth modifier for melt-cast TNT, This work has studied a new method of preparation of HNS, by catalytic oxidation of TNT, to identify the parameters which are significant and to contribute to an understanding of the mechanism involved. A typical reaction is marked by several colour changes, from brown to purple to red-brown then blue. Visible and 1HNMR spectroscopy, hydrogen-deuterium exchange experiments and electrochemical methods have been used to identify intermediates involved in the reaction. The colours have been assigned to a 3-O-complex of TNT, the trinitrobenzyl anion, 3-O- and 1-O-complexes of 2,2',4,4',6,6'-hexanitrobibenzyl (HNBB) and the HNS radical anion respectively. 1HNMR implied the presence of radicals in the system. It was also shown that HNS can form a O-complex on reaction with base and that HNBB will ionise in basic solution with loss of an aromatic proton.HNBB was the main product of the catalytic oxidation of TNT at room temperature, with HNS produced when the reaction mixture was warmed to 55°C. Without catalyst the yield of HNBB and HNS was poor and a more complex mixture of by-products resulted. Oxygen was vital for a good yield of HNBB, a polar, aprotic solvent was also important. The best yield of HNBB came from using sodium methoxide as base in an equimolar amount with TNT. Of the catalysts investigated, a copper sulphate-pyridine slurry was preferred. Dehydrogenation of HNBB to HNS required less base than the oxidation of TNT to HNBB and proceeded without oxygen. The catalytic oxidation of TNT to HNS showed no particular advantages over the established process of HNS production.A probable pathway for the oxidation reaction was proposed which involved a radical mechanism with electron transfer from the trinitrobenzyl anion to an electron acceptor.