TntroductionAs long ago as 1846, Liebig and Wohlerl showed that cyanuric acid depolymerizes on being heated to yield isocyanic acid according to equation ( 1 ) : OH -3HNCOHowever, because the HNCO formed is difficult to keep, the investigation of its properties in the intervening years has been fragmentary. Yet HNCO is often formed when compounds containing the amide group (e.g. NH2CONHz2 and NHzCON:NCONH2 3, are heated, as they often are in industrial processes. An understanding of the nature and properties of isocyanic acid is therefore of considerable importance. The purpose of this review is to bring together what is known and not known about this simple and interesting compound. PreparationA. Depolymerization of Cyanuric Acid.-Liebig and Wohler's method,l shown in equation (1) was further developed, to give a purer product and better yield, by Linhard4 and by Herzberg and Reid.5 Cyanuric acid, on being heated, tends to sublime without reaction. In order to achieve depolymerization, heat must first be applied to the solid to vaporize it and then the resulting vapour must be heated still further.Herzberg and Reid5 gave the cyanuric acid a preliminary heating in vacuo to 200°C in order to remove water. The solid was then volatilized slowly in vucuo at 450 "C and the gas stream was passed through a tube furnace at 700 "C. The bulk of the HNCO was collected in a trap at -80°C. After oxidation by

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A stopped-flow spectrometer has been used to investigate the mechanism of the nitration of toluene, and to determine kinetic parameters for the formation and reaction of nitronium ion, in 77.3 and 78.45 wt.% sulphuric acid. The protonation of nitric acid t o form nitronium ion and water is found to occur at similar rates at both acid concentrations and to have an activation energy of 18.3 t 4.0 kcal mol-l. The activation energy of the reverse reaction, the heat of the reaction, and the activation energy of encounter complex formation between nitronium ion and toluene are estimated to be respectively 10.5 + 4.0, 7.8 F 0.5, and 5.9 f 0

The thermal decomposition of solid sodium bicarbonate has been studied in the temperature range 360-500 K over a range of partial pressures of carbon dioxide. The effect of water vapour has also been studied. Above 440 K the reaction follows contracting-cube kinetics with an activation energy of 32 kJ mol-l and a frequency factor of lo1 s-l. In this temperature range the presence of water or carbon dioxide has little effect on the kinetics. Below 390 K the reaction follows first-order kinetics. In nitrogen, the activation energy is ca. 64 kJ m o P , the frequency factor is lo5 s-l and water vapour has little effect. High partial pressures of carbon dioxide increase the activation energy to ca. 130 kJ mol-l and the frequency factor to 1013e5 s-l.The results of microscopic examination generally confirm the kinetics but show that at low temperatures in nitrogen and carbon dioxide the processes are different in detail.A number of publications have dealt with the thermal decomposition of solid sodium bicarbonate, under various experimental conditions. These include the use of and c l o ~e d ~? ~ sample containers as well as variation in the composition of the atmosphere round the Both isothermal5? 9 9 lo and temperature-programmed heatingll, l2 regimes have been used, and several fluidised-bed studies have been