Rate constants for the title reaction have been measured in both forward and reverse directions leading to rates and equilibrium constants for all the steps in equation (i). The effects of varying the structures of A r A r -0 -C S -N H A r ' e A r -0 -C C N -A r ' s Ar-0- ExperimentalMateriafs.-0-4-Methoxyphenyl chlorothioformate was prepared by adding a solution of 4-methoxyphenol (8.3 g, 67 mmol) in 5% NaOH (60 ml) dropwise to a well stirred, cooled solution of thiophosgene (7.7 g, 67 mmol) in chloroform (40 ml). The reaction was stirred for 1 h at 0-5" and the chloroform layer washed with dilute HCl and water. The chloroform solution was dried with sodium sulphate, evaporated, and the residue distilled to yield a liquid, b.p. 92" at 0.5 Torr (lit.,3h 141-143" at 13 Torr), which solidified on cooling.0-4-Met hoxyphenyl N-arylthioncarbamates were prepared by adding the aniline (32 mmol) dissolved in carbon tetrachloride (5 ml) to a stirred, cooled solution of O-(Cmethoxyphenyl) chlorothioformate (3.3 g, 10 mmol) in carbon tetrachloride (50 ml). The mixture was stirred at room temperature for 24 h, filtered, and the filtrate evaporated to dryness. The thioncarbamate was recrystallised from a suitable solvent (see Table 1).Aryl isothiocyanates were prepared by adding a slight excess of thiophosgene to a mechanically stirred mixture of the appropriate aniline and water. Reaction was almost instantaneous and the isothiocyanate separated as an oil and was extracted with ether. The ether was dried (Na,SO,), evaporated, and the residual oil distilled under high vacuum. The materials prepared above had satisfactory analyses (Table 1) and their structures were confirmed by i.r. (Perkin-Elmer 297 instrument) and 100 MHz 'H n.m.r. spectra. The latter spectra were recorded by Dr. D. 0. Smith using a JEOL instrument, Buffers and reagents used in the kinetics were of analytical quality or were recrystallised or redistilled from bench grade materials. Water used throughout the investigation was distilled twice from glass.Method;F.-Kinetics were followed spectrophotometrically at an. appropriate wavelength determined using a repetitive wavelength scanning spectrophotometer (Unicam SP 800). Constant wavelength studies were carried out using a Unicam SP 800 machine or a Beckman D.B. instrument fitted with Servoscribe potentiometric recorders. The stock solution of substrate (thioncarbamate or isothiocyanate) in ethanol or

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N-Cyanourea is demonstrated as an intermediate in the hydrolysis of dicyanamide which is shown to react as its carbodi-imide tautomer. The hydrolysis of N-cyanourea, dicyanamide, and "'-dimethylcyanoguanidine is specific acid-catalysed. General acid catalysis is demonstrated for the hydrolysis of NN'-dimethylcyanoguanidine and is considered to be specific acid-nucleophilic. Assuming the carbodiimide mechanism also holds for the specific acid-catalysed hydrolysis of cyanamide the tautomeric equilibrium constant for formation of the parent ca+rbodi-imide in water at 25" is estimated to be 0.6 x 1 O-'

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