DAIXS ET AL.: ARYL ISOTHIOCYANATES. 5 



whether the method was really a practical one and to determine if 

 possible the optimum conditions. 



The investigation has shown that the general method suggested 

 by Losanitsch is capable of giving very satisfactory results in the 

 synthesis of aryl isothiocyanates. Yields of mustard oil up to 77 

 per cent based upon the weiglit of the amine have been obtained — a 

 result which is impossible by the usual method. 



REACTIONS INVOLVED IN THE DESULPHURIZATION OF THE 

 ARYL DITHIOCARBAMATES. 



Using aniline as a typical aryl amine the synthesis is best illus- 

 trated by the following reactions: 



I. C6H5NH2 + CSo + NH4OH = C0H5NHCSSNH4 + H2O. 

 II. C6H5NHCSSNH4 + Pb(N03)2 = CeHsNCS + NH4NO3 

 + HNO3+ PbS. 



Equation II does not occur directly, since the addition of the lead 

 nitrate causes the precipitation of the lead salt — 



III. 2C6H3NHCSSNH4 + Pb(NO:j)o = (CcH5NHCSS)2Pb + 2NH4NO3. 



The lead phenyl dithiocarbamate on heating breaks down as 

 follows : 



IV. (CGH5NHCSS)2Pb = C6H5NCS + CeHgNHCSSH + PbS. 



The free phenyl dithiocarbamic acid tends to decompose with the 

 formation of thiocarbanilide, aniline, etc. To prevent this a second 

 mole of lead nitrate is used: 



V. (CoH5NHCSS)2Pb + Pb(N03)2 = 2C6H5NCS + 2PbS + 2HNO3. 



Since the nitric acid diminishes the yield by freeing phenyl dithio- 

 carbamic acid from its NH^ salt, an excess of aimnonium hydroxide 

 is added. The ideal proportions would be: 



VI. 2C6H5NHCSSNH4 + 2Pb(N03)2 + 2NH4OH = 2C6H5NCS 

 + 2PbS + 4NH4NO3. 



For the best results, the solution after the addition of the lead 

 nitrate should be neutral or only slightly acid. An excess of 

 ammonia converts the mustard oil into monophenyl thiourea. 



EXPERIMENTAL. 



PREPARATION AND ISOLATION OF THE AMMONIUM PHENYL 



DITHIOCARBAMATE. 



The following procedure, which is a modification of the method 

 described by Heller and Bauer,* was found to give the best results. 

 Carbon bisulphide (54 gms.) and 28 per cent ammonium hydroxide 



8. J. Prak. Chem. (2) 65, 369 (1902). 



