NOVEMBER, 1950
0
THE PREPARATION OF NITROSOBENZENE P. 5. ROBERTSON and J. VAUGHAN Canterbury University College, Christchurch, New Zealand
THEusual student preparation of nitrosobenzene follows the scheme :
without the isolation of the intermediate phenylhydroxylamine. By following the method given in "Organic Syntheses" (Vol. 25, p. 80) and starting with 25 ml. nitrobenzene the second-year student of organic chemistry can normally obtain a yield of 60 per cent; with 5 ml. starting material the yield is increased to about 65 per cent. There is some thermal breakdown a t the temperature of steam distillation during the final separation of the product, and when the steam-distillation apparatus is not an all-glass one the yield is of course lower due to decomposition losses on the stoppers. (The sum of these losses has been estimated during the present work as about 7 per cent.) Recently a student (P. S. R.), who was carrying out the normal laboratory preparation, inquired whether the high vapor pressure of nitrosobenzene had ever been made the basis of an alternative procedure to steam distillation for purifying the crude product. None of the accessible laboratory texts mentioned any such method although purification by short-path distillation or sublimation should require a lower temperature than that needed for steam distillation. The possibility was investigated as a laboratory assignment, and the results may be of interest to other teachers of chemistry who may be faced with a similar query. Using 5 ml. of nitrobenzene, the final procedure followed was identical with that of "Organic Syntheses"
for the initial reduction and subsequent oxidation of thephenylhydrox-
and introduced into the simple apparatus shown, in which a good water pump could easily maintain the pressure a t 17 mm. he major portion of any moisture retained by the nitrosobenzene apeared to be removed rapidly a t ordinary temSRUDE peratures with no brine MATERIAL flowing through the "cold finger" condenser. The pvdilrinsnm.-tru condenser was then kept a t -lO°C. by gravitational flow of cold brine, and the bulb of the apparatus was half immersed in a water bath maintained a t 65-70°C. Distillation was complete in 30 minutes, which is about 15 to 20 minutes longer than the time required for steam distillation of the same quantity of material. On the stated scale of operation, the yield obtained was about 65 per cent, and the melting point and color of the final material were always a t least as good as those of the, steam-distilled product.
