JOSEPH E. WEBER Bowling Green State University, Bowling Green, Ohio
SINCE dialkylbenzenes sulfonate rapidly with sulfuric sel and continue the suction until all of the supernatant acid and a t moderate temperatures, the recent commer- liquid has been removed. Recrystalhe the crude product from a small amount cial availability of pure xylene isomers affords the organic laboratory instructor a short, easy, and inexpen- (10 ml.) of water. sive sulfonic acid preparation. Until recently the EQUATIONS ortho, meta, and para isomers were commercially availm able only as a mixture of the three. Sulfonation of this mixture was easily accomplished (1) but resulted in an H - L H even more complex mixture of a t least four of the six possible monosulfonic acid derivatives. Sulfonation of + s o H*O some monoalkylbenzenes such as ethylbenzene (9) are 11-C-H easily accomplished also but they too lead to mixed H-C-H products. Para xylene of 95% purity' which is now available in nuantitv a t moderate cost will sulfonate r a.~ i "d hwhen agitated with concentrated sulfuric acid to yield a single product. External heating can be omitted entirely, although the yield is improved somewhat by heating. The following is an experiment regularly performed by students in the second semester of our third year level organic laboratory course.
--
PROCEDURE (8)
Place 12 ml. of para xylene in a dry 200-ml. roundbottomed ring-neck flask and add with swirling agitation 14 m.. of concentrated sulfuric acid. Continue agitation of the mixture with a circular motion and heat gently but directly over a low flame. A thermometer is not needed since the temperature is moderate and not critical but in no case should the boiling point (137") of the xylene be reached. After 20 to 30 minutes the reaction should be complete, as indicated by the clarity of the mixture on cooling. When cooled to room temperature add 10 ml. of water to the contents of the flask and agitate again. The mixture becomes warm once more as a result of the contact between the water and excess acid. While still warm pour the product into a small (100-ml.) beaker to cool and crystallize. When crystallization seems complete a t e r the mass, with suction, on a fritted funnel. Press the crystals down on the fritted plate with a flat-bottomed glass ves1 Pam xylene of 95 per cent purity can be obtained from the Oronite Chemical Company, Richmond, California.
+
1,
pXyleneZsulfonic acid
The average student can complete this preparation of p-xylene-2-sulfonic acid dihydrate and hand in to the instructor a snow white crystalline product of high purity and good yield in 11/2-2 hours from the time that he enters the laboratory. With a single rinsing with water alone the glassware used in the experiment is sparkling clean. Compare this sulfonation experiment with the usual experiment involving the long heating of aniline and sulfuric acid in an oil bath a t 180-185' with its attendant fire hazard, and finally the many decolorizations usually required of the crude product to produce a reasonably pure white sample. Then there is the problem of the removal of the charred material from the reaction flask. LITERATURE CITED (1) J.Chem. Soc., 125, 2489 (1924). (2) Trans. Electrochem. Soc., 93,47 (1948).
(3) WEBER, J. E., AND AUDREY E. MEISTER, "L~boratOry Manual of Organic Chemistry," Bowling Green, Ohio, 1949.
