William D. Ellis Occidental College 10s Angeles, California
Preparation of Diphenylmethane Simplified procedure: purer product
T h e preparation of diphenylmethane a s a n example of the Friedel-Crafts synthesis is included in several manuals of organic laboratory Directions call for the slow addition of anhydrous aluminum chloride to a mixture of dry benzene and benzyl chloride in a n ice bath, followed by refluxing for 30 min. The work-UD is standard: hydrolysis of the reaction mixture followed by washing, drying, and a final atmospheric distillation. The experience in our elementary organic chemistry laboratory was that the diphenylmethane was frequently contaminated with a yellow fluorescent impurity. We assumed that this impurity was anthracene.' A single simple atmospheric distillation usually failed to separate the diphenylmethane from all of the impurity. Fractioustion under reduced pressure was successful in producing a water-white product, but this method is impractical for some classes with limited laboratory experience or those with limited apparatus. I n an effort to eliminate the formation of the impurity, two approaches were tried, both involving a decrease in the amount of aluminum chloride used. Only simple atmospheric distillation was employed in all experiments. One set of experiments followed the standard p r o ~ e d u r evarying ,~ the amount of aluminum rhloride. The second set, again varying the amount of aluminum chloride, was run with a modified procedure involving the elimination of the refluxing period; the entire reaction was completed in the cold in less than 20 min. Summaries of the results of these trials are found in Tahle 1 and Table 2. Table 1 .
Standard Procedure
Moles A1C13/moleC.H.C&CI Per cent yield
0.280 0.140 0.050 0.025 0.015 31 48 55 54 26
As the amount of catalyst was decreased from the standard amount2 of 0.280 moles of aluminum chloride 1 FIESEE,L. F., "Experiments in Organic Chemistry," 3rd ed., I). C. Heath and Company, Boston, 1957, pp. 157-8. ROBERTSON, G . R., AND JACOBS,T. L., "Laboratory Practice of Organic Chemistry," 4th ed., Mmmillan Company, New York, 1962, pp. 2 7 2 3 . WOGEL, A. I., 'iPr&~tioaI Organic Chemistry," 3rd ed., Longmans, London, 1961, pp. 5134.
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Journol of Chemiml Education
per mole of benzyl chloride, it was found that there is a direct qualitative relation between the amount of catalyst used aud the amount of yellow to orange impurity in the dipheuylmethane product; the less aluminum chloride used, the less colored the product. Table 2.
Modified Procedure
Moles AICls/mole CaHsCHICl 0.280 0.140 0.050 0.025 0.015 Per cent vield 4i 44 51 48 22
By eliminating the reflux period a product was obtained which was free of all colored impurity. This modified procedure gives yields which are only slightly lower than those obtained by the standard procedure. As Table 2 shows, the yield is not greatly affected by the level of catalyst above 0.025 moles of aluminum chloride per mole of benzyl chloride, while lesser amounts of catalyst result in decreased yield. One laboratory manual' eliminates refluxing, but suggests letting the reaction mixture stand a t room temperature for 30 min. As shown by the above results, this is not necessary. A good yield of dear product is obtained with no delay, the reaction time being only the time (15-20 min) for the slow addition of the aluminum chloride in the cold bath. As a result of this investigation, the elimiuation of the reflux period as a laboratory technique was tried by two laboratory sections this year and proved successful. The students used only 0.023 moles of aluminum chloride per mole of benzyl chloride, no refluxing or period of standing and distilled the product a t atmospheric pressure from a simple distilling flask. None of the 45 products was the slightest bit colored. I t can be concluded, then, that by elimiuation of the time-consuming reflux period a purer diphenylmethane product can be obtained. Moreover, the amount of aluminum chloride used can be reduced to about onetenth the amounts suggested in the standard proced~re.'-~ A simplified apparatus was used with the modified procedure as the initial reaction need not take place in a reflux apparatus. Instead, the initial reaction was done in a n Erlenmeyer flask which was connected to a trap t o eliminate hydrogen chloride vapors. The author would like to thank Dr. Frank L. Lambert for his encouragement during this investigation.
