Oscar 1. Wright and Lawrence E. Mura
Rockhurst College Kansas City, Missouri
The Bromination of Anthracene
Barnett and cook1 prepared 9-bromoanthracene by adding bromine to a solution of anthrsr cene in carbon bisulfide. The addition product is formed first, 9,l~dibromo-9,10-dihydroanthracene, and this is subsequently dehydrohalogenated (a process requiring five days), steam distilled, and recrystallized from about 300 times its weight of ethanol. Bachman and Kloetze12 improved on the process somewhat. They substituted vacuum distillation for the steam distillation, but they still required four recrystallizations from petroleum ether and one from alcohol in order to get a pure product. The process calls for purified anthracene, which is rather tedious to prepare. It has been observed that crude anthracene can he brominated in ethyl acetate in very high yield, and that the product, 9-bromoanthracene, can be recovered in a high purity. The anthracene is dissolved in ethyl acetate and the bromine added after being dissolved in ethyl acetate. The addition product forms and is dehydrohalogenated by refluxing the ethyl acetate over a period of six to eight hours. This reaction illustrates the property of anthracene of adding the halogen molecule and later losing the elements of HBr to form the monobrominated hydrocarbon. I t is a reaction which can easily be conducted in a beginning organic laboratory, since no special equip ment is required for the process. It can be performed in a beaker on a hot plate (by the repeated additions of ethyl acetate), or by installing a reflux condenser and a gas absorption apparatus. The reactants are mixed and the solvent is refluxed until the evolution of HBr ceases; the solvent is withdrawn by distillation, and the product is recrystallized. Although the process is best for anthracene, 1-bromonaphthalene can also be prepared by the same process. The only product which was observed on the vapor phase chromatogram was the 1-bromonaphthalene. The Experiment
9-Bromoanthracene. Powdered anthracene, 9.0 g (0.05 moles) is suspended in 100 ml of ethyl acetate and 5-10 mg of anhydrous iron (111) chloride is added and the mixture is stirred. A solution of 8 g of bromine (0.05 moles) dissolved in 50 ml of ethyl acetate is added This work was done in an Undergraduate Research Participittion Program sponsored by the National Science Foundation s t Rockhurst College, Kamas City, Missouri. ' BARNEW,E. DEB.,AND COOK, J. W., J. Chem. Sac., 125,1085 (1924).
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over a period of about twenty minutes. During the addition the temperature may rise 5', but the reaction is moderate and easy to control. The mixture is not homogeneous; it contains undissolved anthracene and solution. Only a small amount of HBr is evolved up to this point. An HBr gas disposal apparatus is attached, and the mixture is warmed gently to 60' where there is a copious evolution of HBr and where the solution becomes homogeneous. The solution is maintained at this temperature for three hours as the HBr is evolved, after which 125 ml of the solvent is removed by distillation. The residue from the distillation is added to 200 ml of cold water, and the solid product is filtered off with suction and dried in air. The crude 9-bromoanthracene is recrystallized from hexane yielding 12 grams of a product melting a t 94-97', Further recrystallization from ethanol yields a product which melts at 99-100' in agreement with literature value2. Undoubtedly a purer product is obtained if purified anthracene is used in the reaction. but it is easier to purify the product than the anthracene, and crude anthracene is quite acceptable as starting material. The purity of the ethyl acetate is important. It should contain less than 0.5y0 of ethanol, water, or acetic acid. During the distillation, the evolution of HBr should stop. The first 15-20 ml of ethyl acetate which comes over will fume of HBr in the receiver. Where a larger amount of anthracene is brominated more time must be allowed for the evolution of the HBr. If the fuming persists during the distillation, the reaction may not have run its course. I-Bromonaphthalene. Naphthalene, 6.4 g (0.05 moles), is mixed with 100 ml of ethyl acetate and 4-5 mg of anhydrous iron (111) chloride. A solution of 8 g of bromine (0.05 moles) is added quickly to the mixture with efficient stirring. There is not much apparent reaction, but as the reaction mass is taken to reflux, the evolution of HBr is observed. A gas absorption trap should be applied to the condenser and reflux continued for three to four hours as the HBr is evolved. When the evolution of HBr subsides, 125 ml of the ethyl ac+ tate is distilled from the mixture and the residue is poured into 200 ml of ice water. The product is distilled from the aqueous mixture wit,h steam. about 8-9 g of oil are obtained which show no trace of the 2-bromo isomer by vapor phase chromatography. a
BACHMAN, W. E.,AND KLOETZEL, M. C., J. Org. Chem., 3,58
(1938).
Journal of Chemicol Education
