PERBROMIDE FOR BROMINATIONS PHILIP C. MERKER Long Island University, BrooklynCollege of Pharmacy, Brooklyn, New York JOSEPH A. VONA Pratt Institute of Science and Technology, Brooklyn, New York
TEEchemistry
instructor usually assigns bromina- present because of the persistence of the red bromine tion reactions to the students after much deliberation color. and anxiety. This is quite understandable because of It should be kept in mind that the yields reported are the hazards associated with bromine. The latter not, the result of one experiment; no variables were studied. only causes severe burns but the fumes produce a suf- Because of the complexity of the pyridinium bromide focating effect and are a source of much irritation. perhromide molecule and the influence of the pyridine However, in the last few years various solid brominating ring on the total molecule, it is apparent that the effect agents have become commercially available, notably of temperature and concentration of the reagent should ovridinium bromide ~erbromide,' and N-bromosuccin- have a marked effect on vields. ." imide.2 An excellent discussion of the reactions conducted with N-hromosuccinirnide has been p~blished.~EXPERIMENTAL PART Brominations with pyridiium bromide perbromide Materials. The pyridinium bromide perbromide are safe and convenient. Since pyridinium bromide (PBPB) used in this study was supplied by Jasons perbromide, C6HsNBrBr, is a relatively stable red Drug Company, 1085 Myrtle Avenue, Brooklyn 6, crystalline c ~ m p o u n d , ~having ,~ a melting point of New York. 135-6°C. (dec.), there are no annoying fumes to contend N-Butyl B r ~ m i d e . ~Nineteen grams of n-butyl alwith during handling; no elaborate apparatus is neces- coholwas added to a 500-ml. round-bottom three-necked sary for weigbing out the required amounts of pyridin- flash fitted with a condenser, thermometer, and stirrer. ium bromide perbromide and when spilled it can be One hundred grams of pyridinium bromide perbromide easily brushed off clothing, hands, or face with no was dissolved in 40 ml. of absolute methyl alcohol and accompanying danger of burns. Moreover, the student this solution was added to the reaction mixture in feels safer when handling "solid" hromine. In con- three divided portions over a period of one-half hour. trast to disagreeable and dangerous bromine, pyridinium The mixture was refluxed for two hours. At the end bromide perbromide can be safely stored in the labora- of this period the crude n-butyl bromide was distilled. tory with no danger of spillage and annoyance to stu- The product boiling between 91'and 110' was collected. dents and instructors. The color of the distillate was a bright orange and the The brominations herein reported have been under- remaining material in the flask was composed of a bright taken because they are usually the most common re- red liquid which weighed 71 g. (47ml.) and 15.1 g. of a actions performed in university and college labora- white sediment, probably pyridinium bromide. tories. An attempt has been made to follow the normal The distillate was placed in a separatory funnel and procedures from the references indicated, but substitut- 5 ml. of a saturated solution of sodium hisulfite was ing pyridinium bromide perbromide for the bromine, added. The crude material was then washed with assuming that the perbromide contained 45 per cent of water and concentrated sulfuric acid, followed by a available bromine. The manufacturer has reported washing with sodium bicarbonate. The product a t this 45 to 50 per cent bromine available. However, the time was colorless. Subsequent drying over calcium lower limit has been utilized so as to insure complete chloride was next accomplished and the dried material bromination. From some of the reactions conducted was distilled. it was evident that more than 45 per cent bromine was Fifteen grams of n-butyl bromide was collected a t 99"to 10l0C., corresponding to a yield of 42.6 per cent. ROSEMTWD, K. W., W. KUHNEENN, AND W. LESCH, Ber., 56, This low yield can partly be accounted for by assum1262, 2042 (1923) ing that methyl bromide had formed during the ZIEGLER. K., A. SPAETA, E. SCHMF,W. SCWMANN, AND reaction. E. WINKELMANN, Ann., 551, 80 (1942). p-Brom~acetanilide.~Thirty and one-half grams of a DJERASBI, C., Chem. Rev., 43, 271 (1948). acetanilide was dissolved in 50 ml. of glacial acetic ' ENGLERT, S. M., AND S. M. MCELVAIN, J . Am. Chem. Soc., 51, 863 (1929). ADUS,R., AND J. R. JOANSON, "Elementsry Laboretory Ex6 DJERASSI, C., AND C. R.SCAOLZ, ;hid., 70,417 (1948). periments in Organic Chemistry," Maomillan Company. 61
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614
acid and placed in a 500-ml. round-bottomed flask. Thirty-eight grams of pyridinium bromide perbromide was dissolved in 40 ml. of hot glacial acetic acid. This solution must be kept warm in order to prevent, crystallization of the perbromide. The two solutions upon combination produced some precipitation. This mixture was then allowed to stand, without heating, for 30 minutes. At the end of this time, 2 ml. of saturated sodium bisulfite solution was added to the mixture. The resultmg mass was filtered with suction and was then recrystallized from hot ethyl alcohol (95 per cent) to yield 13 g. (60.8 per cent) of p-bromoacetanilide having a melting point of 168'C. Bromination of Anethole,' CHsCH=CH-C&L OCHa. The bromination of the double bonds of anethole was conducted on a micro and a macro level. A micro preparation was conducted in which 100 mg. of anethole was dissolved in 2 ml. of isopropyl ether and to it was added dropwise 533 mg. of pyridinium bromide perbromide which had been dissolved in 2 ml. of absolute methanol. Precipitation occurred rapidly, but upon continued addition of the total volume of methanolic solution of PBPB, the precipitate dissolved. Cooling in tap water did not cause any crystallization, indicating that the brominated anethole was soluble in methanol. This experiment was repeated with 533 mg. of PBPB dissolved in one-half the amonnt of methanol used in the previous run. The precipitate which formed was recrystallized from 5 ml. of hot petroleum ether. A yield of 120 mg. (57.7) per cent, melting a t 106'C. was obtained. A macropreparation was carried out using ten times the quantities indicated above. After recrystallization from 10 ml. of hot petroleum ether a yield of 1.0 g. - (48.1 per cent) melting - a t 104OC. was obtained. ' CHERONIS, N. D., A N D J. F.ENTRIKIN, "Semjmi~roQualita~
tive Organic Analysis," T. Y. Crawell, New York, 1947.
The above experiments on the bromination of methole were also attempted using petroleum ether as a solvent for the pyridinium bromide perbromide. However, no significant yields were obtained because the perbromide crystallized readily. Br0mobenzene.e Thirty-three grams of benzene and 3 g. of iron tacks were placed in a 500-ml. round-bottom flask equipped with a reflux condenser and a small separatory funnel, thermometer, and stirrer. Sixtyseven grams of pyridinium bromide perbomide was added to the benzene and the flask was warmed gently to start the reaction. An additional 67 g. of the perbomide was dissolved in 60 ml. of absolute methyl alcohol, placed in the s e p aratory funnel, and slowly added to the reaction flask over a period of one-half hour. After the complete addition of the methanolic PBPB solution the reaction was permitted to continueuntil no more red vapors were visible above the liquid. At this point the reaction solution was a deep orangered color with no fumes evolving from the mixture. The reddish-orange product was washed several times with 10 per cent NaOH solution until oolorless. A standard steam distillation apparatus with a water-cooled Liebig condenser was then utilized to effect separation of the crude p-dibromobenzene and the crude bromobenzene. The distillate was purified by first drying over 3 to 4 g. of calcium chloride and then distilling over through an air-cooled condenser. The portion which distilled over between 140" to 170° C. was collected and redistilled. Twenty grams (30.1 per cent) of bromobenzene was collected between 150" and 160°C. In all experiments rubber stoppers wrapped with asbestos were used. These stoppers had been p r e viously coated with carbon black and Dow-Corning silicone grease.
