Organic Syntheses, Coll. Vol. 1, p.25 (1941); Vol. 1, p.3 (1921).CV1P0025

Organic Syntheses, Coll. Vol. 1, p.25 (1941); Vol. 1, p.3 (1921). ALKYL AND ALKYLENE BROMIDES [I. HYDROBROMIC ACID METHOD] Submitted by Oliver Kamm and C. S. Marvel. Checked by H. T. Clarke and Anne W. Davis. 1. Procedure A given primary alcohol is treated with 25 per cent excess of aqueous (48 per cent) hydrobromic acid (Note 1) together with sulfuric acid (Note 2). The mixture is refluxed (Note 3) in order to convert the alcohol as completely as possible into the corresponding bromide, and the latter is then removed from the reaction mixture by distillation. The water-insoluble layer is separated; washed successively with water, cold concentrated sulfuric acid (Note 4), and a sodium carbonate solution; separated; dried with calcium chloride (Note 5); and distilled. Slight variations from this procedure depend upon the physical and chemical properties of the alcohol used, or of the bromide formed in the reaction. For example, in the preparations of ethyl and allyl bromides, the reaction mixture is not refluxed because of the volatility of the former compound, and because of the chemical reactivity of the latter; in the preparation of iso-amyl bromide, too large a proportion of sulfuric acid may produce appreciable decomposition, whereas halides of high molecular weight, because of their low volatility, are separated from the reaction mixture mechanically, instead of by distillation. The use of a modified sodium bromide-sulfuric acid method (Note 6) for the preparation of alkyl bromides is described in connection with the preparation of n-butyl bromide. This method has been used also for the preparations of iso-amyl and trimethylene bromides, but, in general, the yields were found to be somewhat lower than those obtained with the hydrobromic-sulfuric acid method. Preparation of Hydrobromic Acid.—Hydrobromic acid may be prepared conveniently by the interaction of bromine and sulfur dioxide in the presence of water1 (Note 7). In a 3-l. round-bottomed flask are placed 1200 g. (377 cc., 7.5 moles) of bromine, 500 cc. of water, and 1500 g. of crushed ice. A fairly rapid stream of sulfur dioxide is allowed to pass from a pressure tank into the flask, the outlet of the gas-tube being placed below the surface of the bromine layer. The flow of sulfur dioxide is adjusted at such a rate that the gas is completely absorbed. It is advisable to agitate the mixture occasionally during the first stage of the reduction (Note 8). About two hours will serve for the completion of the reduction, at which time the mixture will assume a yellow color (Note 9) which is not removed by further addition of sulfur dioxide, an excess of which is to be avoided (Note 10). To prevent loss by gaseous hydrogen bromide, it is advisable to cool the mixture during the progress of the reduction. When the reduction is completed, the flask is connected with a condenser and the mixture subjected to distillation. The boiling point of constant boiling hydrobromic acid is 125–126° /760 mm., but it must be remembered that, in distilling the product from the sulfuric acid mixture, the thermometer reading should not be relied upon as an index to the composition of the distillate. Towards the end of the distillation the thermometer may rise to 130° and above, when water with only traces of acid distils from the sulfuric acid residue. Upon redistillation of the product the thermometer reading may be relied upon. For many uses a product free from traces of sulfuric acid is not required and one distillation is sufficient. In such cases the progress of the distillation is followed by determinations of the specific gravity of the distillate. According to the above procedure, 20 kg. of 48 per cent hydrobromic acid (92 per cent of the theoretical amount) may be prepared from 10.3 kg. of bromine. The actual time required by one person for the preparation of this quantity is twenty-three hours. For the preparation of alkyl bromides on a relatively large scale the hydrobromic acid need not be distilled but may be used directly for the subsequent preparation. The fact that 0.5 mole of sulfuric acid is present for each mole of hydrobromic acid is not a disadvantage, since the presence of sulfuric acid is desired, and consequently a correspondingly smaller quantity need be added. (A) ALLYL BROMIDE, CH2=CHCH2Br [Propene, 3-bromo-] In a 3-l. round-bottomed flask, a hydrobromic acid solution is prepared (p. 26) by the sulfur dioxide reduction of 480 g. (150.5 cc., 3 moles) of bromine in the presence of 510 g. of ice water or a mixture is made of 1 kg. (5.9 moles) of aqueous 48 per cent hydrobromic acid and 300 g. (162 cc.) of concentrated sulfuric acid. To this is added 385 cc. of aqueous allyl alcohol (p. 42), which, according to bromine titration, contains 233 g. (4 moles) of pure allyl alcohol. The 3-l. round-bottomed flask is fitted with a mechanical stirrer (Note 11), separatory funnel, and an efficient condenser set for downward distillation. Stirring is started, and 300 g. (162 cc.) of concentrated sulfuric acid is added gradually through the separatory funnel to the warm solution. The allyl bromide distils over completely in about one-half to one hour. The crude allyl bromide is washed with dilute sodium carbonate solution, dried over calcium chloride, and then distilled. The yield of product boiling at 69–72° from a number of experiments varies from 445–465 g. (92–96 per cent of the theoretical amount). A small high-boiling fraction is also obtained and examination has shown this to consist of propylene bromide. (B) iso-AMYL BROMIDE, (CH3)2CHCH2CH2Br [Butane, 1-bromo-3-methyl-] In a 5-l. round-bottomed flask, a hydrobromic acid solution is prepared (p. 26) by passing sulfur dioxide into a mixture of 1100 g. of crushed ice and 1 kg. (314 cc., 6.25 moles) of bromine. This is equivalent to a mixture of 2.1 kg. (12.5 moles) of 48 per cent hydrobromic acid and 600 g. of concentrated sulfuric acid. There are then added, in the order mentioned, 880 g. (1086 cc., 10 moles) of iso-amyl alcohol (b.p. 130–132°) and 100 g. (54.5 cc.) of concentrated sulfuric acid. The clear homogeneous solution is refluxed gently during a period of five to six hours. Even during the early stages of the heating, the separation of iso-amyl bromide is observed, and the reaction appears to be complete after about one hour. The product is isolated as in the preparation of n-butyl bromide below. A yield of 1435 g. of crude product is obtained. After purification with concentrated sulfuric acid the product weighs 1410 g. (93 per cent of the theoretical amount). Upon fractionation, however, it is found that appreciable amounts of a high-boiling product are present, and therefore the yield of fractionated material boiling over the range 116–120° varies in different experiments from 1330 to 1360 g. (88–90 per cent of the theoretical amount). (C) n-BUTYL BROMIDE, CH3(CH2)3Br [Butane, 1-bromo-] Hydrobromic-Sulfuric Acid Method.—In a 5-l. round-bottomed flask are placed 1300 g. of crushed ice and 1200 g. (376 cc., 7.5 moles) of bromine. The flask is cooled in an ice-water bath and sulfur dioxide is passed into the mixture until the red color due to free bromine has just disappeared (p. 26). This mixture is equivalent to 2500 g. (14.8 moles) of 48 per cent hydrobromic acid to which 750 g. of concentrated sulfuric acid has been added. To the sulfuric-hydrobromic acid mixture is added 888 g. (1096 cc., 12 moles) of n-butyl alcohol. Following this, 600 g. (324 cc.) of concentrated sulfuric acid is added in several portions, with shaking. The flask is then attached to a reflux condenser and the mixture is refluxed during a period of five to six hours, during which time the formation of butyl bromide is carried practically to completion. The flask is now fitted with a condenser set downward and the product removed from the reaction mixture by direct distillation (about one hour). The water-insoluble layer is separated, washed first with water, then with 200 g. (109 cc.) of cold concentrated sulfuric acid, and finally with a sodium carbonate solution (50 g. of sodium carbonate in 500 cc. of water). The product is separated as completely as possible from the aqueous layer, dried during several hours with a small quantity (15–25 g.) of calcium chloride, and distilled. The yield of product boiling between 101–104° is 1560 g. (95 per cent of the theoretical amount). Sodium Bromide Method.—In a 5-l. round-bottomed flask is placed 1350 cc. of water, and then with stirring 1545 g. (15 moles) of finely powdered sodium bromide is added. It is advisable to add the salt to the water in this manner in place of the reverse procedure, in order to avoid caking of the sodium bromide. First, 888 g. (12 moles) of n-butyl alcohol and then gradually 2 kg. (1087 cc.) of concentrated sulfuric acid are added. The last half of the acid is added through a dropping funnel after the flask has been connected with a reflux condenser. The mixture is shaken occasionally during the addition of the sulfuric acid because of a tendency to separate into layers, and is finally refluxed during a period of two hours. The condenser is then set downward and the butyl bromide removed by distillation. The product is purified as in the preceding experiment for the preparation of n-butyl bromide by the hydrobromic- sulfuric acid method. The yield of n-butyl bromide boiling between 101–104° is 1480 g. (90 per cent of the theoretical amount). (D) n-DODECYL BROMIDE, CH3(CH2)10CH2Br [Lauryl Bromide] In a 250-cc. round-bottomed flask are placed 70 g. (0.42 mole) of hydrobromic acid (48 per cent) (p. 26) and 22 g. (12 cc.) of concentrated sulfuric acid. To this mixture is added 40 g. (0.22 mole) of lauryl alcohol (b.p. 188–192° /110 mm.) (Org. Syn. 10, 62), and the mixture is then refluxed for five to six hours. The bromide is isolated as described in the preparation of n-octyl bromide (p. 30). The product is distilled under reduced pressure and is collected from 175–180° /45 mm. The yield is 49 g. (91 per cent of the theoretical amount) (Note 12). (E) ETHYL BROMIDE, CH3CH2Br [Ethane, bromo-] In the preparation of hydrobromic acid for the manufacture of ethyl bromide, particular care must be taken to avoid the presence of any excess of sulfur dioxide gas. The evolution of gas during the distillation of the ethyl bromide will invariably result in a large loss of this volatile product (b.p. 38– 39°). A hydrobromic acid solution is prepared (p. 26) in a 5-l. round-bottomed flask by the reduction of 1 kg. (314 cc., 6.25 moles) of bromine in the presence of 1.1 kg. of cracked ice. A mixture of 2075 g. (12.3 moles) of 48 per cent hydrobromic acid and 600 g. (324 cc.) of concentrated sulfuric acid may be used in place of the above reduction mixture. After the addition of 500 g. (622 cc., 10 moles) of 95 per cent ethyl alcohol, the flask is attached to a long condenser set ready for distillation, and 1 kg. (544 cc.) of concentrated sulfuric acid is slowly added through a separatory funnel. Because of the volatility of ethyl bromide, the mixture is not heated under a reflux condenser, but is subjected instead to slow distillation. The end of the condenser is provided with an adapter tube, and the distillate is collected in a flask containing ice water. The crude ethyl bromide, weighing 1055 g., is purified as directed under the n-butyl bromide experiment (p. 28). The washing with concentrated sulfuric acid is almost superfluous unless a product of special purity is desired; for instance, in the present experiment a washing with 300 g. of concentrated acid results in a decrease in weight of only 10 g. The ethyl bromide is distilled from a water bath and boils at 38.5–39.5° provided that chips of porous plate are added to prevent superheating. Final yields vary from 980 to 1035 g. (90–95 per cent of the theoretical amount) according to the precautions taken to prevent losses due to evaporation. (F) n-OCTYL BROMIDE, CH3(CH2)6CH2Br [Octane, 1-bromo-] In a 500-cc. round-bottomed flask are placed 240 g. (1.4 moles) of hydrobromic acid (48 per cent) (p. 26), 62 g. (34 cc.) of concentrated sulfuric acid, and 71 g. (0.55 mole) of n-octyl alcohol (b.p. 135– 140° /100 mm.). The mixture is boiled under reflux for five to six hours. The solution is diluted with water and the bromide layer is separated, washed once with a little cold concentrated sulfuric acid, then with water and finally with dilute sodium carbonate solution, after the procedure described for n-butyl bromide. The crude yield is 102 g.; this is dried over a little calcium chloride and distilled. The product is collected at 196–200° (91–93° /22 mm.) and amounts to 96 g. (91 per cent of the theoretical amount). (G) TRIMETHYLENE BROMIDE, Br(CH2)3Br [Propane, 1,3-dibromo-] Hydrobromic-Sulfuric Acid Method.—In a 5-l. round-bottomed flask are placed 1200 g. (377 cc., 7.5 moles) of bromine and 1300 g. of crushed ice, and the bromine is reduced with sulfur dioxide to hydrobromic acid (p. 26). In place of the above reduction mixture, there may be used a mixture of 2.5 kg. (14.8 moles) of aqueous 48 per cent hydrobromic acid and 750 g. (408 cc.) of concentrated sulfuric acid. First, 456 g. (433 cc., 6 moles) of trimethylene glycol (b.p. 210–215°) is added, and then 1200 g. (652 cc.) of concentrated sulfuric acid, the sulfuric acid being added slowly. The mixture is refluxed (Note 13) during a period of five to six hours and is then subjected to distillation until no water- insoluble product appears in the distillate (about one hour). The trimethylene bromide is purified (Note 14) in accordance with the method used for n-butyl bromide (p. 28). A yield of 1065–1142 g. boiling at 162–165° (88–95 per cent of the theoretical amount) is obtained. Sodium Bromide Method.—The yields of trimethylene bromide by the sodium bromide method as described for n-butyl bromide (p. 29) are slightly lower than those given above. Thus, from 1350 g. of water, 1545 g. (15 moles) of sodium bromide, 456 g. (6 moles) of trimethylene glycol, and 2500 g. of sulfuric acid, a yield of 1110 g. of crude product is obtained, from which, after purification (Note 14) and distillation, a yield of 1030 g. of bromide (85 per cent of the theoretical amount) is obtained. 2. Notes 1. When an alcohol is heated with aqueous 48 per cent hydrobromic acid, a partial conversion takes place into the corresponding bromide. The reaction is more rapid and more complete, however, in the presence of sulfuric acid. Although the constant boiling hydrobromic acid obtainable on the market may be used in all the preparations described, its formation by the sulfur dioxide reduction of bromine will be considerably less expensive and equally convenient, provided that a cylinder of sulfur dioxide is available. For use in the preparation of alkyl bromides, distillation of the bromine-sulfur dioxide reduction mixture is superfluous. 2. The procedure described is quite general for the preparation of primary bromides. The presence of sulfuric acid would usually be objectionable in the preparation of secondary and tertiary bromides because of the ease of dehydration of the corresponding alcohols and the formation of isomers. This is also true of iso-butyl bromide and to some extent of iso-amyl bromide. These bromides may be obtained in good yields by the phosphorus tribromide method described in Org. Syn. 13, 20. In the preparations described, a more dilute hydrobromic acid solution may be used, provided that the proportion of sulfuric acid is increased. Aqueous solutions of alcohols may also be used if the proportion of sulfuric acid is suitably adjusted. In the allyl alcohol experiment, material was used as obtained from the glycerol-formic acid preparation (p. 42) after one salting out with potassium carbonate. 3. The reaction mixture is heated under the reflux condenser for several hours preliminary to the first distillation of the alkyl bromide. This is done in order to convert the alcohol as completely as possible into the corresponding bromide and thus to prevent its volatilization with the bromide. Direct distillation of the reaction mixture without refluxing will usually result in a decrease in yield of 5–15 per cent. Alkyl bromides of low molecular weight may, however, be distilled directly from the reaction mixture without the necessity of refluxing, providing the process of distillation is conducted very slowly. Alkyl halides of high molecular weight are separated from the reaction liquors mechanically instead of by distillation. This is done in order to avoid the decomposition due to heating the slightly volatile material with a gradually increasing concentration of sulfuric acid. 4. The main impurities usually found in alkyl halides are the corresponding alcohols and ethers. Cold concentrated sulfuric acid is an efficient reagent for the removal of these impurities in all cases where the alkyl halide itself is not attacked by this reagent. Whenever a product contains a considerable quantity of unchanged alcohol, several washings with the cold concentrated acid may be required. 5. In many organic preparations too large a quantity of drying agent is usually employed, with the resulting loss of a considerable amount of material due to absorption by the drying agent. In the present experiments it is found that after a careful separation of the alkyl halide from the water layer as small a quantity as 15 g. of calcium chloride is sufficient for the drying of 1500 g. of alkyl halide. 6. The favorable results obtained in the preparation of alkyl bromides with aqueous hydrobromic acid to which sulfuric acid has been added suggest that practically the same result might be accomplished by the use of sodium bromide, water, and sulfuric acid in such ratios as approximate the proportions used in the first instance. In actual practice this modified sodium bromide method was found fairly satisfactory for the preparation of n-butyl bromide and of trimethylene bromide. Slightly lower yields are due to the decreased solubilities of the alcohols in the reaction mixtures because of the presence of dissolved salts. One would therefore predict that, with alcohols of still higher molecular weights, even lower yields would be obtained with the sodium bromide method. This prediction is substantiated in experiments with iso-amyl alcohol, where the sodium bromide method gives yields of only 70 per cent of the theoretical amount, whereas the hydrobromic acid method gives yields of almost 90 per cent. The sodium bromide method is therefore not recommended for the preparation of alkyl bromides of high molecular weight. 7. In the sulfur dioxide reduction of bromine, it should be noted that the proportion of water used depends upon whether the reduction mixture is to be distilled for the preparation of 48 per cent hydrobromic acid, or whether it is to be used directly for the manufacture of alkyl bromides. 8. During the first stage of the reduction, the flask should be shaken from time to time in order to avoid the accumulation of sulfur dioxide, or possibly of sulfuryl bromide, which would result in a violent reaction owing to a large quantity of the material reacting at one time. In more than a hundred reduction experiments conducted with quantities of bromine varying from 0.5 to 2 kg., this sudden reaction was noted in only one or two instances, in spite of the fact that there was usually no agitation other than that furnished by the entering gas stream. Mechanical stirring is frequently important in obtaining successful yields. In Fig. 2, A and B represent two convenient types of stirring devices2 where refluxing and stirring are desired at th