Org. Synth. 1921, 1, 3
DOI: 10.15227/orgsyn.001.0003
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 water
1 (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.
DOI: 10.15227/orgsyn.001.0003
(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.
DOI: 10.15227/orgsyn.001.0003
(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).
DOI: 10.15227/orgsyn.001.0003
(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).
DOI: 10.15227/orgsyn.001.0003
(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).
DOI: 10.15227/orgsyn.001.0003
(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.
DOI: 10.15227/orgsyn.001.0003
(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).
DOI: 10.15227/orgsyn.001.0003
(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 devices
2 where refluxing and stirring are desired at the same time. When the stopper of the flask is so small that it will hold only the mechanical stirrer, a Y-tube such as is shown in Fig. 3 B fitted with either form of stirrer is suitable. It often happens that a separatory funnel or thermometer must also be inserted in the flask in addition to the reflux condenser. A
three-necked flask such as is shown in Fig. 3 A is especially convenient, the middle opening being used for the stirrer, the two smaller ones for thermometer and reflux condenser.
Fig. 2.
Fig. 3.
9.
The intensity of the color in the reduced
bromine solution depends somewhat on the quality of
bromine used, and with the technical product special care is needed in noting the end point of the reduction, i.e., the transition from the
bromine color to a yellowish-brown color.
10.
An excess of
sulfur dioxide is to be avoided for the reason that evolution of gas during the distillation of the product will result in a considerable loss of alkyl bromide through volatilization. This loss is especially marked in the preparations of ethyl and allyl bromides if this precaution is not observed.
11.
In the preparation of
allyl bromide, appreciable loss may occur, not only because of carbonization but primarily because of the reaction of the
allyl bromide with
hydrobromic acid to form
propylene bromide. Stirring during the formation of the alkyl bromide prevents the formation of two layers in the reaction mixture and thus assures a rapid and smooth distillation. An efficient condenser is of course essential.
12.
The troublesome emulsions often encountered when
lauryl bromide is shaken with
sulfuric acid may be avoided by the following alternative method of purification. The reaction mixture, after the heating is completed, is diluted with water and the layer of
lauryl bromide is separated. The bromide is washed with water and then with
potassium carbonate solution. The washing with water should be sufficiently thorough so that there is no appreciable foaming on shaking with the carbonate. The bromide is then dried with
calcium chloride and distilled.
Lauryl bromide and the bromides of other primary alcohols of high molecular weight can also be prepared by saturating the alcohol at 100–120° with hydrogen bromide as described in Org. Syn. 15, 24. This hydrogen bromide procedure has been reported to be superior for preparing lauryl bromide to the hydrobromic acid-sulfuric acid method described above.
13.
Trimethylene bromide, prepared by direct distillation of the reaction mixture without the preliminary refluxing period, contains appreciable amounts of
trimethylene bromohydrin.
14.
In the purification of
trimethylene bromide with
sulfuric acid the high specific gravity of the former (1.987) must be borne in mind. Vigorous shaking with
sulfuric acid may result in the formation of emulsions.
[II. PHOSPHORUS AND BROMINE METHOD]
Submitted by R. H. Goshorn, Thomas Boyd, and E. F. Degering.
Checked by R. L. Shriner and F. J. Wolf.
1. Procedure
DOI: 10.15227/orgsyn.001.0003
(A) ETHYL BROMIDE, CH3CH2Br
[Ethane, bromo-]
A 1-l. three-necked, round-bottomed flask is fitted with an Allihn condenser and a dropping funnel, the stem of which reaches to the bottom of the flask. By means of a bent glass tube the top of the Allihn condenser is connected to a Liebig condenser the end of which dips just beneath the surface of a mixture of ice and water (about 800 g.) contained in a 2-l. beaker (Note 1).
In the reaction flask are placed 363 g. of 95 per cent ethyl alcohol (7.5 moles), 25 g. of red phosphorus, and 15 g. of yellow phosphorus (1.29 gram atoms) (Note 2). The mixture is heated by means of the oil bath until gentle refluxing takes place (bath temperature at 100–110°), and 377 g. of bromine (121 cc., 2.36 moles) is slowly added through the dropping funnel over a period of three hours (Note 3), (Note 4). The water is drained from the Allihn condenser, and distillation is continued at a rate of forty to fifty drops per minute up to a temperature of about 94° (a thermometer may be placed in the flask in place of the dropping funnel) (bath at about 155°). When the distillation is complete the two condensers are disconnected first and the burner removed (Note 5).
The distillate is washed with 20 cc. of ice-cold 5 per cent sodium hydroxide solution, then with 50 cc. of ice-water. The product is dried over anhydrous calcium chloride and distilled through a short column, the distillate being collected in a dry receiver packed in an ice-salt mixture. The yield of colorless ethyl bromide distilling between 37° and 40° is 420–465 g. (82–90 per cent of the theoretical amount based on the bromine) (Note 6).
DOI: 10.15227/orgsyn.001.0003
(B) iso-PROPYL BROMIDE, (CH3)2CHBr
[Propane, 2-bromo-]
The procedure for the preparation of iso-propyl bromide is similar to that for ethyl bromide except for the quantities of the reactants and the temperature of the bath. A mixture of 180 g. (228 cc., 3.0 moles) of iso-propyl alcohol, 6 g. of red phosphorus, and 4 g. of yellow phosphorus (0.3 gram atom) is placed in the reaction flask. Then, 135 g. (43.2 cc., 0.84 mole) of bromine is added over a period of three hours, the temperature of the oil bath being maintained between 85° and 95° (preferably at 90°). The distillate is collected up to a temperature of 145° (bath temperature 155–185°). In the second distillation after washing and drying the crude product, the fraction boiling between 59° and 62° is collected. The yield of product is 143–152 g. or 69–73 per cent of the theoretical amount (Note 7).
DOI: 10.15227/orgsyn.001.0003
(C) n-PROPYL BROMIDE, CH3CH2CH2Br
[Propane, 1-bromo-]
The procedure for the preparation of n-propyl bromide is similar to that for ethyl bromide except for the quantities of the reactants and the temperature of the bath. A mixture of 180 g. (224 cc., 3.0 moles) of n-propyl alcohol, 6 g. of red phosphorus, and 4 g. of yellow phosphorus (0.3 gram atom) is placed in the reaction flask. Then 135 g. (43.2 cc., 0.84 mole) of bromine is added slowly over a period of three hours, the temperature of the oil bath being maintained between 125° and 140°. The distillate is collected up to a temperature of 140° (bath temperature 190–210°). In the second distillation, after washing and drying the crude product, the fraction boiling between 70–73° (Note 8) is collected. The yield of n-propyl bromide is 170–186 g. (82–90 per cent of the theoretical amount).
DOI: 10.15227/orgsyn.001.0003
(D) n-BUTYL BROMIDE, CH3(CH2)3Br
[Butane, 1-bromo-]
The same apparatus as described in the preparation of ethyl bromide may be used with the omission of the Liebig condenser.
In the reaction flask are placed 138 g. (170 cc., 1.86 moles) of n-butyl alcohol, 6 g. of red phosphorus, and 4 g. of yellow phosphorus (0.3 gram atom). The oil bath is heated to 175°, and 94 g. (30.1 cc., 0.58 mole) of bromine is added slowly through the dropping funnel at such a rate that the dense white vapors which form remain near the top of the Allihn condenser. The optimum period for the addition of bromine is about one hour, during which time the bath temperature slowly decreases to 165°. The mixture is distilled directly from the reaction flask which is attached to a condenser set for downward distillation. When the distillation becomes slow about 25 cc. of water is added through the dropping funnel to assist in carrying over the butyl bromide. The final bath temperature is about 210°. The distillate is washed with 20 cc. of 5 per cent sodium hydroxide solution and then with 50 cc. of water. The product is dried with calcium chloride and distilled through a Vigreux column. The fraction boiling from 97° to 99.5° is collected and amounts to 145–150 g. (90–93 per cent of the theoretical amount).
DOI: 10.15227/orgsyn.001.0003
(E) sec.-BUTYL BROMIDE, CH3CH2CHBrCH3
[Butane, 2-bromo-]
The procedure for secondary butyl bromide is similar to that used in the preparation of n-butyl bromide, except that bromine is added at a bath temperature of 140–150°, and the distillate (bath temperature up to 190°) from the reaction mixture is washed with water and dried over anhydrous sodium sulfate before redistilling. The yield (b.p. 86–87°) is about 135–137 g. (84–85 per cent of the theoretical amount).
DOI: 10.15227/orgsyn.001.0003
(F) tert.-BUTYL BROMIDE, (CH3)3CBr
[Propane, 2-bromo-2-methyl-]
In a 500-cc. three-necked, round-bottomed flask fitted with a dropping funnel and a mechanical stirrer is placed 5 g. (0.16 gram atom) of red phosphorus. The flask is cooled on a bath of cold water, and 47 g. (15 cc., 0.29 mole) of bromine is added very slowly at first (Note 9). After the initial reaction has moderated the remainder of the bromine is added over a period of one and a half to two hours. The stirrer is started, and 69 g. (0.93 mole) of tert-butyl alcohol is placed in the separatory funnel and added slowly to the phosphorus bromide. The stirrer and separatory funnel are removed; the flask is connected to a condenser and arranged for vacuum distillation. By means of a water pump the pressure is gradually reduced to 20–30 mm. and the bromide distilled by raising the bath temperature slowly to 70–80°. The distillate is washed quickly with 50 cc. of ice-water, dried with anhydrous calcium chloride, and redistilled. The fraction boiling from 69° to 71° is collected. The yield is 32 to 34 g. (39–42 per cent of the theoretical amount).
2. Notes
1.
A diagram of the apparatus used is shown in Fig. 4.
Fig. 4.
2.
Nearly as good yields are obtained if
yellow phosphorus is used alone. However, a much smoother and more readily controllable reaction is obtained if some
red phosphorus is used along with the
yellow phosphorus. Care must be exercised in handling
yellow phosphorus since it ignites spontaneously in the air. It should be kept under water and transferred quickly to the flask which already contains the alcohol.
3.
A vigorous reaction, with some bumping and the formation of white vapors, accompanies the first addition. As the density of the vapors decreases, the rate of addition of
bromine may be increased to the extent that some of the white vapors are carried over into the Liebig condenser.
4.
There is a tendency for water to be sucked from the ice bath into the apparatus because of sudden changes in pressure within the system. This difficulty may be avoided by adding the
bromine at a constant rate, by maintaining a constant reflux rate, and by maintaining the temperature of the oil bath nearly constant, allowing any changes in temperature, especially lowering, to take place slowly. If water is sucked into the reaction flask, the addition of
bromine may as well be discontinued because the alcohol has become diluted and the yield will be low.
5.
Care must be exercised in the disposal of the residue in the flask since it sometimes contains unreacted
phosphorus which ignites spontaneously when dry and exposed to the air. After the residue has cooled, it should be drowned in a large volume of water and the mixture washed down the drain with a good stream of water.
6.
Ethyl bromide is quite volatile, and hence care must be taken to keep it cold during the washing operations and to use efficient condensing systems.
7.
In washing the product a lachrymator is noticed, and in the final distillation a residue of 30–36 g. possessing highly lachrymatory properties is observed.
8.
The residue has lachrymatory properties and a pungent odor.
9.
Care should be taken to add the
bromine very slowly at first since sometimes the heat of the reaction is sufficient to ignite the
phosphorus and it burns until all the
oxygen is removed from the flask.
3. Discussion
Alkyl bromides are prepared most conveniently from the corresponding alcohols. The reagents commonly employed are:
phosphorus bromide,
3 a metallic bromide and
sulfuric acid,
4 very concentrated or fuming
hydrobromic acid, often under pressure,
5 and aqueous
hydrobromic acid in accordance with the method suggested by Norris.
6 Allyl,
propyl, and
isobutyl bromides have been prepared
7 by a modification of this last method:
bromine was reduced by means of
sulfur dioxide, the alcohol was added, and the mixture distilled. The procedure described has two essential modifications: the addition of concentrated
sulfuric acid and a refluxing period preliminary to the distillation. The
sodium bromide method has been improved by similar modifications. Purer alkyl bromides have been reported by a slight variation in the method of purification.
8 Since
trimethylene glycol is now a by-product from the purification of
glycerol, it is a convenient compound for the preparation of
trimethylene bromide. A method involving the use of gaseous
hydrobromic acid has been described,
9 but the aqueous hydrobromic-sulfuric acid method
10 has been found to be a considerable improvement.
Comparative studies of the preparation of a number of alkyl bromides show that
phosphorus tribromide gives better results than red
phosphorus and
bromine,
11 while a comparison of different methods of preparing
n-butyl bromide shows that the best yields are obtained with
hydrogen bromide and
sulfuric acid.
12Other procedures which have been described for the preparation of alkyl bromides include: the addition of
hydrogen bromide to ethylenic hydrocarbons in the presence of an absorbent hydrous metal oxide gel;
13 heating an alcohol or its
sulfuric acid ester with
hydrobromic acid in aqueous solution at 110–180°, under pressure to avoid formation of a vapor phase;
14 treatment of a dialkyl sulfate with
hydrogen bromide in the vapor phase;
15 heating an alcohol saturated with
hydrogen bromide in the hydrogen bromide salt of a nitrogenous heterocyclic compound such as
pyridine hydrobromide;
16 and cleaving aliphatic ethers with
hydrogen bromide and steam in the presence of a dehydrating catalyst.
17 Alkyl bromides have been prepared from alcohols of high molecular weight by saturating the alcohol with
hydrogen bromide at 100°C.
18This preparation is referenced from:
- Org. Syn. Coll. Vol. 1, 46
- Org. Syn. Coll. Vol. 1, 117
- Org. Syn. Coll. Vol. 1, 131
- Org. Syn. Coll. Vol. 1, 149
- Org. Syn. Coll. Vol. 1, 156
- Org. Syn. Coll. Vol. 1, 203
- Org. Syn. Coll. Vol. 1, 207
- Org. Syn. Coll. Vol. 1, 248
- Org. Syn. Coll. Vol. 1, 250
- Org. Syn. Coll. Vol. 1, 296
- Org. Syn. Coll. Vol. 1, 306
- Org. Syn. Coll. Vol. 1, 435
- Org. Syn. Coll. Vol. 1, 521
- Org. Syn. Coll. Vol. 2, 128
- Org. Syn. Coll. Vol. 2, 179
- Org. Syn. Coll. Vol. 2, 184
- Org. Syn. Coll. Vol. 2, 246
- Org. Syn. Coll. Vol. 2, 292
- Org. Syn. Coll. Vol. 2, 323
- Org. Syn. Coll. Vol. 2, 358
- Org. Syn. Coll. Vol. 2, 366
- Org. Syn. Coll. Vol. 2, 406
- Org. Syn. Coll. Vol. 2, 547
- Org. Syn. Coll. Vol. 2, 602
- Org. Syn. Coll. Vol. 3, 159
- Org. Syn. Coll. Vol. 3, 363
- Org. Syn. Coll. Vol. 3, 523
- Org. Syn. Coll. Vol. 3, 601
- Org. Syn. Coll. Vol. 3, 692
- Org. Syn. Coll. Vol. 3, 848
- Org. Syn. Coll. Vol. 4, 763
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
red phosphorus
yellow phosphorus
ethyl alcohol (64-17-5)
calcium chloride (10043-52-4)
potassium carbonate (584-08-7)
sulfuric acid (7664-93-9)
sodium hydroxide (1310-73-2)
glycerol (56-81-5)
HYDROBROMIC ACID,
hydrogen bromide (10035-10-6)
sodium carbonate (497-19-8)
bromine (7726-95-6)
sulfur dioxide (7446-09-5)
Allyl bromide,
Propene, 3-bromo- (106-95-6)
Allyl alcohol (107-18-6)
propylene bromide (78-75-1)
Butane, 1-bromo-3-methyl-,
iso-amyl bromide (107-82-4)
Butane, 1-bromo-,
Butyl bromide,
n-butyl bromide (109-65-9)
sodium bromide (7647-15-6)
Lauryl Bromide,
n-DODECYL BROMIDE (143-15-7)
Lauryl alcohol (112-53-8)
Ethyl bromide,
Ethane, bromo- (74-96-4)
Octane, 1-bromo-,
n-octyl bromide (111-83-1)
Trimethylene bromide,
Propane, 1,3-dibromo- (109-64-8)
trimethylene glycol (504-63-2)
sulfuryl bromide
trimethylene bromohydrin (627-18-9)
PHOSPHORUS (7723-14-0)
Propane, 2-bromo-,
iso-PROPYL BROMIDE (75-26-3)
Propane, 1-bromo-,
Propyl bromide,
n-PROPYL BROMIDE (106-94-5)
Butane, 2-bromo-,
2-Bromobutane,
sec.-BUTYL BROMIDE (78-76-2)
sodium sulfate (7757-82-6)
Propane, 2-bromo-2-methyl-,
tert.-BUTYL BROMIDE (507-19-7)
oxygen (7782-44-7)
phosphorus tribromide (7789-60-8)
pyridine hydrobromide (18820-82-1)
formic acid (64-18-6)
n-butyl alcohol (71-36-3)
iso-propyl alcohol (67-63-0)
n-propyl alcohol (71-23-8)
iso-amyl alcohol (123-51-3)
n-octyl alcohol (111-87-5)
Isobutyl bromide,
iso-butyl bromide (78-77-3)
tert-butyl alcohol (75-65-0)
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