Ethyl Bromide - Method 11
- 100 grms potassium bromide
- 100 grms (54 mL) conc. sulphuric acid
- 60 grms (75 mL) absolute alcohol
Fit up the apparatus as shown. The distilling flask should have a capacity of not less than 1 litre, and is attached to a long condenser. An adapter is fixed to the end of the
condenser, dipping into a conical flask (250 mL), which serves as receiver. The alcohol and sulphuric acid are mixed in the distilling flask and cooled to the ordinary
temperature tinder the tap. The potassium bromide, coarsely powdered, is then added. The flask, which is closed with a cork, is fixed to the condenser and heated on the sand-bath.
A sufficient quantity of water is poured into the receiver to close the end of the adapter. After a short time the liquid in the flask begins to boil and froth up, and the ethyl
bromide, in the form of heavy drops of colourless liquid, distils and collects at the bottom of the receiver. If the liquid threatens to froth over, the flask must be raised from
the sand-bath for a moment. The distillation is continued until no further drops of oil appear at the end of the condenser. As the ethyl bromide has a low boiling point (38-39°C),
it is desirable to surround the receiver with ice during this operation. The distillate is now removed and poured into a separating funnel, and the lower layer of ethyl bromide
separated. The water is thrown away and the ethyl bromide poured back together with about an equal bulk of dilute sodium carbonate solution and shaken up. The ethyl bromide is
withdrawn, as before, and again shaken up with water. Finally, it is carefully separated from the water and run into a dry distilling flask. The small quantity of water which
remains. and renders the liquid turbid, is removed by adding a dehydrating agent.
Dehydration
Moisture can be readily removed from liquids by adding a solid hygroscopic substance which does not act chemically upon the liquid. The common dehydrating agents are
calcium chloride, potassium carbonate, sodium sulphate (anhydrous), quicklime, etc. Alkalis can not of course be used for dehydrating organic acids, nor can calcium chloride be
employed in conjunction with alcohols or organic bases, with which it combines. In the present instance it can be used. A few small pieces of the granulated or fused calcium
chloride are added to the liquid. The flask is corked and left to stand for some hours until the liquid becomes clear. It is then distilled. A thermometer is inserted into the neck
of the flask with the bulb just below the side tube. The flask is attached to a condenser and heated gently on the water bath, so that the liquid distils at a moderate speed (2-3
drops a second). The temperature is noted and the portion boiling at 35-43°C collected in a separate flask. This consists of ethyl bromide which may contain a little ether. Yield 75-80 grams.
Ethyl Bromide - Method 22
The preparation of ethyl bromide has assumed great importance of late, and hence any improvements in the methods for increasing its yield are to be welcomed.
The laboratory methods - based on the method of De Vrij1 — as given in the usual practical books on "organic preparations" fall short in the following respects:
- A large excess of alcohol is used, giving a yield of less than 50% of ethyl bromide on the alcohol used.
- The formation of ether, owing to (a) the large excess of alcohol used, and (b) the high temperature used the reaction. This necessitates special treatment of the crude ethyl bromide for the removal of the ether, with concomitant loss of ethyl bromide.
- The use of potassium bromide instead of the less expensive sodium bromide.
By using the quantities given below, and paying attention to the rate of limiting the reaction mixture, a minimum yield of pure ethyl bromide of 80% of the theoretical on either the sodium bromide or alcohol used can be obtained, and with special care yields of 90%, are possible; also, it is possible to obtain these yields without the formation of even a trace of ether, thus obviating loss of time and material in purifying the crude product.
Experimental
Preparations of Ethyl Bromide
- Molecular proportions by weight of absolute alcohol, sulphuric acid (d. 1.84), and sodium bromide.
- Water to the extent of 5-10% of the weight of alcohol taken.
- Excess of sulphuric acid to the extent of 5% of the theoretical weight required.
Method
Typical results obtained
by the method described
EtOH | H2O | H2SO4 | NaBr | EtBr | Yield |
92g | — | 200g | 206g | 185g | 84.0% |
138g | — | 300g | 309g | 270g | 85.6% |
184g | — | 400g | 412g | 370g | 84.9% |
276g | 15g | 620g | 618g | 550g | 84.0% |
276g | 20g | 625g | 618g | 556g | 85.0% |
The alcohol and water are mixed in a round-bottom flask of requisite size, and the sulphuric acid is added, the mixture being well cooled during the process. To the cold mixture, the sodium bromide, in very coarsely powdered form, is added. The flask is then supported in a capacious water-bath fixed to a good condenser, the outlet of which dips into water contained in a stout, wide-mouthed bottle standing in a trough of cold water. The water-bath is heated very gently, so that the rise of temperature is slow, until drops of ethyl bromide are seen to fall from the condenser into the receiver; the temperature is only allowed to rise as the rate of formation of ethyl bromide slackens, until the water is boiling; when the rate of formation of ethyl bromide at this temperature slackens, the water-bath is removed and a sand-bath put in its place; again the temperature is carefully raised, so that the ethyl bromide distils over regularly, the heating being continued until no more ethyl bromide is obtained. By paying careful attention to the heating, no frothing is obtained, and for quantities of six gram-molecular weights of each of the reacting substances a 2-litre round-bottomed flask is of ample size. The only loss, so far as has been ascertained, is that, of hydrogen bromide during the first hour of heating, and this doubtless would be brought to a minimum by increasing the quantity of water, within limits, and allowing longer time for heating. Most of the hydrogen bromide is dissolved in the water contained in the receiver, and can be recovered as sodium bromide. The crude ethyl bromide is separated from the acid water, washed with very dilute sodium carbonate solution, and then once with water, and allowed to remain over fused calcium chloride for half an hour. On distilling slowly from a water-bath, it will be found that more than 99% of the crude ethyl bromide will distil between 38.5°C and 39.5°C.
Ethyl Bromide - Method 33
In a recent paper2 F. E. Weston discusses the best conditions for the preparation of ethyl bromide. As the present author has recently had occasion to prepare this compound on a fairly large scale, and, as Mr. Weston truly remarks, its preparation has assumed great importance, the following notes are possibly worth recording.
The problem presented was how to prepare the greatest quantity of ethyl bromide from any given weight of sodium or potassium bromides, for, owing to the high price of bromides compared with alcohol or sulphuric acid, a moderate waste of the latter substances was immaterial.
The great difficulty to contend with was the production of ether, since its separation from ethyl bromide by means of concentrated sulphuric acid and subsequent washing until free from acid is both tedious and wasteful.
Experimental
The usual mixture was prepared as follows:
To 1500 mL of concentrated sulphuric acid were gradually added 900 mL of water, and, after cooling, 1500 mL of absolute alcohol were slowly run care being taken to prevent the temperature rising too much. When cold, 1100 to 1200 grams of potassium bromide or an equivalent quantity of the sodium salt were added. [When sodium bromide (NaBr·2H2O) was employed the volume of water added to the acid was slightly diminished]. The whole mixture was then heated in a 14-litre flask on a sand-bath at the lowest temperature at which ethyl bromide would distil over. The distillation required about eight or nine hours. The ethyl bromide was collected in water, washed by repeated shaking with water, and dried over calcium chloride.
Determination of the boiling point in order to test the purity of the compound is unsatisfactory, as a mixture of ether and ethyl bromide of constant boiling point can be obtained, so the purity of the product was determined by its specific gravity. Usually the ethyl bromide was practically free from ether.
The best yield obtained was 96% calculated on the weight of anhydrous bromide taken, and it rarely fell below 90%, the variations undoubtedly arising from irregularities in the temperature at which the distillation was carried out. Practically no hydrobromic acid distilled over.
The largest amount of reaction mixture employed in any single preparation was about 8000 grams, and the distillation, although tedious, presented no difficulties.
It is clear from both the preparations of Mr. Weston and the present author that low temperature with consequent slow distillation and a large quantity of a not too concentrated alcohol sulphuric acid-water mixture are necessary for good yields. Frothing never gave the least trouble.
