Boiling Hydrobromic Acid

Add 120 g. of powdered potassium bromide to 200 ml. of water. Place the container in cold water, and slowly add 90 ml. of cone, sulfuric acid (1.7 mols). The temperature should not go appreciably above 75°C. otherwise a small amount of free bromine may be formed. However, the formation of a small amount of bromine is not particularly serious, since it will come over with the distillate boiling between 100 to 115°C. Cool the solution to room temperature, and remove the potassium acid sulfate by filtering through a hardened filter paper in a Buchner funnel. Place the filtrate in a 500-ml. distilling flask connected to a water condenser fitted with an adapter, and heat over a wire gauze. If the presence of 0.01 to 0.015 per cent of sulfate ion is not objectionable, reserve the distillate that starts to come over 1° below the temperature of the constant-boiling mixture. The distillation should be stopped when the temperature drops. The specific gravity of the solu- 

The yield of the product was considerably lowered by reducing the ratio of potassium bromide to sulfuric acid. 

Blanchard, and Hall Laboratory Methods of Inorganic 

Organic Syntheses Collective Vol. I, p. 23, John Wiley Sons, Inc., 

Two procedures are given here one. of the hydrogen sulfide method, and the other the catalytic union of the elements followed by the absorption of the gas in water. 

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The thin film of oxide which remains in the casserole is dissolved by treating with a little constant-boiling hydrobromic acid (Note 7) the solution obtained may then be evaporated and the residue added to the next fusion mixture. 

The upper aqueous layer contains considerable unchanged hydrobromic acid. If this layer is distilled, about 150 g. (0.9 mole) of constant-boiling hydrobromic acid (b.p. 123-124°/748 mm.) may be recovered. 

The hydrogen, bromide may be absorbed in water and constant-boiling hydrobromic acid formed (Org. Syn. Coll. Vol. 

Constant-boiling hydrobromic acid can be recovered by fractionating the distillate at atmospheric pressure. 

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° at 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... 

Alkyl Bromides General Discussion. When an alcohol is heated with aqueous 48 per cent hydrobromic acid, a partial conversion takes place into the corresponding bromide. The reaction is, however, more rapid and more complete in the presence of sulfuric acid. Although the constant boiling hydrobromic acid obtainable on the market may be used in all the above experiments, its preparation by the sulfur-dioxide reduction of bromine will be considerably cheaper and equally convenient, provided 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. 

The formation of alkyl bromides is more ready than that of the alkyl chlorides. Hence secondary as well as tertiary bromides can be obtained directly from the corresponding alcohols by heating with constant boiling hydrobromic acid (e.g. Expt 5.53), although in the case of primary alcohols the presence of sulphuric acid results, as a rule, in a more rapid reaction and in improved yields. 

Hydrobromic Acid. Hydrobromic acid may be prepared conveniently by the interaction of bromine and sulfur dioxide in the presence of water.7 An even simpler procedure for the preparation of constant-boiling hydrobromic acid consists of the action of sulfuric acid upon a bromide under conditions such that no oxidation takes place (procedure C)8 or by the bromination of a 1 1 mixture of water and tetrahydronaphthalene (procedure A, note). 

If 48 per cent (constant boiling) hydrobromic acid is used the diazotization is very difficult to control. The reaction becomes very vigorous and forces out the stopper. 

To the reaction flask containing the crude trimethylethylene dibromide is added 540 cc. of water, and the flask is fitted with a long reflux condenser and a mercury-sealed stirrer (Notes 3 and 4). The mixture is refluxed with stirring for three to five hours until hydrolysis is practically complete (Note 5). The reflux condenser is then replaced by a condenser for distillation, and the crude methyl fio-propyl ketone is removed from the reaction mixture by direct distillation, with stirring, until the temperature rises (Note 6) and the oil nearly stops coming over (about one and one-half hours), or until the oil coming over is heavier than water. The residue in the flask can be distilled to yield about 380 cc. of constant-boiling hydrobromic acid. 

Bromination of the parent heterocycle with bromine in acetic acid gives the 1-bromo derivative with one equivalent of bromine in the cold and the 1,3-dibromo compound with two equivalents of bromine at room temperature. Three equivalents of bromine in boiling hydrobromic acid give a mixture of the 2,3-dibromo and the 1,2,3-tribromo compounds. High-temperature chlorination with a mixture of phosphoryl chloride and phosphorus pentachloride gives a mixture of the 1-chloro and 1,2,3-trichloro derivatives. ... 

Amino acids are accessible by use of ethyl (acetylamino)malonate. Dittmer, Hert, and Chambers treated 2-(chloromethyl)thiophene with ethyl (acetyl-amino)malonate in the presence of sodium ethoxide, obtaining an 87% yield of -(acetylamino)-a>(2-thienyl)malonate which was converted into /3-(2-thienyl)alanine in 78% yield by boiling hydrobromic acid 390... 

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