Hydrobromic acid, constant-boiling

Properties—Hydrogen iodide is a colourless gas. It is very soluble in water and fumes in moist air (cf. hydrogen chloride), to give hydriodic acid. Its solution forms a constant boiling mixture (cf. hydrochloric and hydrobromic acids). Because it attacks mercury so readily, hydrogen iodide is difficult to study as a gas, but the dissociation equilibrium has been investigated. 

Method 2 (from potassium bromide and sulphuric acid). Potassium bromide (240 g.) is dissolved in water (400 ml.) in a litre flask, and the latter is cooled in ice or in a bath of cold water. Concentrated sulphuric acid (180 ml.) is then slowly added. Care must be taken that the temperature does not rise above 75° otherwise a little bromine may be formed. The solution is cooled to room temperature and the potassium bisulphate, which has separated, is removed by flltration through a hardened Alter paper in a Buchner funnel or through a sintered glass funnel. The flltrate is distilled from a litre distilling flask, and the fraction b.p. 124 127° is collected this contains traces of sulphate. Pure constant boiling point hydrobromic acid is obtained by redistillation from a little barium bromide. The yield is about 285 g. or 85 per cent, of the theoretical. 

A solution of cuprous bromide may be prepared either by dissolving the solid in hot constant boiling point hydrobromic acid or by refluxing a mixture of 63 g. of crystallised copper sulphate, 20 g. of copper turnings, 154 g. of sodium bromide dihydrate, 30 g. (16-3 ml.) of concentrated sulphuric acid and 1 litre of water for 3-4 hours. If the colour of the solution has not become yellowish after this period of heating, a few grams of sodium sulphite should be added to complete the reduction. 

By slow distillation of the alcohol with constant boiling point (48 per cent.) hydrobromic acid, for example ... 

By treatment of the alcohol with a mixture of constant boiling point hydrobromic acid and concentrated sulphuric acid the presence of sulphuric acid results, as a rule, in more rapid reaction and improved 3delds. A typical example is ... 

Mix 40 g. (51 ml.) of isopropyl alcohol with 460 g. (310 ml.) of constant boiling point hydrobromic acid in a 500 ml. distilling flask, attach a double surface (or long Liebig) condenser and distil slowly (1-2 drops per second) until about half of the liquid has passed over. Separate the lower alkyl bromide layer (70 g.), and redistil the aqueous layer when a further 7 g. of the crude bromide will be obtained (1). Shake the crude bromide in a separatory funnel successively with an equal volume of concentrated hydrochloric acid (2), water, 5 per cent, sodium bicarbonate solution, and water, and dry with anhydrous calcium chloride. Distil from a 100 ml. flask the isopropyl bromide passes over constantly at 59°. The yield is 66 g. 

The residue in the flask may be mixed with the aqueous layer of the first distillate, 40 g. of isopropyl alcohol added, and the slow distillation repeated. The yield of crude isopropyl bromide in the second distillation is only slightly less than that obtained in the original preparation. Subsequently most of the residual hydrobromic acid may be recovered by distillation as the constant boiling point acid (126°). 

This solution should be returned to the storeroom for subsequent recovery as constant boiling point hydrobromic acid. If time, permits the students should carry out this operation. Distil slowly from a distilling flask and when the tern-... 

A considerable quantity of constant boiling point hydrobromic acid may be obtained by distilling these solutions. 

Owing to the comparatively negligible difference in the cost of bromine and the equivalent quantity of constant boiling point hydrobromic acid, there is little to be gained—apart from the instructional value— in preparing the hydrobromic acid from bromine in the preparation of alkyl bromides. 

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. 

Method 2. Prepare 40 g." of cuprous bromide according to Section 11,50,2 (about 75 g. of crystallised copper sulphate are required) and dissolve it in 40 ml. of constant boiling point hydrobromic acid (48% HBr) contained in a 2 5 litre roimd-bottomed flask. 

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. 

Synthetic methods. / / -Dimethylglutaric acid (prepared from mesityl oxide) was converted into the di-silver salt, which, by an improvement of the method of Windaus and Klanhardt, was converted into / / -dimethyl-y-butyrolactone. The latter on treatment with constant-boiling hydrobromic and sulphuric acid gave y bromo /i/ dimethylbutyric acid which readily gave its ethyl ester. The pure fluoro ester was obtained from this by heating with silver fluoride, although the yield was low. 

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. 

The 2 1. (2750 g.) of 40 per cent hydrobromic acid may advantageously be replaced by a corresponding quantity (1550 cc.) of constant-boiling 48 per cent hydrobromic acid, should this be available. ... 

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