Aq Hydrobromic Acid

Recipes for making HBr (from Vogel's "Practical Organic Chemis-try"[37])

"Hvdrobromic acid. Method 1 (from bromine and sulphur dioxide). A mixture of 600g. (or 188.5ml.) of bromine, 250ml. of water and

750g. of crushed ice is placed in a 1.5 litre round-botomted flask and a rapid stream of sulphur dioxide (from a siphon of the liquified gas) is passed into the flask, care being taken that the outlet of the gas-delivery tube id below the surface of the bromine layer. The rate of flow of the gas is adjusted so that it is completely adsorbed. It is advisable to cool the flask in ice and also to shake the contents from time to time. The reduction is complete when the mixture assumes uniform yellowish-brown or yellowish colour, which is unaffected by further introduction of sulphur dioxide; excess of the latter gas should be avoided as it will be evolved during the subsequent distillation The flask is then connected with a short stilll head and condenser, and the mixture is distilled. The main product will pass over at 125-12& / 760mm., but the temperature may rise to 130°; the distillation is then stopped. The residue is sulphuric acid. The distillate is redistilled from a little barium bromide in order to remove traces of sulphuric acid, and the fraction, b.p. 125-12&1-constant boiling point hydrobromic acid containing 48 per cent. HBr- collected. The yield is about 1150g. or 90 per cent, of the theoretical.

Br2 + S02 + 2H20 = H2S04 +2HBr.

Method 2 (from potassium bromide and sulphuric acid). Potassium bromide (240g.) is dissoleved in water (400ml.) in a litre flask, and the latter is cooled in ice or in a bath of cold water. Concentrated sulphuric acid (180ml.) is then slowly added. Care must be taken that the temperature does not rise above 7S3 otherwise a little bromine may be formed. The solution is cooled to room temperature and the potassium bisulphate, which has separated, is removed by filtration through a hardened filter paper in a Buchner funnel or through a sintered glass funnel. The filtrate 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 285g. or 85 per cent, of the theoretical.

Method 3 (from bromine and sulphur). (1). A 1-litre three necked flask is charged with 27g. of flowers of sulphur and 550 ml. of

water. The flask is equipped with a dropping funnel (with tip below the surface of the water), a water sealed mechanical stirrer and an Allihn reflux condenser; Ground glass joints are preferable, but used rubber stoppers are generally satisfactory. The flask is immersed in a bath of water at 60°, the mixture stirred vigorously and, when the temperature inside the flask is about 50°, 400g. (125.5ml) of bromine are introduced from the dropping funnel during about 20 minutes. The temperature of the reaction mixture rises rapidly as the reaction proceeds; the flask is cooled momentarily in a bath of cold water if the condensed bromine vapour is near the top of the condensor. When all the bromine has been added, the mixture is heated on a boiling water bath for 15 minutes. The reaction product is cooled, and filtered from residual sulphur and other solid matter through a sintered glass funnel. The filtrate is distilled and the constant b.p. hydrobromic acid collected at 125-126° / 760mm. The yield is 805g.

Note. (1) The reaction between bromine and sulphur in the presence of water may be represented by the equation:

3Br2 + S + H20 = 6HBr + H2S04

It is rather slow at moderate temperatures and the hydrobromic acid formed in the initial stages of the reaction inhibits its further progress. By carrying out the reaction at 50-7 (f or above in the presence of a large excess of water, the inhibition observed at lower temperatures does not occur.

Cognate preparation taken as one intended for HI (hydroiodic acid, Hey! If it works for HI, it probably works for HBr).

Hvdroiodic acid. A 1.5L three-necked flask is charged with a mixture of 480g. of iodine and 600mL water. The central aperature is fitted with a stopper carrying an efficient mechanical stirrer leading almost to the bottom of the flask, and the smaller apperatures respectively with a lead-in tube for hydrogen sulfide extending well below the surface of the liquid and with an exit tube attached to an inverted funnel just dipping into 5% sodium hydroxide solution. The mixture is vigorously stirred and a stream of hydrogen sul phide (either form a freshly charged Kipp's apparatus or from a cylinder of the gas) passed in as rapidly as it can be absorbed. After several hours the liquid assumes a yellow colour (sometimes it is almost colourless) and most of the sulphur sticks together in the form of a hard lump. The sulpur is removed by filtration through a funnel plugged with glass wool (or through a sintered glass funnel), and the filtrate is boiled until the lead acetate paper test for hydrogen sulphide is negative. The solution is filtered again, if necessary. The hydriodic acid is then distilled from a 500ml. Claisen flask, and the fraction b.p. 125.5- 126.5? / 760mm. is collected. This is the constant boiling point hydriodic acid and contains 57% of hydrogen iodide. The yield of the constant boiling acid is 785g. or 90% of the theoretical.

Note. The hard lump of sulphur remaining in the flask is best removed by boiling with concentrated nitric acid in the fume cupboard."

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