Hydrobromic acid

Hydrobromic acid is used in the manufacture of bromide, as an alkylation catalyst, and in organic synthesis. 

Hydrobromic acid or hydrogen bromide is a colorless and corrosive liquid or gas, respectively, with a sour taste and acrid odor. The gas liquefies at —66.5°C (—87°F) and freezes at —86°C (—122°F). The constant-boihng acid is an aqueous solution of 47.5% HBr, boiling at 126°C (258°F). Saturated aqueous solution contains 66% HBr at 25°C (77°F) the aqueous solutions are strongly acidic. 

Hydrobromic acid is a corrosive hquid. The gas is a strong irritant to the eyes, nose, and mucous membranes. In humans, exposure to 5 ppm for a few minutes cau cause irritation of the nose. Irritation of the eyes and lungs may be felt at higher concentrations. The detectable odor threshold is 2 ppm. 

Hydrogen bromide is a noncombustible gas. The acid or the gas in contact with common metals and in the presence of moisture produces hydrogen. Violent reactions occur with ammonia and ozone (Mellor 1946, Suppl. 

The obvious method for preparing hydrobromic acid would be to treat solid sodium bromide with concentrated sulphuric acid. This is not a successful method, however, owing to the fact that hydrogen bromide reacts to some extent with concentrated sulphuric acid according to the equation 

The method we shall use starts with bromine and allows it to react with red phosphorus and water. Phosphorus and bromine combine very easily to form PBr3, but the phosphorus bromide hydrolyzes completely and hydrogen bromide passes over to the absorption flask. 

4 solid rubber stoppers to fit small U-tubes. one 1-hole rubber stopper to fit large U-tube. glass delivery tubes as in diagram, glass wool. 

Why would not a metal like zinc, which will combine vig-orously with bromine, serve instead of phosphorus  

Explain why at the outset of the process bubbles pass outward through the large U-tube, and later they pass in the opposite direction. 

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Hydrobromic acid is rather easily oxidised when exposed to light and becomes brown due to the bromine liberated. Otherwise, its properties are those of a strong acid, similar to hydrochloric acid. 

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. 

Addition of concentrated sulphuric acid to a solid bromide produces hydrobromic acid, but also some bromine (brown vapour). 

Hydrobromic acid. Method 1 (from bromine and sulphur dioxide). A mixture of 600 g. (or 188-6 ml.) of bromine, 250 ml. of water and 760 g. of crushed ice is placed in a 1 6 litre round-bottomed flask and a rapid stream of sulphur dioxide (from a siphon of the liquefied gas) is passed into the flask, care being taken that the outlet of the gas-delivery tube is below the surface of the bromine layer. The rate of flow of the gas is adjusted so that it is completely absorbed. 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 a uniform yellowish-brown or yellow colour, which is unaffected by further introduction of sulphur dioxide excess of the latter gas should be avoided as it will be... 

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. 

It is rather slow at moderate temperatures sind the hydrobromic acid formed in the initial stages of the resLCtion inhibits its further progress. By carrying out the reaction at 60-70 or above in the presence of a large excess of water, the inhibition observed at lower temper, atuies does not occur. 

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 ... 

The hydrobromic acid sulphuric acid solution may be prepared by the reduction of bromine with sulphurous acid (Section 11,49, 1) distillation of the reaction product is unnecessary ... 

The dibrotnides of aliphatic glycols are best prepared by mixing the glycol with a cold hydrobromic acid - sulphuric acid mixture, allowing to stand for 24 hours, and heating on a steam bath for three hours ... 

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°). 

Trimethylene Dibromide. In a 1-litre round-bottomed flask place 500 g. (338 ml.) of 48 per cent, hydrobromic acid and add 150 g. (82 ml.) of concentrated sulphuric acid in portions, with shaking. Then add 91 g. of trimethylene glycol (b.p. 210-215°), followed by 240 g. (130-5 ml.) of concentrated sulphuric acid slowly and with shaking. Attach a reflux condenser to the flask and reflux the mixture for 3 hours. Arrange for downward distillation and distil, using a wire gauze, until no more oily drops pass over (30—40 minutes). Purify the trimethylene dibromide... 

Allyl Bromide. Introduce into a 1-litre three-necked flask 250 g. (169 ml.) of 48 per cent, hydrobromic acid and then 75 g. (40-5 ml.) of concentrated sulphuric acid in portions, with shaking Anally add 58 g. (68 ml.) of pure allyl alcohol (Section 111,140). Fit the flask with a separatory funnel, a mechanical stirrer and an efficient condenser (preferably of the double surface type) set for downward distillation connect the flask to the condenser by a wide (6-8 mm.) bent tube. Place 75 g. (40 5 ml.) of concentrated sulphuric acid in the separatory funnel, set the stirrer in motion, and allow the acid to flow slowly into the warm solution. The allyl bromide will distil over (< 30 minutes). Wash the distillate with 5 per cent, sodium carbonate solution, followed by water, dry over anhydrous calcium chloride, and distil from a Claisen flask with a fractionating side arm or through a short column. The yield of allyl bromide, b.p. 69-72°, is 112 g. There is a small high-boiling fraction containing propylene dibromide. 

Dibromobutane from 1 4 butanediol). In a 500 ml. threenecked flask fltted with a stirrer, reflux condenser and dropping funnel, place 154 g. (105 ml.) of 48 per cent, hydrobromic acid. Cool the flask in an ice bath. Add slowly, with stirring, 130 g. (71 ml.) of concentrated sulphuric acid. To the resulting ice-cold solution add 30 g. of redistilled 1 4-butanediol dropwise. Leave the reaction mixture to stand for 24 hours heat for 3 hours on a steam bath. The reaction mixture separates into two layers. Separate the lower layer, wash it successively with water, 10 per cent, sodium carbonate solution and water, and then dry with anhydrous magnesium sulphate. Distil and collect the 1 4-dibromo-butane at 83-84°/12 mm. The yield is 55 g. 

Anhydi ous pinacol (I) is catalytically decomposed by aqueous hydrobromic acid into dimethyl butadiene (II) and pinacoloiie (III) separation is effected by distillation through an efficient fractionating column ... 

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-... 

Continue the passage of bromine vapour until the solution in A assumes a distinctly yellow colour (2-3 hours) the reaction is then complete. Filter the tribromoaniline on a Buchner funnel, wash it thoroughly with water to remove hydrobromic acid, and suck as dry as possible. Ilecr3 stallise from methylated (or rectified) spirit. The yield is 22 g. m.p. 120°. 

In the preparation of bromo compounds by the Sandmeyer reaction, the amine is generally diazotised in sulphuric acid solution (or in hydrobromic acid solution), and the resulting aryldiazonium sulphate (or bromide) is treated with a solution of cuprous bromide in excess of hydrobromic acid the addition... 

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

In a 1 or 1-5 htre round-bottomed flask prepare a solution of 53-5 g. of o-toluidine in 170 ml. of 48 per cent, hydrobromic acid, cool to 5° by immersion in a bath of ice and salt. Diazotise by the gradual addition of a solution of 36 -5 g. of sodium nitrite in 50 ml. of water stopper the flask after each addition and shake until all red fumes are absorbed. Keep the temperature between 5° and 10°. When the diazotisation is complete, add 2 g. of copper powder or copper bronze, attach a reflux condenser to the flask, and heat very cautiously on a water bath. Immediately evolution of gas occurs, cool the flask in crushed ice unless the... 

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

The last-named reaction provides an excellent method for the preparation of a-substituted glutaric acids the intermediate alkyl (aryl) -2-cyanoethyl-malonate is both hydrolysed and decarboxylated re ily by boiling with an excess of 48 per cent, hydrobromic acid solution. 

Aminopyridine (I) is converted by diazotisation in the presence of bromine and concentrated hydrobromic acid Into 2 broraopyridine (II) the latter upon treatment with copper powder in the presence of p-cymene yields 2 2 -dipyridyl (III). 

The basic premise for making bromosafrole has been to mix sa-frole with Hydrobromic Acid (a.k.a. hydrogen bromide, HBr). That s it. The HBr does what is called a Markovnikov addition reaction whereby the HBr sees the allyl double bond of safrole and preferentially attaches its hydrogen to the gamma carbon and its bromine to the middle beta carbon (don t ask). 

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. 

Technically, the chemist could avoid the complex glassware apparatus of this procedure for a more crude approach [104]. This report shows some dudes de-methylating an amphetamine with concentrated HCI in a pressure cooker. A similar approach with good yields was also employed in ref. 83 and should work as well or better on guaiacol. Hydroiodic acid or hydrobromic acid will work better than hydrochloric acid but, you know, whatever floats the chemist s boat. To do this the chemist can just plain reflux HI or HBr with the guaiacol for a few hours and process as before or she can use HI, HBr or HCI and place the reactants in a pipe bomb for a few hours. 

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