SYNTHESIS OF BUTYL BROMIDE (1-Bromobutane)

• To 250 g of 48 per cent hydrobromic acid contained in a 500-ml round-bottomed flask add 75 g (41 ml) of concentrated sulphuric acid in portions with shaking; some hydrogen bromide may be evolved. 

• Add 88 g (110 ml, 1.2 mol) of butan-1-ol, followed by 60 g (32.5 ml) of concentrated sulphuric acid in several portions with shaking, and finally a few chips of porous porcelain. 

• Attach a reflux condenser to the flask and reflux the mixture gently on a wire gauze for 2-3 hours; during this period the formation of butyl bromide is almost complete and a layer separates above the acid (1). 

• If the preparation is carried out in the open laboratory, fit an absorption device to the top of the condenser in order to absorb any hydrogen bromide and sulphur dioxide which may be evolved. 

• Allow the contents of the flask to cool, remove the condenser and set it for downward distillation. 

• Distil the mixture until no more oily drops of butyl bromide pass over (30-40 minutes). 

• Transfer the distillate to a separatory funnel and remove the halide which forms the lower layer. 

• Wash it successively with water, an equal volume of concentrated hydrochloric acid (2), water, 5 per cent sodium hydrogen carbonate or sodium carbonate solution, and water. 

• Separate the water as completely as possible and dry with 2-3 g of anhydrous calcium chloride or magnesium sulphate; the desiccant should be left in contact with the bromide for at least 30 minutes and shaken occasionally. 

• Filter the dried product through a small funnel supporting a fluted filter paper into a 200-ml flask, add a few chips of porous porcelain and distil either from an air bath or on a ceramic-centred wire gauze. 

• Collect the portion boiling at 100-103 °C. The yield is 155g (95%).

Notes to keep in mind:

1. A suitable hydrogen bromide medium can be prepared by dissolving 240 g of potassium bromide in 400 ml of warm water, cooling and adding 200 ml of concentrated sulphuric acid slowly and with constant stirring, so that the temperature does not rise above 40 °C. After further cooling to 15°C, the mixture is filtered and the butan-1-ol is added to the filtrate. A further 120 ml of concentrated sulphuric acid is then added carefully and the mixture is heated under reflux for 3-4 hours.

2. The crude bromide contains a little unchanged alcohol and is said to contain some dibutyl ether (b.p. 141 °C). The former is removed by washing with concentrated hydrochloric acid and this purification process is satisfactory for most purposes. Both the alcohol and the ether are removed by washing with 1 1-12 ml of concentrated sulphuric acid; the butyl bromide is not affected by this reagent.

Cognate preparations: s-Butyl bromide (2-bromobutane)

• The quantities required are as for butyl bromide but with butan-2-ol (b.p. 99-100 °C) replacing the butan-1-ol. 

• Two to three washings with concentrated hydrochloric acid are necessary, i.e. until the volume of the acid layer remains unchanged on shaking with halide. The yield of s-butyl bromide, b.p. 90.5-92.5°C,isl50g(92%).

1-Bromopentane (pentyl bromide): 

• Use 210 g (142 ml) of 48 per cent hydrobromic acid, 60 g (33 ml) of concentrated sulphuric acid, followed by 88 g (108ml, lmol) pentan-1-ol (b.p. 135-136 °C) and lOg (5.5ml) of concentrated sulphuric acid. 

• Distil the product through a short fractionating column, and collect the 1-bromopentane at 127-130 °C (135 g, 89%).

l-Bromo-3-methylbutane: 

• Proceed as for 1-bromopentane, but use 88 g (109ml, lmol) of 3-methylbutan-l-ol, b.p. 129.5-131 °C. 

• Distil the purified product through a fractionating column and collect the l-bromo-3- methylbutane at 1 17-120 °C (125 g, 83%).

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; finally add 58 g (68 ml, 1 mol) of pure allyl alcohol. 

• Fit the flask with a separatory funnel, a mechanical stirrer and an efficient condenser (preferably of the double surface type) set for downward distillation. 

• 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 flask through a short fractionating column. 

• The yield of allyl bromide, b.p. 69-72 °C, is 1 1 2 g (93%). 

• There is a small high-boiling fraction containing 1,2-dibromopropane.

1,3-Dibromopropane (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 propane-l,3-diol (b.p. 210-215 °C), followed by 240g (130.5ml) of concentrated sulphuric acid slowly and with shaking. 

• Attach a reflux condenser to the flask and reflux the mixture for 3-4 hours. 

• Arrange for downward distillation and distil, using a wire gauze, until no more oily drops pass over (30-40 minutes). 

• Purify the 1,3-dibromopropane as detailed for butyl bromide above. About 220 g (91%) of the pure dibromide, b.p. 162-165 °C, are obtained.

1,4-Dibromobutane (from butane- 1 ,4-diol):

• In a 500-ml three-necked flask fitted with a stirrer, reflux condenser and dropping funnel, place 154g (105 ml) of 48 per cent hydrobromic acid. 

• Cool the flask in an ice bath. Add slowly, with stirring, 130g (71ml) of concentrated sulphuric acid. 

• To the resulting ice-cold solution add 30g (0.33 mol) of redistilled butane- 1,4-diol 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 magnesium sulphate. 

• Distil and collect the 1,4-dibromobutane at 83-84 °C/12mmHg. The yield is 55 g (76%).

1,4-Dibromobutane (from tetrahydrofuran):

• Place a mixture of 250 g (170 ml) of 48 per cent hydrobromic acid and 75 g (41 ml) of concentrated sulphuric acid in a 500-ml round -bottomed flask, add 18.1 g (20.5 ml, 0.25 mol) of re-distilled tetrahydrofuran (b.p. 65-66 °C), attach a reflux condenser and reflux gently for 3 hours. 

• Separate the lower layer of dibromide and purify as in the previous preparation. 

• The yield of 1,4-dibromobutane, b.p. 83-84°C/12mmHg is 40 g (74%).

1 ,5-Dibromopentane (from pentane-1 ,5-diol):

• Proceed as for 1,4-dibromo-butane but use 35 g (0.33 mol) of redistilled commercial pentane-l,5-diol. The yield of 1,5-dibromopentane, b.p. 99°C/13mmHg, is 39 g (51%).

1 ,5-Dibromopentane (from tetrahydropyran): 

• Proceed as for 1,4-dibromo-butane (from tetrahydrofuran) but use 21.5 g (24.4 ml, 0.25 mol) of redistilled tetrahydropyran (b.p. 86.5-87.5 °C). The yield of 1,5-dibromopentane, b.p. 99°C/13mmHg, is 46 g (80%).