SYNTHESIS OF ISOBUTYL BROMIDE (l-Bromo-2-methylpropane)

• Place 92.5g (115ml, 1.25 mol) of isobutyl alcohol (2-methylpropan-l-ol) and 8.55 g (0.275 mol) of purified red phosphorus in a 500 ml three-necked flask fitted with a sealed mechanical stirrer, a reflux condenser and a dropping funnel containing 100 g (32 ml, 0.62 mol) of bromine (for precautions in the use of bromine). 

• Start the stirrer, heat the flask (e.g., in an oil bath) so that the contents reflux gently, and introduce the bromine at such a rate that it appears to react completely so that there is little bromine vapour above the surface of the reaction mixture, and the reaction is under control. 

• When all the bromine has been added, reflux the mixture gently for 1 5-30 minutes more. 

• Remove the stirrer, arrange the condenser for downward distillation and distil off most of the isobutyl bromide (1). 

• Then add about 50 ml of water through the dropping funnel and continue the distillation to remove the remainder of the product. 

• Separate the crude bromide and wash it successively with water, an approximately equal volume of concentrated hydrochloric acid, water, 10 per cent sodium carbonate solution, and finally water. 

• Dry the product over anhydrous calcium chloride and distil, collecting the isobutyl bromide which passes over at 91-94 °C. The yield is 150 g (91%).

Notes to keep in mind:

1. It is not advisable to distil the mixture almost to dryness since the formation of flammable alkene may then occur. This is avoided by conducting the distillation in two stages as described.

Cognate preparations:* 1-Bromohexane 

• Use 152.5 g (186.5 ml, 1.49mol)of hexan-1-ol, 9.3 g (0.3 mol) of purified red phosphorus and 120g (38.5ml, 0.95 mol) of bromine;t b.p. 154-1 56 °C.

1-Bromoheptane:

• Use 1 73 g (209 ml, 1 .49 mol) of heptan- 1 -ol, 9.3 g (0.30 mol) of purified red phosphorus and 120g (38.5 ml, 0.95 mol) of bromine; b.p. 180 °C.

1-Bromooctane:

• Use 81 g (98.5 ml, 0.623 mol) of octan-1-ol (b.p. 193-194 °C), 5.18 g (0.1 67 mol) of purified red phosphorus and 55 g (18 ml, 0.343 mol) of bromine; b.p. 198-201 °C.

l-Bromododecane:

• Use 116g (0.623 mol) of dodecan-1-ol (lauryl alcohol), m.p. 24 °C, 5.18g (0.167 mol) of purified red phosphorus and 55g (18ml, 0.343 mol) of bromine. 

• Heat the alcohol-phosphorus mixture to about 250 °C with vigorous stirring and add the bromine slowly. 

• Allow the mixture to cool after all the bromine has been introduced. 

• Add ether, filter off the excess of phosphorus and wash the ethereal solution of the bromide with water and dry over anhydrous potassium carbonate. 

• Remove the ether on a water bath, and distil the residue under reduced pressure, b.p. 149-151 °C/18mmHg.

1-Bromotetradecane:

• Use 107 g (0.5 mol) of tetradecan-1-ol (m.p. 38 °C), 3.4 1 g (0. 1 1 mol) of purified red phosphorus and 44 g ( 1 4.5 ml, 0.275 mol) of bromine and proceed as under 1-bromododecane; b.p. 178.5-179.5 °C/ 20mmHg, m.p. 5°C.

1-Bromohexadecane:

• Use 121 g (0.5 mol) of hexadecan-1-ol (cetyl alcohol), m.p. 48 °C, 3.41 g (0.1 1 mol) of purified red phosphorus and 44 g (0.275 mol) of bromine, and proceed as for 1-bromododecane; filter off the excess of phosphorus at 16-20°C; b.p. 202-203 "C/21 mmHg; m.p. 14 °C.

I-Bromo-2-phenylethane:

• Use 1 52.5 g (148 ml, 1.25 mol) of 2-phenylethanol, b.p. 216.5-217 °C, 10.35 g (0.33 mol) of purified red phosphorus and 1 10 g (35.5 ml, 0.68 mol) of bromine. Isolate the l-bromo-2-phenylethane as detailed for 1-bromododecane; b.p. 98°C/12mmHg.

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

• Use 45 g (0.5 mol) of redistilled butane- 1,4-diol, 6.84 g (0.22 mol) of purified red phosphorus and 80 g (26 ml, 0.5 mol) of bromine. 

• Heat the glycol-phosphorus mixture to 100-1 50 °C and add the bromine slowly; continue heating at 100-150 °C for 1 hour after all the bromine has been introduced. 

• Allow to cool, dilute with water, add 100 ml of ether and remove the excess of red phosphorus by filtration. 

• Separate the ethereal solution of the dibromide, wash it successively with 10 per cent sodium thiosulphate solution and water, then dry over the anhydrous potassium carbonate. 

• Remove the ether on a water bath and distil the residue under diminished pressure. Collect the 1,4-dibromobutane at 83-84 °C/ 12mmHg; the yield is 73 g (67%).

1,6-Dibromohexane:

• Proceed as for 1,4-dibromobutane but use 58 g (0.49 mol) of hexane-l,6-diol. The yield of 1,6-dibromohexane, b.p. 114-115 °C/12 mmHg, is 85 g (71%).

1,4-Dibromobutane (from tetrahydrofuran):

• Place 18.1 g (20.5 ml, 0.25 mol) of redistilled tetrahydrofuran (b.p. 65-66 °C), 3.41 g (0.11 mol) of purified red phosphorus and 4.5 g of water in the flask. 

• Heat the mixture gently and add 40g (13 ml, 0.25 mol) of bromine at such a rate that there is little bromine vapour above the surface of the reaction mixture. 

• Heat at 100-150 °C for 45-60 minutes after all the bromine has been introduced. 

• Work up as for the butane- 1,4-diol preparation. The yield of 1,4-dibromobutane, b.p. 83-84 °C/ 12 mmHg, is 42 g (72%).

1,5-Dibromopentane (from tetrahydropyran):

• Proceed as in the previous preparation but replace the tetrahydrofuran by 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/13 mmHg, is 43 g (75%).