SYNTHESIS OF 1,4-DIIODOBUTANE

• In a 500-ml three-necked flask, equipped with a thermometer, a sealed stirrer unit and a reflux condenser, place 32.5 g of phosphorus pentoxide and add 1 15.5 g (67.5 ml) of 85 per cent  orthophosphoric acid (1). 

• When the stirred mixture has cooled to room temperature, introduce 166g (1 mol) of potassium iodide and 22.5 g (0.25 mol) of redistilled butane- 1,4-diol (b.p. 228-230°C or 133-135 °C/18 mmHg). 

• Heat the mixture with stirring at 100-120 °C for 4 hours. 

• Cool the stirred mixture to room temperature and add 75 ml of water and 125 ml of ether. 

• Separate the ethereal layer, decolourise it by shaking with 25 ml of 10 per cent sodium thiosulphate solution, wash with 100 ml of cold saturated sodium chloride solution, and dry with magnesium sulphate. 

• Remove the ether by flash distillation on a steam bath and distil the residue from a flask with fractionating side-arm under diminished pressure. 

• Collect the 1,4-diiodobutane at 110°C/6mmHg, the yield is 65g(84%).

• Alternatively, add 18 g (20 ml, 0.25 mol) of redistilled tetrahydrofuran (b.p. 65-66 °C) to a mixture of 32.5 g of phosphorus pentoxide, 1 15.5 g (67.5 ml) of 85 per cent orthophosphoric acid and 1 66 g of potassium iodide, heat for 3-4 hours, cool and isolate the 1,4-diiodobutane as above. The yield of product, b.p. 1 10 °C/6 mmHg, is 70 g (90%).

Notes to keep in mind:

1. The orthophosphoric acid must be adjusted to a concentration of 95 per cent H3PO4. Alternatively, the commercial 100 per cent orthophosphoric acid may be diluted with water to this concentration. The 95 per cent acid is claimed to be the most efficient for the preparation of iodides from alcohols and glycols, and for effecting cleavage of tetrahydrofuran and tetrahydropyran. Anhydrous orthophosphoric acid does not give such good results because of the limited solubility of hydrogen iodide in the reagent.

Cognate preparations: 1,5-Diiodopentane (from pentane-l,5-diol)

• Proceed as for 1,4-diiodobutane but use 26 g (26.5 ml, 0.25 mol) of redistilled pentane-1,5-diol (b.p. 238-239 °C) in place of the butane- 1,4-diol. The yield of 1,5-di-iodopentane, b.p. 142-143 °C/16mmHg, is 65 g (80%).

1,5-Diiodopentane (from tetrahydropyran):

• Use 21.5 g (24.4 ml, 0.25 mol) of redistilled tetrahydropyran (b.p. 86.5-87.5 °C) in place of the tetrahydrofuran, otherwise proceed as for 1,4-diiodobutane. The yield of 1,5-diiodo-pentane, b.p. 142-143 °C/16mmHg, is 71 g (88%).

1,6-Diiodohexane:

• Proceed exactly as detailed for 1,4-diiodobutane but replace the butane- 1,4-diol by 29.5 g (0.25 mol) hexane-l,6-diol, m.p. 41-42 °C. The yield of 1,6-diiodohexane, b.p. 150°C/10mmHg, m.p. 10 °C, is 70g (83%).

Butyl iodide:

• Use 37 g (46 ml, 0.5 mol) of butan-1-ol together with the quantities of the other reactants used above; a 2-hour reaction time is sufficient. The yield of butyl iodide, b.p. 129-130 °C, is 64 g (70%).

Iodocyclohexane:

• Proceed as for butyl iodide using 50 g (0.5 mol) of redistilled cyclohexanol (b.p. 160-161 °C). Distil the iodocyclohexane under reduced pressure, b.p. 67-69 °C/9mmHg; the yield is 90 g (86%).