SYNTHESIS OF ISOPROPYL IODIDE (2-lodopropane)

• Mix 30 g (38 ml, 0.5 mol) of propan-2-ol with 450 g (265 ml) of constant boiling point hydriodic acid (57%) in a 500-ml distilling flask, attach a condenser for downward distillation, and distil slowly (1-2 drops per second) from an oil or air bath. 

• When about half the liquid has passed over, stop the distillation. Separate the lower layer of crude iodide (70 g, 82%). 

• Redistil the aqueous layer and thus recover a further 5 g of iodide from the first quarter of the distillate (1). 

• Wash the combined iodides with an equal volume of concentrated hydrochloric acid, then, successively, with water, 5 per cent sodium carbonate solution and water. 

• Dry with anhydrous calcium chloride and distil. The isopropyl iodide distils constantly at 89 °C.

Notes to keep in mind:

1. A further quantity of isopropyl iodide, only slightly less than that obtained in the first distillation, may be prepared by combining the residues in the distilling flask, adding 30 g (38 ml) of propan-2-ol, and repeating the distillation. Finally, the residues should be distilled and the 57 per cent constant boiling point acid recovered.

Cognate preparations: Isobutyl iodide (1 -iodo-2-methylpropane)

• Use 30 g (37.5 ml, 0.37 mol) of 2-methylpropan-l-ol and 273 g (161ml) of 57 per cent hydriodic acid; 65 g (96%) of the crude iodide are obtained. If the crude iodide is dark in colour, add a little sodium metabisulphite. B.p. 1 19-120 °C.

s-Butyl iodide (2-iodobutane):

• Use 30 g (37.5 ml, 0.37 mol) of butan-2-ol and 273 g (161 ml) of 57 per cent hydriodic acid; 63 g of crude iodide are obtained, b.p. 117.5-1 19 °C.

Iodocyclopentane 

• Use 43 g (45.5 ml, 0.5 mol) of cyclopentanol and 340 g (200ml) of 57 per cent hydriodic acid; 89 g (91%) of crude iodide are obtained, b.p. 58 °C/22 mmHg.

Allyl iodide

• Use 29 g (34 ml, 0.5 mol) of allyl alcohol and 340 g (200 ml) of 57 per cent hydriodic acid; 74 g (88%) of crude iodide are obtained. 

• Upon adding 29 g (34 ml) of allyl alcohol to the combined residue in the flask and the aqueous layer and distilling as before, a further 72 g of crude allyl iodide may be isolated, b.p. 99-101 °C (mainly 100 °C). 

• The compound is very sensitive to light; the distillation should therefore be conducted in a darkened room and preferably in the presence of a little silver powder.