Preparation of alkyl iodides from alcohols

Alkyl iodides are the most easily formed of the alkyl halides and the slow distillation of the alcohol with constant boiling hydriodic acid is a general method of preparation (e.g. Expt 5.57). As with the corresponding chlorides and bromides (q.v.), the yields of the required alkyl iodides in this reaction may be diminished in the case of certain (tertiary and secondary) alcohols as a result of skeletal rearrangement. 

An alternative reagent, which is particularly effective for the conversion of diols into diiodo compounds, is a mixture of potassium iodide and 95 per cent orthophosphoric acid (Expt 5.58). The reagent also cleaves tetrahydrofuran and tetrahydropyran to yield the corresponding a, co-diiodo compounds [cf. the hydrobromic acid-sulphuric acid reagent, Section 5.5.2, p. 559]. 

Reaction of an alcohol with the reagent o-phenylene phosphorochloridite followed by treatment of the alkyl o-phenylene phosphite so obtained with iodine in dichloromethane at room temperature results in a good yield of alkyl iodide. This method, exemplified by the preparation of 1-iodoheptane (Expt 5.60), is the preferred procedure when acid-sensitive functional groups are present. 

Mix 30 g (38 ml, 0.5 mol) of propan-2-ol with 450 g (265 ml) of constant boiling point hydriodic acid (57%) (Section 4.2.32, p. 436) 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. 

(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. 

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Stone and Shechter have developed high-yield procedures for the preparation of alkyl iodides from alcohols, ethers, and olefins by reaction with a reagent described as 95% orthophosphoric acid in combination with potassium iodide. For the conversion of 1,6-hexanediol into 1,6-diiodohexane, a mixture of 65 g. of P2O5 and 231 g. (135 ml.) of 85% phosphoric acid is stirred mechanically and let cool to room temperature, potassium iodide and 1,6-hexanediol are added, and the mixture is stirred and heated as indicated. The initially homogeneous solution separates into... 

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