SYNTHESIS OF BUTYL IODIDE (1-lodobutane)

• Place 12.5 g (0.4 mol) of purified red phosphorus and 78 g (96 ml, 1.05 mol) of butan-1-ol in a 250-ml round-bottomed flask fitted with a Liebig-type reflux condenser. 

• Heat the mixture to gentle refluxing with a Bunsen flame over a wire gauze. 

• Remove or lower the flame and add 127g (0.5 mol) of coarsely powdered iodine in approximately 2 g portions down the centre of the condenser. 

• Complete the entire addition of the iodine fairly rapidly (i.e. in about 20-30 minutes), but allow the mildly exothermic reaction to subside after the addition of each portion. 

• Finally continue heating under reflux for 30-60 minutes; little or no iodine should then be visible.

• Arrange the condenser for downward distillation and distil off most of the crude product. 

• When the volume of liquid in the flask has been reduced to about 1 5-20 ml, add about 40 ml of water and continue the distillation until no more oily drops pass over into the receiver (1). 

• Separate the crude alkyl iodide and wash it successively with approximately equal volumes of water, concentrated hydrochloric acid (2), water, 10 per cent sodium carbonate solution and water (3). 

• Dry the product over anhydrous calcium chloride and distil, collecting the butyl iodide which passes over at 129-131 °C; the yield is 165g(90%)(4).

Notes to keep in mind:

1.  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.

2. The washing with concentrated hydrochloric acid removes any unchanged alcohol

which may be present.

3. If the washed organic phase is darker than a pale brown-pink, add a few crystals of

sodium thiosulphate to the final wash-water to remove traces of iodine.

4. The purified iodide may be preserved in a bottle containing a short coil of clean

copper wire.

Cognate preparations:

• The following may be prepared by the above procedure in similar yield using the same quantities of red phosphorus and of iodine:

Propyl iodide (1-iodopropane) - from 63 g (78 ml, 1.05 mol) of propan-1-ol. (B.p. 102-103 °C).

Isopropyl iodide (2-iodopropane) - from 63 g (80 ml, 1.05 mol) of propan-2-ol. A little hydrogen iodide is evolved. (B.p. 89-90 °C).

s- Butyl iodide (2-iodobutane) - from 78 g (97 ml, 1.05 mol) of butan-2-ol. A little hydrogen iodide is evolved. (B.p. 118-120°C).

1-Iodopentane - from 92 g (1 13 ml, 1.05 mol) of pentan-1-ol. (B.p. 153-156 °C).

1-Iodohexane- from 107 g (130 ml, 1.05 mol) of hexan-l-ol.(B.p. 178-180 °C).

1-Iodoheptane - from 122 g (148 ml, 1.05 mol) of heptan-1-ol. (B.p. 198-201 °C, 62.5 °C/3.5mmHg).

1-Iodooctane - from 137g (166ml, 1.05 mol) of octan-1-ol. (B.p. 219-222 °C, 86.5 °C/5 mmHg).

Iodocyclohexane - from 105 g (110ml, 1.05 mol) of cyclohexanol. Carry out the reaction as above, dilute the cooled reaction product with ether and filter. Wash and dry the organic phase, remove the ether and distil under reduced pressure. (B.p. 81-83 °C/20 mmHg).

l-Iodo-2-phenylethane (2-phenylethyl iodide) - from 149 g (149 ml, 1.05 mol) of 2-phenylethanol. Proceed as for iodocyclohexane; a little hydrogen iodide is evolved towards the end of the reaction. (B.p. 114-1 16 "C/12 mmHg).

1 ,3-Diodopropane - from 40 g (38 ml, 0.525 mol) of propane- 1,3-diol (tri- methylene glycol). Proceed as for iodocyclohexane; stop heating as soon as all the iodine has been added. (B.p. 88-89 °C/6 mmHg).