SYNTHESIS OF 1-FLUOROHEXANE

CAUTION: Alkyl fluorides are said to be highly toxic. Great care should be taken not to inhale the vapours; conduct the entire operation in an efficient fume cupboard.

• In a dry 500-ml three-necked flask, equipped with a sealed stirrer unit, a 100-ml dropping funnel and a short fractionating column (1), place a mixture of 116g (2mol) of anhydrous, finely powdered potassium fluoride (2) and 200 g of dry ethylene glycol (3). 

• Connect the fractionating column (which carries a thermometer) to a downward double surface condenser fitted with a receiving flask with the aid of a side-arm adapter. 

• Heat the flask in an oil bath at 160-170 °C and introduce 165 g (141 ml, 1 mol) of 1-bromohexane dropwise, with stirring, during 5 hours. 

• A liquid passes over intermittently at 60-90 °C. 

• When the addition is complete, allow the bath temperature to fall to 1 10-120 °C; replace the dropping funnel by a tube of narrow bore dipping just below the surface of the liquid, attach the side-arm of the receiver adapter to a water pump, and draw a slow stream of air through the apparatus while maintaining the stirring. It is advisable to interpose a trap (e.g. a Drechsel bottle) cooled in ice between the water pump and receiver in order to recover any uncondensed liquid. 

• Distil the combined distillates through an efficient fractionating column (4); after a small forerun (0.5 g) of hex-1-ene collect the crude 1-fluorohexane at 92-97 °C. 

• Purify the crude product by cooling in ice and adding 1 ml portions of a solution containing 9.0 g of bromine and 6.0 g of potassium bromide in 50 ml of water until the organic layer acquires an orange colour: shake the mixture vigorously for a minute or so after each addition. The volume of KBr/Br 2 solution required is usually less than 5 ml. Separate the aqueous layer, wash the organic layer with saturated aqueous potassium bromide solution until colourless, and finally with water. Dry the liquid with magnesium sulphate and fractionate. 

• Collect the fraction 92-94 °C: the yield is 44 g (42%). The colourless liquid keeps unchanged for long periods.

Notes to keep in mind:

1.  Any fractionating column of moderate efficiency is satisfactory, e.g. a Dufton column (20 cm long containing a spiral 10 cm in length, 2 cm in diameter with 8 turns of the helix) or a Vigreux column (20-25 cm long).

2. Grind finely pure laboratory grade, anhydrous potassium fluoride, and heat it in an electrically heated oven at 180-210 °C; store in a desiccator. Before use, dry the powdered salt at 180 °C for 3 hours and grind again in a warm (c. 50 °C) glass mortar.

3. Redistil laboratory grade ehtylene glycol under reduced pressure and collect the fraction of b.p. 85-90 °C/7 mmHg for use as a solvent for the potassium fluoride.

4. A Widmer column (spiral 18 cm in length, 1.5 cm in diameter with 20 turns of the helix) is satisfactory.

Cognate preparation: 1-Fluoropentane

• Use 116 g (2 mol) of dry potassium fluoride in 200 g of dry ethylene glycol: heat in an oil bath at 140-150 °C and add 151 g (124 ml, 1 mol) of 1-bromopentane during 5 hours with stirring. The reaction product distils intermittently at 50-85 °C. The yield of 1-fluoro-pentane, b.p. 63.5-65 °C, is 25 g (28%).