SYNTHESIS OF 2-METHYLCYCLOHEXANONE

CAUTION: This experiment should be carried out in a fume cupboard.

• Equip a 250-ml three-necked round-bottomed flask with a thermometer, a reflux condenser protected by a calcium chloride guard-tube and (in the central neck) a 100-ml pressure-equalising dropping funnel fitted with an inlet adapter to allow flushing with a stream of dry nitrogen. 

• The apparatus should be thoroughly dried in an oven and assembled under a stream of dry nitrogen. 

• Adjust the nitrogen flow to a slight trickle, which should be maintained throughout the reaction, and in the flask place a magnetic follower bar, 30ml of dry ether, 4.8g (0.050mol) of 1-methylcyclohexene and 0.5 g (0.0225 mol, 20% excess) of lithium borohydride (1). 

• In the dropping funnel place 5 ml of dry ether and 0.95 ml (0.075 mol, 20% excess) of purified boron trifluoride-etherate. 

• Add the boron trifluoride-etherate solution to the stirred mixture dropwise during 15 minutes, keeping the temperature at 25-30 °C by cooling in a water bath. 

• Stir for 2 hours at room temperature and then destroy excess hydride by adding carefully 5 ml of water. 

• Place in the dropping funnel a solution of chromic acid, prepared from 11.0g (0.0369 mol) of sodium dichromate dihydrate and 8.1ml (0.1 474 mol) of 98 per cent sulphuric acid made up to 45 ml with water. 

• Add the chromic acid solution portionwise to the reaction mixture during 15 minutes while maintaining the temperature at 25-30 °C by cooling in an ice-water bath. 

• Heat the dark mixture under reflux for 2 hours, cool and separate the upper ether layer. 

• Extract the aqueous acid layer with two 15 ml portions of ether, combine the organic extracts, wash once with 5 ml of saturated sodium chloride solution and dry over magnesium sulphate. 

• Filter, remove the ether from the filtrate by flash distillation and carefully distil the residual straw-coloured liquid. 

• Collect the 2-methylcyclohexanone having b.p. 160-164 °C. The yield is about 3.5 g (63%). 

• The purity may be checked by g.l.c. on a 1.5-m column of 10 per cent Silicone oil on Chromosorb W held at 128 °C, with a nitrogen flow rate of 40 ml/minute; t R is about 1.7 minutes.

Notes to keep in mind:

1.  Reaction of lithium borohydride with water may be rapid and violent; do not expose to high humidity and avoid contact with eyes, skin and clothing (contact with cellulosic material may cause combustion). It should be handled with the same caution as is afforded to lithium aluminium hydride.