SYNTHESIS OF OCTAN-2-ONE (Hexyl methyl ketone)

• Equip a 500-ml three-necked flask with a mechanical stirrer, a thermometer and a two-way adapter carrying a dropping funnel and condenser; arrange for the occasional cooling of the flask in an ice-water bath. 

• Place a solution of 32.5 g (0.25 mol) of octan-2-ol in 100 ml of ether in the flask and 125 ml (0.083 mol) of chromic acid solution (1) in the dropping funnel. 

• Add the chromic acid solution dropwise during 15 minutes to the vigorously stirred ether solution of the ketone, keeping the temperature between 25 and 30 °C by cooling as necessary. 

• Continue stirring at room temperature for a further 2 hours and then transfer the reaction mixture to a separating funnel. 

• Separate the ether layer and extract the dark green aqueous layer with four 60 ml portions of ether. 

• Combine the ether extracts and wash with 40 ml of saturated sodium hydrogen carbonate solution and then with 40 ml of saturated sodium chloride solution; finally dry the ether extract over anhydrous sodium sulphate. 

• Filter, remove the ether on a rotary evaporator and distil the residue at atmospheric pressure. 

• Collect the octan-2-one (2) at 170-172 °C. The yield is 26 g (81%). The purity may be checked by g.l.c. using a 1.5-m, 10 per cent Silicone oil column at 110°C and with a nitrogen flow rate of 40 ml/ minute. 

• The ketone has a retention time of 3.1 minutes.

Notes to keep in mind:

1. The chromic acid solution may be prepared as follows. Dissolve lOOg (0.33 mol) of sodium dichromate dihydrate in 300 ml of water and slowly add 134g (73 ml, 1.34 mol) of concentrated sulphuric acid (98%, d 1.84). Cool the solution and dilute to 500 ml with water in a graduated flask.

2. The ketone has a cheese-like, rather pervading, odour.