SYNTHESIS OF 5-METHYLHEX-5-EN-2-ONE

• Equip a 1 -litre two-necked round-bottomed flask with a sealed stirrer unit and a reflux condenser protected with a guard-tube containing anhydrous calcium sulphate. 

• Place in the flask 500 ml of anhydrous ethanol, 75 g (0.75 mol) of freshly distilled pentane-2,4-dione (b.p. 136-137 °C), 63.4 g (0.70 mol) of 3-chloro-2-methylpropene (methallyl chloride) and 96.8 g (0.70 mol) of anhydrous potassium carbonate. 

• Heat the stirred mixture under gentle reflux for 16 hours. 

• Allow the mixture to cool a little and replace the condenser by a still-head and condenser arranged for downward distillation. 

• Distil the stirred mixture until about 370 ml of ethanol and the ethyl acetate formed during the reaction has collected, then cool the residue and add sufficient ice-water to dissolve the suspended salts (about 550 ml is required). 

• Transfer to a separatory funnel and extract with three 200 ml portions of ether. 

• Wash the combined extracts with two 100ml portions of saturated aqueous sodium chloride and then dry the ethereal solution over anhydrous sodium sulphate. 

• Filter, and remove the ether by flash distillation. 

• Fractionally distil the residue using a well-lagged fractionating column of about 1 2 cm length filled with glass helices. 

• Collect the unsaturated ketone as a fraction of b.p. 148-153 °C; (mainly 148-150 °C) (1). 

• The yield of 5-methylhex-5-en-2-one is 33.1 g (39%); its purity may be checked by g.l.c. on a 10 per cent Silicone oil on Chromosorb W 1.5-m column, held at 82 °C, nitrogen flow rate 40 ml/minute, t R 1 minute.

Notes to keep in mind:

1.  The forerun consists of residual ethanol and ethyl acetate together with some of the unsaturated ketone. A substantial high boiling residue remains.