SYNTHESIS OF PENTANE-2,4-DIONE (Acetylacetone)

• Fit a 1 -litre three-necked flask with a gas inlet tube and a gas outlet leading to a gas absorption device charged with aqueous alkali to trap excess boron trifluoride, and stopper the third neck. 

• Place 58 g (73 ml, 1 mol) of pure, anhydrous acetone and 255 g (236 ml, 2.5 mol) of acetic anhydride in the flask and cool in a freezing mixture of ice and salt. 

• Connect the gas inlet tube through an empty wash-bottle to a cylinder of commercial boron trifluoride, and bubble the gas through the reaction mixture at such a rate that 250 g is absorbed in about 5 hours (2 bubbles per second). 

• Pour the reaction mixture into a solution of 400 g of crystallised sodium acetate in 800 ml of water contained in a 2.5-litre round-bottomed flask. 

• Steam distil the mixture, and collect the distillate in the following portions; 500 ml, 250 ml and 250 ml. 

• In the meantime prepare a solution of 120g of pure crystallised copper(n) acetate in 1500 ml of water at about 85 °C; if the solution is not clear, filter from any basic acetate. 

• Precipitate the copper complex of acetyl-acetone by adding 700 ml of the hot copper(n) acetate solution to the first portion of the steam distillate, 350 ml to the second, 250 ml to the third and 200 ml to the fourth portion. 

• Allow to stand for 3 hours, or better overnight, in the ice chest.

• Filter off the salt at the pump, wash once with water and suck as dry as possible. 

• Transfer the copper complex to a separatory funnel, add 400 ml of 20 per cent by weight sulphuric acid and 400 ml of ether, and shake. 

• Remove the ether layer. Extract the aqueous layer with two 150 ml portions of ether. 

• Dry the combined extracts with 125 g of anhydrous sodium sulphate (or the equivalent quantity of magnesium sulphate), and distil off the ether. 

• Distil the residue through a short fractionating column and collect the acetyl-acetone at 134-136°C. The yield is 80 g (80%).

Cognate preparations: 2-Acetylcyclohexanone

• Place a mixture of 24.5 g (0.25 mol) of cyclohexanone (regenerated from the bisulphite compound) and 51 g (47.5 ml, 0.5 mol) of acetic anhydride in a 500-ml three-necked flask, fitted with an efficient sealed stirrer, a gas inlet tube reaching to within 1-2 cm of the surface of the liquid, and in the third neck a thermometer immersed in the liquid, combined with a gas outlet tube leading to a trap (1). 

• Immerse the flask in a bath of Cardice-acetone, stir the mixture vigorously and pass in the boron trifluoride as fast as possible (10-20 minutes) until the mixture, kept at 0-10 °C, is saturated (copious evolution of white fumes when the outlet tube is disconnected from the trap). 

• Replace the Cardice-acetone bath by an ice bath and pass the gas in at a slower rate to ensure maximum absorption. 

• Stir for 3.5 hours while allowing the ice bath to attain room temperature slowly. 

• Pour the reaction mixture into a solution of 136g of hydrated sodium acetate in 250 ml of water, reflux for 60 minutes (or until the boron fluoride complexes are hydrolysed), cool in ice and extract with three 50 ml portions of light petroleum, b.p. 40-60 °C (2), wash the combined extracts free of acid with sodium hydrogen carbonate solution, dry over anhydrous calcium sulphate, remove the solvent by flash distillation and distil the residue under reduced pressure. 

• Collect the 2-acetylcyclohexanone at 95-97 °C/10mmHg. The yield is 27 g (77%).

Notes to keep in mind:

1.  Alternatively the reaction may be effected by adding the ketone and the acetic anhydride to 100g (0.75 mol) of a 1:1 acetic acid-boron trifluoride complex.

2.  Light petroleum is preferable to ether because it removes smaller amounts of acetic acid from the aqueous phase.