SYNTHESIS OF MESITYL OXIDE (4-Methylpent-3-en-2-one)

4-MethyI-4-hydroxypentan-2-one (diacetone alcohol):

• Fit a 1-litre round-bottomed flask with a large Soxhlet extractor and attach an efficient double surface condenser to the latter. 

• Place 595g (750 ml, 10.25 mol) of commercial acetone, preferably dried over anhydrous potassium carbonate, and a few fragments of porous porcelain in the flask. 

• Select as large a Soxhlet thimble as the extractor will accommodate and three-quarters fill it with barium hydroxide (1). 

• Fill the remaining space in the thimble with glass wool. 

• Insert the charged thimble into the extractor. Heat the flask on a water bath or steam bath so that the acetone refluxes back into the extractor rather rapidly. 

• Continue the heating until the acetone no longer refluxes when the flask is almost completely immersed in the boiling water bath (72-120 hours). 

• The refluxing may be interrupted at any time for as long as desired without influencing the preparation. 

• Equip the flask with a fractionating column attached to an efficient double surface condenser set for downward distillation. 

• Immerse the flask in an oil bath and raise the temperature gradually to 125 °C; maintain this temperature as long as acetone distils over. 

• The recovery of acetone is complete when the temperature at the top of the column is about 70 °C. 

• Distil the residue(2) under diminished pressure (3); a little acetone passes over first, followed by the diacetone alcohol at 71-74 °C/ 23mmHg (or 62-64°C/13mmHg). The yield is 450g (75%).

Mesityl oxide:

• Fit a 750-ml round-bottomed flask with a fractionating column attached to a condenser set for downward distillation. 

• Place 400g (3.44 mol) of diacetone alcohol (the crude product is quite satisfactory), 0.1g of iodine and a few fragments of porous porcelain in the flask. 

• Distil slowly with a small free flame (best in an air bath) and collect the following fractions: (a) 56-80 °C (acetone and a little mesityl oxide); (b) 80-126 °C (two layers, water and mesityl oxide); and (c) 126-131 °C, which is almost pure mesityl oxide. 

• Separate the water from fraction (b), dry with anhydrous potassium carbonate or anhydrous sodium sulphate and fractionate from a small flask. 

• A further quantity of mesityl oxide is thus obtained. The total yield is about 320g (95%).

Notes to keep in mind: 

1. If crystallised barium hydroxide [Ba(OH)₂ ,8H₂O] is employed, this becomes dehydrated after one run; the anhydrous compound is just as satisfactory and may be used repeatedly.

2. The residual liquid contains about 95 per cent of diacetone alcohol and is satisfactory for the preparation of mesityl oxide.

3. Diacetone alcohol partially decomposes when distilled under normal pressure.