SYNTHESIS OF BUTANE-2,3-DIONE DIOXIME (Dimethylglyoxime)

• In a 500-ml three-necked flask, supported on a water bath in a fume cupboard and provided with a dropping funnel, a reflux condenser and a thermometer, place 72 g (90 ml, 1 mol) of dried and redistilled ethyl methyl ketone. 

• Introduce a follower bar and stir magnetically. 

• Add 3 ml of concentrated hydrochloric acid and warm the liquid to 40 °C. 

• Then add 103 g (115 ml, 1 mol) of butyl nitrite (b.p. 76-79 °C) or 1 17 g (134 ml, 1 mol) of isopentyl nitrite (b.p. 96-99 °C) (Section 4.2./, p. 413) slowly, maintaining the temperature at 40-50 °C; the mixture must be stirred vigorously. 

• Heat is generated in the reaction so that cooling may now be required. 

• Continue the stirring, without cooling, for 30 minutes after all the nitrite has been added. 

• The reaction mixture now consists of a solution of butane-2,3-dione monoxime in butan-1-ol or 3-methylbutan-l-ol. 

• To remove any unused ketone, treat the mixture with a cold solution of 45 g of sodium hydroxide in 100 ml of water and stir for 20-30 minutes. 

• Transfer the reaction mixture to a separatory funnel and extract the reddish-brown solution twice with 50 ml portions of ether: the alcohol may be recovered, if desired, by fractionation of the ethereal extracts. 

• Keep the aqueous layer; it contains the sodium salt of butane-2,3-dione monoxime (1). 

• Prepare a solution of 70 g (lmol) of hydroxylamine hydrochloride or of 82 g (1 mol) of hydroxylamine sulphate in about three times its weight of water, and add sodium hydroxide solution until the solution is neutral to litmus. 

• Place the aqueous solution of the sodium salt of the monoxime in a 1-litre round-bottomed flask and add the hydroxylamine solution with stirring. 

• Heat the mixture on a water bath for about 45 minutes. Filter off the precipitated dioxime (2) while the solution is still hot, wash it with hot water and drain well. 

• Recrystallise the crude product from about 10 times its weight of rectified spirit. The yield of pure dimethylglyoxime (a white, crystalline solid, m.p. 240 °C) is 55 g (47%).

Notes to keep in mind:

1.  If it is desired to isolate the monoxime, cool a portion (say one-fifth part) of the aqueous solution of the sodium salt in an ice-salt bath and carefully neutralise by adding concentrated hydrochloric acid (about 20 ml) with vigorous stirring, keeping the temperature below 15°C. During the addition, the resulting slurry becomes difficult to stir efficiently; filter off the solid product at this stage and continue to neutralise the filtrate to obtain the remainder of the crude monoxime. The pale brown solid thus obtained contains some sodium chloride; crystallisation from water gives almost colourless crystals, m.p. 73-74 °C.

2.  If the product is coloured, dissolve it in 2 m sodium hydroxide solution on a water bath. Filter the hot almost saturated solution, and to the hot filtrate add a concentrated solution of ammonium chloride in excess of the amount required to precipitate all the dimethylglyoxime, i.e. employ an amount greater than the equivalent of the sodium hydroxide used. Filter at once with suction, and wash with boiling water. Recrystallise the white product from rectified spirit.