SYNTHESIS OF PHENYLGLYOXAL

CAUTION: Selenium and its compounds are toxic; carry out this and the following preparations in an efficient fume cupboard.

• Fit a 500-ml three-necked flask with a sealed stirrer, a reflux condenser and a thermometer. 

• Place 300 ml of dioxane (1), 55.5 g (0.5 mol) of pure selenium dioxide and 10 ml of water in the flask, heat the mixture to 50-55 °C and stir until the solid has dissolved. 

• Remove the thermometer momentarily and add 60 g (0.5 mol) of acetophenone in one lot; replace the thermometer. 

• Reflux the mixture, with stirring, for 4 hours; after about 2 hours the solution becomes clear and little further precipitation of selenium is observable. 

• Decant the hot solution from the precipitated selenium through a fluted filter paper, and remove the dioxane and water by distillation through a short column. 

• Distil the residual phenylglyoxal under reduced pressure and collect the fraction boiling at 95-97 °C/25 mmHg. 

• The yield of pure phenylglyoxal (a yellow liquid) is 48 g (72%); this sets to a stiff gel on standing, probably as a result of polymerisation, but may be recovered without appreciable loss by distillation. 

• The aldehyde is best preserved in the form of the hydrate, which is prepared by dissolving the yellow liquid in 3.5-4 volumes of hot water and allowing to crystallise. Phenylglyoxal hydrate (m.p. 91 °C) also crystallises from chloroform, ethanol or ether-light petroleum (b.p. 60-80 °C); upon distillation under diminished pressure, the free aldehyde is obtained.

Notes to keep in mind:

1.  Rectified spirit can also be used as solvent. The dioxane can, however, be recovered and used in a subsequent run.

Cognate preparations: Cyclohexane-l,2-dione

• Equip a 1 -litre, three-necked flask with a reflux condenser, thermometer and dropping funnel. 

• Place 250 g (2.55 mol) of pure cyclohexanone in the flask, heat to 70-80 °C and add a solution of 280 g (2.52 mol) of pure selenium dioxide in 1500 ml of rectified spirit from the dropping funnel over a period of 2 hours, maintaining the temperature at 70-80 °C. 

• Reflux the reaction mixture for a further 2 hours. 

• Distil off as much of the alcohol as possible and decant the liquid residue from the elemental selenium. 

• Wash the latter several times with ether, and combine the ether extracts with the decanted liquid. 

• Remove the ether by distillation and distil the residue under reduced pressure (c. 25 mmHg): about 200 g of an oil, consisting of cyclohexane-l,2-dione, cyclohexane and water, is obtained. 

• Dissolve the oil in 1 litre of ether, and extract thrice with ice-cold 10 per cent potassium hydroxide solution; the total amount of potassium hydroxide solution should be equivalent to 1.5 times that necessary to react with the oil assumed to be the pure dione in the monoenol form (about 1.51). 

• Shake the alkaline extract once with ether to remove cyclohexanone, acidify with ice-cold hydrochloric acid and then saturate with salt. 

• Extract the hydrochloric acid solution with ether, dry the ethereal extract with magnesium sulphate, remove the ether by distillation at normal pressure and distil the residue under reduced pressure. 

• Collect the cyclohexane- 1,2-dione (a pale green liquid) at 96-97 °C/25 mmHg; the compound decomposes slightly on keeping. The yield is 55-56 g (19%).

• It is important that the synthesis should be carried out as quickly as possible, particularly the washing with alkali at °C, since the latter tends to convert the product into 1-hydroxy-cyclo-pentanecarboxylic acid.

Indane-1 ,2,3-trione hydrate (ninhydrin)

• In a 500-ml three-necked flask, fitted with a reflux condenser and mechanical stirrer, place 1 1 g (0. 1 mol) of pure selenium dioxide dissolved in 240 ml of dioxane and 5 ml of water. 

• Heat the stirred solution to 60-70 °C, remove the source of heat, add 15 g (0.1 mol) of crude indane-1, 3-dione (Expt 7.10) and reflux the resulting mixture for 6 hours. 

• A solid separates during this period. Filter the mixture, transfer the filtrate to a distilling flask and distil off about 180 ml of dioxane; then add 100 ml of water, boil the solution to coagulate the red tarry precipitate and remove it by filtration.

• Concentrate the filtrate to about 50 ml and filter. 

• Boil the filtrate with 0.2-0.3 g of decolourising carbon, filter again, concentrate to 20-25 ml and keep at room temperature. 

• Collect the crystals of crude ninhydrin by suction filtration, and recrystallise from hot water with the addition of a little decolourising carbon, if necessary (1). The yield of colourless ninhydrin is 6g (34%); the crystals turn red between 125 and 130°C and melt at 242-243 °C.

Notes to keep in mind:

1.  Recrystallisation from water with the use of decolourising carbon should yield almost colourless crystals; selenium-containing contaminants may, however, give rise to discoloration. In this case a further recrystallisation with the addition of decolourising carbon and a little tin(n) chloride should be carried out.