Indole-3-aldehyde.

Indole (20 g.) was dissolved in a mixture of chloroform (150 ml.) and 96 % alcohol (400 ml.) contained in a 2L flask fitted with a rubber stopper carrying a reflux condenser, a stirrer and a dropping funnel. The mixture was kept gently boiling over an electric hot plate and stirred while a solution of potassium hydroxide (250 g.) in water (300 ml.) was gradually dropped in over a period of 4-5 hours. The mixture was boiled for 30 minutes after the last addition of potassium hydroxide. When the contents of the flask had cooled, the potassium chloride was collected on a Buchner funnel and washed with alcohol. The combined filtrate and washings were then steam-distilled, the receiver being changed when the chloroform and most of the alcohol had passed over. The distillation was continued for 30 minutes after the distillate was no longer turbid owing to the presence of 3-chloroquinoline.

The hot aqueous liquid in the flask was decanted from the tarry residue and set aside to cool. The tarry material remaining was dissolved in the minimum quantity of hot alcohol, the alcoholic solution poured into a litre of hot water, the whole again boiled until the tarry globules had coalesced (an action which can be hastened by the addition of a little sodium chloride to the solution) and then filtered through a hot fluted filter-paper. The tar remaining on the filter was extracted yet another time in the same way. Aldehyde separated from all three aqueous solutions. This was filtered off and the combined filtrates amounting to some 2.5-3.0 litres were concentrated to about 300 ml. This concentrated solution on cooling yielded a further crop of aldehyde. The total yield of aldehyde thus obtained amounted to 7-5 g. (31 % of theory). The crude material melted at 1940 and after recrystallisation from water at 1980 (cf. Ellinger and Flamand, 195'). It contained 9.57% N. C9H7NO requires N9.66%.