INDOLE AND ITS DERIVATIVES 343 



Properties. 



Indole crystallises in glistening platelets which melt at 52. It is 

 volatile in steam and these vapours have a peculiar and unpleasant 

 smell. It is fairly soluble in alcohol, ether, chloroform, benzene, 

 ligroin. 



It is a weak base, a secondary amine, and combines with strong 

 acids to form salts. 



Reactions. 



(1) A pine shaving moistened with hydrochloric acid and intro- 

 duced into an alcoholic solution of indole becomes red. 



(2) On the addition of a few drops of nitric acid and, drop by drop, 

 a few drops of very dilute potassium nitrite solution ('I per cent.) to 

 a solution of indole, the solution becomes red and in strong solutions 

 a precipitate of nitrosoindole is formed. (Baeyer.) 



(3) On adding half the volume of a 2 per cent, alcoholic solution 

 of p-dimethylaminobenzaldehyde to a solution of indole and, drop by 

 drop, 25 per cent, hydrochloric acid until a red colour appears, the 

 further addition of a few drops of 0-5 per cent, sodium nitrite solution 

 gives a dark red colour. (Ehrlich.) 



(4) On adding sodium nitroprusside solution to indole solution 

 until it is of a yellow colour and then .a few drops of caustic soda a 

 deep violet-blue colour is obtained ; the addition of acetic acid changes 

 it to pure blue. (Legal.) 



(5) If under a solution of indole treated with glyoxylic acid a 

 layer of concentrated sulphuric acid be run, a red colour is produced at 

 the point of contact. (Hopkins.) This reaction is sensitive to I in 

 500,000. 



(6) If formaldehyde be used instead of glyoxylic acid a similar 

 colour is produced. This reaction is sensitive to I in 700,000 

 (Kondo.) 



