194 



Professor Kolbe. 



[Mar. 12, 



stronger acids regenerate isatin, placed it in analogy with oxindol or 

 cumarin, 8fc, which show a similar behaviour. 



These facts alone, as already remarked, were not sufficient upon which 

 to build a solid hypothesis of the constitution of isatin. That this has 

 nevertheless been attempted simply proves how much the necessity was felt 

 of finding for isatin its proper place among those compounds to which it 

 4s most nearly related. 



Oxidation of Isatin by Chromic Acid. Isatoic Acids. 



The first experiments upon the oxidation of isatin, which led to the 

 discovery of the so-called isatoic acid, and also the behaviour of the 

 latter, have been already shortly detailed.* Some of the statements 

 then made as to the supposed composition of the compounds derived 

 from that acid, require correction. Since then isatoic acid and its 

 derivatives have been the subjects of careful investigation. 



If the supposition formerly expressed, viz., that isatin is a com- 

 pound of formyl and nitrogen-benzoyl be correct, then one might 

 expect that it would, by suitable oxidation, be converted into a 

 ■carboxylic acid, according to the following equation : — 



(CO,C 6 H 4 N)COH + = (CO,C 6 H 4 N)COOH. 



Nitrogen-benzoyl formyl. Nitrogen-benzoyl carboxylic acid. 



An acid of this atomic composition is in fact formed from isatin ; 

 ^this is the compound termed in the preceding communication Isatoic 

 acid, which name will be retained for it throughout this more com- 

 plete paper. 



The isatin used for its preparation was made in part by myself, by 

 oxidising (natural) indigo by nitric or chromic acid, and partly pro- 

 cured from Schuchardt (in Gorlitz) and from Kahlbaum ( Berlin) .f 



A solution of chromic acid in acetic acid has shown itself to be 

 the only suitable oxidising agent for the conversion of isatin into 

 isatoic acid. The following mode of procedure, the result of many 

 trials, has been found to be the best. About 600 grams of glacial 

 acetic acid (the half of the total quantity to be used"), are poured over 

 100 grams of finely powdered isatin placed in a large flask. The 

 chromic acid (200 grams) is brought, in moderate portions at a time 

 <(each time about one- sixth of the total quantity), into a funnel loosely 

 stoppered by means of a glass rod, and a part of the remaining 

 600 grams of acetic acid poured over it ; by lifting up the glass rod, 



* " Journ. fur Prakt. Chem." [2], 30, 84, and 124. 



f The fact, perhaps already observed by other chemists, that the isatin got with 

 chromic acid is darker than that got with nitric, is explained by the tenacious 

 adhesion to the first of small quantities of chromic oxide ; by dissolving this pre- 

 paration in hot caustic soda solution and precipitating the filtrate by dilute sul- 

 phuric acid, absolutely pure isatin is obtained from it. 



