66o 



SCIENCE PROGRESS 



HO 



QH 4 



0°" 



CO 



/ 



o 



C 6 H<< 



\COONa 



These formulae have been disputed by von Baeyer 1 as being 



not completely in accord with their chemical behaviour, and 



quite recently Silberrad 2 has brought forward evidence against 



the quinonoid character of the salts. Similar to the phthaleins, 



Silberrad has produced melliteins and pyromelliteins, highly 



coloured bodies produced from mellitic and pyromellitic acids. 



Some derivatives of these substances which by analogy should 



possess a quinonoid structure due to rupture of a lactone ring, 



cannot, according to Silberrad, be formulated as quinones, and 



admit of no tautomerism. For example we have — 



I 



K salt of octabrom 3 : 3' : 6 : 6' : 9 : 9' hexahydroxydixanthylbenzene 



tetracarboxylic acid. 

 Br O Br Br O Br 



KOr^^^N^^iOK KO/V N V' s \OK 



Br 



Br 



Br 



C-OH 



and 



JBr 



C-OH 



CO.Kr^^ 

 C0 2 kL^/ 



C0 2 K 

 C(OH) 



Br OK 

 >Br 



/ 



C0 2 K /" "No 



Br OK 



;«(?/a-xanthyl derivative. 



Arguing from these halogenated derivatives to the free 

 hydroxyl compounds (which, by the way, can be formulated 

 as quinones) and thence to the phthaleins in general, Silberrad 

 regards the present formulae of the latter as unsatisfactory and, 

 favouring the theory of Baeyer, ascribes latent chromophoric 

 properties to the 9-carbon atom of the xanthyl derivatives. 

 It must be remembered, however, that these substances are of 



1 Ber. d. d. Chem. Ges. 38, 569 (1905). 



2 J 'our n. Chem. Soc. 89, 1787 (i9°o)- 



