ON HYDRO-AROMATIC COMPOUNDS WITH SINGLE NUCLEUS. 129 



Knoevenagel ^ has shown that by the condensation of ethy] aceto- 

 acetate, &c., with alkyl diiodides or aldehydes in presence of diethylamine 

 or piperidine, esters of 1 : 5-diketocarboxylic acids are produced, which 

 readily form ring compounds with loss of the elements of water ; and the 

 free acids generated from these bodies on hydrolysis lose carbon dioxide, 

 giving ketotetrahydrobenzenes 



Some of the more important reactions of these substances have been 

 mentioned ; a further property of peculiar interest is their ready transfor- 

 mation into aromatic derivatives ; e.</., l-keto-3-methyl-A2-tetrahydro- 

 benzene readily absorbs two atoms of bromine ; the product formed is 

 unstable, losing two molecules of hydrogen bromide, giving meta-cresol,^ 



^C(CH3) . CH3 XBrCCH^) . CH^ .C(CHg) . CH 



CH ^CH„ -» CHBr /^CH, -^ CH ^CH 



\C0 - CH„ ^^"^"^-^ CO - CHj \c(OH) = CH 



l-keto-S-methyl-o-isopropyl-Aj-tetrahydrobenzene under similar con- 

 ditions gives sy i7i-carva,cro\.^ 



Halogen substitution productsof ketotetrahydrobenzenes result from the 

 action of chlorine on phenols, aniline, &c.'' 



Ketodihydrohenzenes. — Neither of the two theoretically possible 

 i.somerides is known ; but tetra- and hexa-chloro-derivatives have been 

 prepared/' 



Carboxtlic Acids. 



Ilexahydrobenzoic acid has been obtained by the reduction (i.) of 

 )>enzoic acid,^ (ii.) of the hydrobromide of Aj-tetrahydrobenzoic acid,^ and 

 (iii.) of hydroxyhexahydrobenzoic acid,^ and also by the hydrolysis 

 and distillation of the product resulting from the action of ethyl sodio- 

 malonate and pentamethylenedibromide,'-* and by the distillation of 

 pentamethylenedicarboxylic acid.^ Tetrahydrobe^izoic acids have been 

 prepared by the removal of hydrogen bromide from monobromohexa- 

 hydrobenzoic acid,"* and by the direct reduction of benzoic '' and dihydro- 



■ Annaien, 1894, 281, 2o ; 1895, 288, 321 : and 1895, 289, 131. 

 - Bfir., 1893, 26, 1951 ; Aimalen, 1894, 281, 98. 

 » Ber., 1894, 27, 2347. 



' Ber., 1890, 23, 3777 ; 1892, 26, 2688; 1894, 27, 547, 550, 560. 

 ^ Zincke, Ber., 1894, 27, 546. 

 « Markownikoff, Ber., 1892, 26, 3357. 



' Aachan, Annaien, 1892, 271, 261. « Bncberer, Ber., 1894, 27, 1231, 



" Perkin and Haworth, J.C.H., 1894, 65, lOo. 

 '" Annaien, 271, 261. " Jhid., 234. 



1902. K 



