HYDRO-AROMATIC COMPOUNDS. 



Benzene, its homologues and derivatives, though they form a special 

 group of compounds with special properties, behave nevertheless in 

 some respects like unsaturated compounds. They can be reduced 

 under certain conditions and they will combine by addition with the 

 halogens on exposure to sunlight. The reduced compounds are 

 known as the hydro-aromatic compounds, and the halogen addition 

 compounds are regarded as derived by substitution from reduced ben- 

 zene. The aromatic compounds are most easily reduced by the method 

 of Sabatier, which consists in passing their vapour mixed with hydrogen 

 over nickel dust heated to about 170. 



Benzene gives three reduction products : 



CH 2 CH 2 CH 



H 2 cr NCH2 



H 2 C\JCH 2 

 CH 2 



Hexahydrobenzene, Tetrahydrobenzene, Dihydrobenzene, 



or" or or 



Hexamethylene, or Cyclohexene. Cyclohexadiene. 



Cyclohexane. 



These compounds correspond to the carbocyclic compounds con- 

 taining 3, 4, 5, etc., atoms of carbon (p. 237), and have properties like 

 the aliphatic compounds. 



The number of compounds in this group is not very large, but the 

 natural compounds, ths terpenes, inositols, cholesterol, cholalic acid are 

 hydro-aromatic compounds. 



THE INOSITOLS. 



The small number of natural compounds included under this heading 

 are hydroxy-derivatives of hexahydrobenzene. 



Quercitol, or cyclohexanepentol, is found in acorns and in the leaves 

 of Chamaerops humilis, a variety of palm. It is a 

 colourless solid which melts at 235 and is dextro- 

 rotatory, [a] D = + 24-16. A laevorotatory quercitol has 

 been found in the leaves of Gymnema sylvestre. It 

 CHOH melts at 174 and has [a] D = - 73-9. 



Inositol. Several varieties of inositol have been found in nature: 

 an inactive form, 2 active forms and a racemic form. 

 CHOH Seven inactive forms are theoretically capable of exist- 

 HOHCr //S ^CHOH ence - i- Inositol is found in heart-muscle and other 

 animal organs, but is present in larger amounts in unripe 

 [C V^CHOH beans and peas. It is present in the free state and also 

 CHOH in combination with phosphoric acid as ester in the 

 husks of various cereals. The calcium and magnesium 

 of this acid is termed Phytin. d-Inositol is obtained by the reduction of 

 pinitol with hydriodic acid. l-Inositol is obtained from quebrachitol by 

 reduction. 



cyllitol, an inactive inositol, is present in the organs of various 

 elasmobranch fish the dog fish, skate and shark. 



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