CHAPTER VIII 



AROMATIC COMPOUNDS 



The aromatic compounds may be defined as substances containing the 

 benzene carbon ring or a similar ring. A very great number occur 

 among the higher plants but of these many are restricted in distribution, 

 and may only be found in a few genera or even in one genus: others, on 

 the other hand, are widely distributed. At present our knowledge of the 

 part they play in general plant metabolism is slight. 



The more widely distributed aromatic plant products may be grouped 

 as: 



1. The phenols, and their derivatives. 



2. Inositol and phytin. 



3. The aromatic acids, alcohols and aldehydes. 



4. The tannins. 



5. The "essential oils" and resins. 



6. The flavone, flavonol and xanthone pigments, known as the soluble 

 yellow colouring matters. 



7. The anthocyan pigments, known as the soluble red, purple and 

 blue colouring matters. 



In connexion with the aromatic compounds it should be noted that 

 many of them contain hydroxyl groups, and one or more of these groups 

 may be replaced by the glucose residue, CeHnOg — , with elimination of 

 water and the formation of a glucoside, in the way already described 

 (see p. 50). The majority of such compounds are sometimes classed 

 together as a group — the glucosides — regardless of the special nature of 

 the substance to which the glucose is attached (this course has been 

 followed to some extent in Chapter x with compounds, the chief interest 

 of which lies in their glucosidal nature). In treating of the aromatic 

 substances in the following pages, mention will be made when they occur 

 as glucosides, this combination being in these cases only a subsidiary 

 point in their structure. 



The various groups of aromatic substances will now be considered in 

 detail. 



Phenols. 



There are three dihydroxy phenols, resorcinol, catechol and quinol, 

 but only the two latter are known to exist in the £ree state in 

 plants. Resorcinol frequently occurs as a constituent of complex plant 



