Chapter 4 



AROMATIC COMPOUNDS 



The chemical concept of aromaticity, of course, has nothing to do with aroma. For 

 present purposes it will be sufficient to define aromatic compounds as those whose struc- 

 tural formulas contain at least one benzene ring. A great variety of plant constituents 

 may be classed as aromatic compounds, and several groups of these are included in other 

 chapters. The unity within the present chapter comes from the presumption that all com- 

 pounds included are biosynthetically derived via 5-dehydroquinic acid. Flavonoids and 

 aromatic amino acids are also derived from 5-dehydroquinic acid but are more conven- 

 iently discussed in Chapters 9 and 10 respectively. Aromatic compounds, such as anthra- 

 quinones and chromones, which are probably derived from acetate, are covered in Chap- 

 ter 6; and terpenoid aromatic compounds are in Chapter 8. 



SIMPLE PHENOLS AND AROMATIC ACIDS 



The simple phenols are colorless solids when pure but usually oxidize and become 

 dark on exposure to air. Water solubility increases with the number of hydroxyl groups 

 present, but solubility in polar organic solvents is generally high. Phenols which are 

 only slightly soluble in water are readily soluble in dilute, aqueous solutions of sodium 

 hydroxide; but under basic conditions their rate of oxidation is increased considerably, so 

 that any prolonged treatment with strong alkali should be avoided. 



There are a few naturally occurring aromatic acids which have carboxyl as their 

 only functional group. However, most natural aromatic acids also have phenolic groups 

 and thus share properties with other phenols. Water insoluble phenolic acids can be dis- 

 tinguished from other water insoluble phenols by the fact that they may be dissolved in 

 sodium bicarbonate solution whereas the less acidic phenols require more alkaline sol- 

 vents. Many natural phenolic compounds have at least one hydroxyl group combined as 

 an ether, ester or glycoside rather than free. Ethers or esters are less soluble in water 

 than the parent phenols while the glycosides are more water-soluble. 



The natural aromatic compounds are usually characterized by having at least one 

 aliphatic side chain attached to the aromatic ring. The variety of possible side chains 

 combined with the structural variations already mentioned creates a bewildering array of 

 substances in this class. In certain cases the complexity of the aliphatic side chain makes 

 the aromatic portion of the molecule appear to be an almost incidental structural feature. 

 Table 1 depicts a few simple aromatic compounds found in plants, but it is impossible in 

 so little space to more than hint at the variety found in nature. 



Several simple aromatic compounds are of physiological or economic interest. 

 Vanillin, methyl salicylate and piperonal are responsible for the odors of vanilla, winter- 

 green (Gaultheria procumbens) and heliotrope respectively. 



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