62 AROMATIC COMPOUNDS 



A substance named "humic acid" has been identified as a natural constituent of sev- 

 eral plants by Raudnitz (30). The relationship of this to the humic acid of soil is not clear. 

 Humic acid was purified from leaves of Rhododendron ponticum and described as a water- 

 soluble^ surface -active phosphate ester having a molecular weight of ca. 1200. Plant 

 parts containing this humic acid turn red on heating with acid. In this respect humic acid 

 resembles the leucoanthocyanidins (q. v. ). However, the red pigment formed from humic 

 acid may be recognized by its absorption spectrum which shows bands at 459 and 548 m\x. 



ISOLATION 



Inasmuch as the compounds described in this chapter vary considerably in their 

 properties, no single isolation procedure will suffice to separate all of them as a group 

 from all other plant constituents. In devising isolation schemes the special properties of 

 the particular category under investigation must be considered. 



Many of the simple aromatic compounds occurring in plants have free phenolic hy- 

 droxyl groups, carboxyl groups, or both. Carboxylic acids may be extracted from plant 

 material or an ether extract of plant material with 2% sodium bicarbonate solution. When 

 this solution is acidified, the acids often precipitate or may be extracted with ether. After 

 removal of carboxylic acids, phenols may be extracted with 5% sodium hydroxide solution. 

 Like the acids, they may be precipitated or extracted into ether after acidification. Be- 

 cause many phenols are highly sensitive to oxidation under alkaline conditions, it may be 

 advisable to exclude air or add a reducing agent like sodium dithionite during the alkali 

 treatment. 



Some of the lower molecular weight aromatic compounds may be purified by distil- 

 lation or sublimation under atmospheric or reduced pressure. Phenols are usually not 

 steam distillable, but phenol ethers or esters, being less polar than the parent hydroxyl 

 compounds, can often be distilled with steam. Coumarin, for example, is customarily 

 isolated by steam distillation. 



Solvent extraction procedures find widespread application in purification of natural 

 aromatic compounds. Common organic solvents like acetone, ether and benzene are 

 often employed. Multiple partition between water or buffer solutions and an immiscible 

 organic solvent has been used to purify compounds with suitable solubility properties. 

 The hydrolyzable tannins and glycosides may be extracted with hot water or water-ethanol 

 mixtures. 



In purifying coumarins a crude preparation can be treated with warm dilute alkali 

 to open the lactone ring and form a water-soluble sodium coumarinate. Neutral organic 

 impurities may then be extracted with ether. On acidification of the water solution the 

 coumarin reforms so that the coumarin with any acidic compounds can be extracted into 

 ether. Acid impurities can then be removed from the ether by shaking with sodium bicar- 

 bonate solution. 



Lignans may be extracted with acetone or ethanol and are often precipitated as 

 slightly soluble potassium salts by adding concentrated, aqueous potassium hydroxide to 

 an alcoholic solution. As a variant on this procedure Freudenberg and Knot (31) converted 

 lignans to insoluble, crystalline products using potassium acetate in ethanol. After col- 

 lecting the crystalline material free lignans could be regenerated by decomposing with 

 water. Acids or alkalies are to be avoided in preparing lignans, as they often produce 

 isomerization. 



The problem of isolating native lignin has called forth several special and ingenious 

 approaches. Older procedures relied on either removing cellulose and other polysaccha- 



