AROMATIC COMPOUNDS 63 



rides with strong sulfuric or hydrochloric acids, or else dissolving the lignin with alkali. 

 Such harsh procedures doubtless cause considerable degradation of the native lignin. A 

 so-called "native lignin" can be extracted from sawdust using acetone or alcohol at room 

 temperature. This extraction procedure, however, removes only 1/2 to 3% of the total 

 lignin. The solubility of lignin can be increased somewhat by grinding the wood flour very 

 fine in a ball mill. Another technique has been to remove cellulose from wood by allowing 

 fungi or purified cellulase to act on sawdust. By such treatments 25-30% of the total lig- 

 nin can be obtained in a soluble form. It is believed by some workers that any soluble 

 lignin is by definition not native lignin. Nevertheless the study of soluble lignins is nec- 

 essary for an understanding the chemical nature of lignin. 



Traditional methods for preparation of plant tannins have used extraction with hot 

 water, salting out with sodium chloride, reextraction of the precipitate into acetone and 

 removal of lipids from the acetone-extractable material with ether. By adding sodium 

 chloride in successive small portions some fractional precipitation of a tannin mixture 

 can be achieved. Lead or zinc acetates (10%) are often used to precipitate tannins which 

 may be recovered from the precipitate by decomposing it with hydrogen sulfide. Gelatin 

 also forms a precipitate with aqueous solutions of tannins. Ethanol can then be used to 

 redissolve tannin from this precipitate. Precipitation by adding an alcoholic solution 

 solution of potassium acetate to an alcoholic solution of tannin is often of preparative value. 

 in tannin isolation. 



Chromatographic procedures have been applied to the purification of practically all 

 the types of compounds discussed in this chapter. Chromatography on silicic acid has 

 been used to separate such compounds as lignan glycosides (32) or caffeic and chlorogenic 

 isomers from various plants (33). Chromatography on alumina using such solvents as 

 ethyl acetate or ethyl acetate -methanol mixtures has been used for lignans (34) and cou- 

 marins (35), Tannins have been purified on Solka-Floc, developing with 5% acetic acid (36). 



CHA RA CTERIZA TION 



Because of the large number of different substances included in this chapter there 

 is a vast number of specific reactions which have been applied to their characterization. 

 Only a few can be mentioned here for each class of compounds. 



A large proportion of the natural aromatic compounds have phenolic hydroxyl groups 

 and are therefore distinguished by the weakly acidic nature of this group. Thus, they are 

 often only slightly soluble in water or sodium bicarbonate solution but readily soluble in 

 dilute aqueous sodium hydroxide. The aromatic compounds with free carboxylic groups 

 are (like other organic acids) slightly soluble in water but easily soluble with efferves- 

 cence in sodium bicarbonate solution. If all phenolic groups are combined as esters or 

 ethers, the oxygenated benzene ring may still be recognized by the formation of colored 

 azo dyes on reaction with diazotized suUanilic acid or p-nitroaniline (Pauli reaction). 

 Many phenols also reduce Fehling's solution or ammoniacal silver nitrate. Production of 

 color with a 1% ferric chloride solution is also characteristic of many phenols. Other 

 color reactions will be found in the general references. Many paper chromatographic 

 studies of phenolic compounds have been made. Bate-Smith (37) surveyed the leaves of 

 many plants and found that the best solvent system for separating the phenolic compounds 

 was acetic acid/hydrochloric acid/water 30:3: 10. Appearance in ultraviolet light and 

 treatment with several different spray reagents could be used to identify different classes 

 of compounds. Other papers on chromatography of plant phenolics are by Inglett and 

 Lodge (38), Ibrahim and Towers (39), and Billek and Kindl (40). The most generally ap- 

 plicable detection methods are observation in ultraviolet light and spraying with diazonium 

 reagent. Pridham (41) has described paper chromatography and electrophoresis of phenols. 

 During paper chromatography caffeic acid can be converted into the coumarin, esculetin, 



