AROMATIC COMPOUNDS 59 



CH^OH 



/CHgOH 



CH3O ^ >-C H— CH 



According to this formulation lignin made up solely of sinapyl units cannot occur because 

 the methoxyl group at C-5 blocks condensation with the side chain of another molecule. 



Plants contain varying amounts of lignin ranging from a few per cent (herbaceous 

 plants) to about 30% (conifers). Ferns and club mosses apparently contain true lignin, 

 but its occurrence is doubtful in Thallophytes, Bryophytes or Equisetum spp. Primitive 

 dicots contain a smaller proportion of sinapyl units than advanced ones (21). The great 

 majority of chemical research on lignin has been carried out using spruce lignin, and 

 most statements regarding lignin may be interpreted as applying strictly only to this par- 

 ticular type of lignin which is composed almost entirely of coniferyl alcohol units. 



In addition to the general references there are reviews on lignin by Brauns (22, 23), 

 Kremers (24) and Nord and de Stevens (25). 



HYDE OLYZ ABLE TANNINS 



A variety of phenolic plant constituents possess an astringent taste and the ability 

 to tan leather, but chemically the plant tannins are divided into two groups. Condensed 

 or catechin tannins are discussed in Chapter 9. The so-called hydrolyzable tannins con- 

 tain ester linkages which may be hydrolyzed by boiling with dilute hydrochloric acid. The 

 alcoholic component of the ester is usually a sugar, but in Tara tannin it is quinic acid. 

 Structures of some of the phenolic acids found in tannins are shown in Table 6. Gallic 

 acid is probably the one of most common occurrence. Ellagic acid is a secondary product 

 formed on hydrolysis of some tannins which are actually esters of hexaoxydiphenic acid. 

 It appears as a "bloom" on the surface of leather which has been processed with ellagitan- 

 nins. Similarly, chebulic acid can be a secondary product of tannin hydrolysis, formed 

 by lactonization of a carboxyl group which in the native tannin is esterified with a sugar. 



The hydrolyzable tannins are often complex mixtures containing several different 

 phenolic acids esterified to different positions of the sugar molecule. The "tannic acid" 

 of commerce is actually a mixture of free gallic acid and various galloyl esters of glucose, 

 Chinese gallotannin is probably the most thoroughly investigated hydrolyzable tannin. It 

 is found in aphid galls on a sumac plant (Rhus semialata) native to southwestern Asia. 

 Chinese gallotannin is a mixture of galloyl esters of glucose. The basic structure is 1, 2, 

 3,4, 6-pentagalloyl-/3-glucose with 3-5 additional galloyl groups attached by depside link- 

 ages to form a chain of 2 or 3 m-galloyl groups (26). 



An example of an ellagitannin is chebulagic acid of dividivi, the dried fruit of 

 Caesalpinia coriaria. On hydrolysis this tannin yields glucose, gallic acid, ellagic acid, 

 and chebulic acid. Its structure has been formulated as: 



