vn] OXIDIZING ENZYMES 89 



Coniferyl alcohol when oxidized yields the aldehyde, vanillin (so 

 much used for flavouring), which occurs in the fruits of the Orchid 

 ( Vanilla planifolia). 



AROMATIC ACIDS. 



Of this group the following are some of the best known representa- 

 tives : 



Salicylic acid is a monohydroxybenzoic acid. It occurs both in 

 the form of esters and in the free state in various plants. 



Protocatechuic acid is a dihydroxybenzoic acid. It has been found 

 in the free state in a few plants, but is more widely distributed as a 

 constituent of many plant products. As will be shown later it forms 

 the basis of one of the series of tannins. 



COOH COOH 



OH 



Salicylic acid Protocatechuic acid 



Gallic acid is a trihydroxybenzoic acid : 



COOH 



OH 



It occurs free in gall-nuts, in tea, wine, the bark of some trees and 

 in various other plants. It forms a constituent of many tannins. It is 

 a crystalline substance not very readily soluble in cold but more soluble 

 in hot water. In alkaline solution it rapidly absorbs oxygen from the 

 air and becomes brown in colour. 



Expt. 88. The extraction and reactions of gallic acid. Take 100 gms. of tea, dry 

 in a steam oven and grind in a mortar. Put the powder into a flask and cover well 

 with ether. The preliminary drying and grinding can be omitted, but if carried out 

 will make the extraction more complete. After at least 24 hrs. filter off" the extract, 

 and either distil or evaporate off the ether. The ether will be coloured deep green 

 by the chlorophyll present in the dried leaves, and a green residue will be left. A.dd 

 about 20 c.c. of distilled water to the residue, heat to boiling and filter. Heating 

 is necessary because the gallic acid is only sparingly soluble in cold water. Keep the 

 residue for Expt. 91. With the filtrate make the following tests; for (a), (6) and (c) 

 dilute a few drops of the filtrate in a porcelain dish : 



(a) Add a drop of ferric chloride solution. A blue-black coloration is given. 



