104 AROMATIC COMPOUNDS [ch. 



The corresponding derivative of cinnamic acid, i.e. caffeic acid (see 

 also p. 123) is probably widely distributed. It is related to dihydroxy- 

 phenylalanine (see p. 152). 



Coniferyl alcohol is related to caffeic acid (see p. 103). Coniferyl 

 alcohol, when oxidised, yields the aldehyde, vanillin (so much used for 

 flavouring) which occurs in the fruits of the Orchid (Vanilla plani- 

 folia). (See also p. 166.) 



Gallic acid is a trihydroxy-benzoic acid : 



COOH 



HO 



V 



OH 



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. 99. 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. Add 

 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. 103. 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 5% ferric chloride solution. A blue-black coloration is given. 



(6) Add a drop or two of iodine solution. A transient red colour appears. 



(c) Add a drop or two of lime water. A reddish or blue coloration will be given. 



{d) To a few c.c. of the filtrate in a porcelain dish add a little 5 7o lead acetate 

 solution. A precipitate is formed which turns red on addition of caustic potash solu- 

 tion, and dissolves to a red solution with excess of potash. 



(e) To a few c.c. of the filtrate in a test-tube add a little 1 ^/o potassium cyanide 

 solution. A pink colour appears, but disappears on standing. On shaking with air it 

 reappears. 



(/) To a few c.c. of the filtrate in a test-tube add a few drops of 10% gelatine 

 solution. No precipitate is formed. 



{g) To a few c.c. of the filtrate in a test-tube add a little 5 o/q lead nitrate solution. 

 No precipitate is formed. 



