Structure of Simpler Organic Compounds. 1 57 



Trihydroxy Phenols. 

 The following are known 



OH OH OH 



Pyfogalfol Phlwoytucindl 



Pyrogallol is generally prepared by heating to about 

 210 gallic acid, which has the formula 



OH 



OH 

 OH 



It then decomposes according to the equation 

 C 6 H 2 (OH) S COOH = C0 2 + C 6 H 3 (OH) 3 



Gallic acid is obtained from gall-nuts, and is closely 

 related to tannin or tannic acid. 



Pyrogallol is largely used in the laboratory and in 

 photography on account of its reducing properties. In 

 alkaline solution it readily absorbs oxygen, and forms a 

 dark-coloured pigment. It is a colourless, crystalline 

 substance, with melting point 115. 



Aldehydes. 



Benzaldehyde, C 6 H 5 -CHO, is the chief representative 

 of this class. It is sometimes known as "oil of bitter 

 almonds." Bitter almonds contain a glucoside known 

 as amygdalin, which under the influence of ferment 

 " emulsin " decomposes into hydrocyanic acid, glucose, 



