PROXIMATE CONSTITUENTS OF THE ANIMAL BODY 51 



An ester treated with an alkali is decomposed with, the formation of an 

 alkaline salt of the acid, and the corresponding alcohol which, being volatile, 

 is given off on warming the mixture. Thus : 



Et.C 2 H 3 2 + NaHO = NaC 2 H 3 2 + Et.OH. 



(ethyl acetate) (potassium acetate) (alcohol) 



This process of decomposition of an ester with the formation of the alkaline 

 salt of an acid is often spoken of as saponification, i.e. soap formation, though 

 the term ' soap ' is applied only to the compounds of alkalies with the higher 

 fatty acids. The series of alcohols we have just dealt with containing one 

 OH group replaceable by metals or acid radicals are known as monatomic 

 alcohols. If in the molecule of the paraffin two or more atoms of hydrogen 

 have been replaced by the group OH, we speak of diatomic or polyatomic 

 alcohols. Thus, derived from the paraffin propane C 3 H 8 we may have the 

 monatomic alcohol C 3 H 7 OH, propyl alcohol, or the tri atomic alcohol C 3 H 5 (OH 3 ), 

 which is known as glycerin, or glycerol. 



Other alcohols of physiological importance are cholesterol and cetyl alcohol. 

 Cholesterol is a monatomic alcohol with the formula C 27 H 45 OH. It is very 

 complex in structure, and belongs to the aromatic series. Recent work points 

 to an affinity of cholesterol with the terpenes, which have hitherto been found 

 only as the product of the metabolism of plant cells. Cholesterol is a constant 

 constituent of protoplasm. It occurs in large quantities in the medullary 

 sheath of nerves ; it is a normal constituent of bile and may form concretions 

 (biliary calculi) in the gall bladder. In combination with fatty acids it is an 

 important constituent of sebum and of wool fat. 



CH 3 



I 

 Another alcohol cetyl alcohol C 16 H 34 O = (CH 2 ) 14 occurs in the feather 



I 

 CH 2 OH 



glands of the duck and forms an important constituent of the wax, spermaceti, 

 obtained from a cavity in the skull of the sperm whale. 



ALDEHYDES. By oxidation of any of the alcohols we obtain another 

 group of compounds the aldehydes. From ethyl alcohol, for instance, by 

 warming with potassium bichromate and dilute sulphuric acid, ethyl aldehyde 



/H 



is produced and given off. In these aldehydes the group C H is converted into 



H I XOH 



the group 0=0, and it is the possession of this group which determines the 



aldehyde character of any compound, as well as the reactions which are typical 



of this class of compounds. 



Some of the typical reactions of aldehydes may be here shortly summarised : 

 (1) They act as reducing agents, the CHO group being converted into the 



group COOH, which is distinctive of an acid. We may therefore say that on 



oxidation aldehydes are converted into the corresponding fatty acids as follows j 



CH 3 CH 3 



| +0=1 

 CHO ' COOH 



(ethyl aldehyde) (acetic acid) 



