

PROXIMATE CONSTITUENTS OF THE ANIMAL BODY 47 



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 or more atoms of hydrogen have been replaced by the group OH, we speak of diatomic 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 triatomic alcohol C 3 H 5 (OH) 3 , 

 which is known as glycerin, or glycerol. 



Other alcohols of physiological importance are cholesterol and cetyl alcohol. Cho- 

 lesterol 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 con- 

 stituent 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 Ci 6 H 34 = (CH 2 ) 14 occurs in the feather glands of 



I 

 CH 2 OH 



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 w r e obtain another group of 

 compounds the aldehydes. From ethyl alcohol, for instance, by warming with potas- 

 sium bichromate and dilute sulphuric acid, ethyl aldehyde is produced and given off. In 



H 



/ H . I 



these aldehydes the group C -H is converted into the group C = O, and it is the 



' I OH | 



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 : 



CH 3 CH 3 



I +0= | 

 CHO COOH 



(ethyl aldehyde) (acetic acid) 



On account of the ease with which this oxidation takes place, aldehydes act as strong 

 reducing agents. Warmed with an alkaline solution of cupric hydrate, they take up 

 oxygen, reducing the cupric to a red precipitate of cuprous hydrate. If warmed with 

 an ammoniacal solution of silver (i.e. silver nitrate solution to which ammonia has 

 been added until the precipitate first formed is just redissolved), they reduce the silver 

 nitrate with the formation of a mirror of metallic silver on the surface of the glass 

 vessel in which they are heated. 



(2) On warming with phenyl hydrazine, they give the typical compounds, hydra- 

 zones and osazones, which are also given by the sugars and will be mentioned in 

 connection with these bodies. 



(3) They also form addition products. With ammonia, they yield the group of 

 compounds known as aldehyde ammonia. Thus : 



CH 3 CH 3 



| + NH 3 = | NH 2 

 CHO C^H 



N OH 



