24 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



the body temperature. Fats are all soluble in hot alcohol, ether, and 

 chloroform, but insoluble in water. 



Chemical Constitution of the Fats. The fats are compounds of 

 fatty acids with glycerin, and may be termed glycerides or gly eerie 

 ethers. 



The fatty acids form a series of acids derived from the mona- 

 tomic alcohols by oxidation. Thus, to take ordinary ethyl alcohol, 

 C 2 H 5 HO, the first stage in oxidation is the removal of two atoms 

 of hydrogen to form aldehyde, CH 3 .COH : on further oxidation an 

 atom of oxygen is added to form acetic acid, CH 3 .COOH (see also 

 p. 14). 



A similar acid can be obtained from all the other alcohols, 

 thus: 



From methyl alcohol CH 3 .HO, formic acid H.COOH is obtained 



ethyl C 2 H 5 .HO, acetic CH 3 .COOH 



propyl C 3 H 7 .HO, propionic,, C 2 H 5 .COOH 



butyl C 4 H 9 .HO, butyric C 3 H 7 .COOH 



amyl C 5 H U . HO, valeric C 4 H 9 .COOH 



hexyl C 6 H ]3 .HO,caproic C 5 H U .COOH 



and so on. 



Or in general terms : 



From the alcohol with formula C M H 2n+1 .HO the acid with formula 

 C^Ha^.CO.OH is obtained. The sixteenth term of this series has 

 the formula C 15 H 3 ,.CO.OH, and is called palmitic acid; the 

 eighteenth has the formula C n H 35 .CO.OH, and is called stearic acid. 

 Each acid, as will be seen, consists of a radical, C^Ha^CO, united 

 to hydroxyl (HO). 



Oleic acid, however, is not a member of the fatty acid series 

 proper, but belongs to a somewhat similar series of acids known as 

 the acrylic series, of which the general formula is C H _ ] H 2w _ 3 COOH. 

 It is the eighteenth term of the series, and its formula is C\ V H 33 .CO.OH. 



The first member of the group of alcohols from which this acrylic series of 

 acids is obtained is called ally I alcohol (CH 2 : CH.CH 2 OH) ; the aldehyde of 

 this is acrolein (CH 2 : CH.CHO), and the formula for the acid (acrylic acid) is 

 CH 2 : CH.COOH. It will be noticed that two of the carbon atoms are united 

 by two valencies, and these bodies are therefore imsaturated ; they are un- 

 stable and are prone to undergo by uniting with another element a conversion 

 into bodies in which the carbon atoms are united by one bond only. This 

 accounts for their reducing action, and it is owing to this construction that 

 the colour reactions with osmic acid and Sudan III. are due. Fat which 

 contains any member of the acrylic series like oleic acid blackens osmic acid, 

 by reducing it to a lower (black) oxide. Fats like palmitin and stearin do 

 not give this reaction. 



