36 A TEXTBOOK OF PHYSIOLOGY 



3. If three molecules of oxygen be introduced into methane, water 

 again splits off: 



HO O H OH 



\ 

 C -> H - C or H.COOH 



H OH 



O 



Formic acid 



A body containing the characteristic group COOH is obtained. 

 This is called the carboxyl group ; its possession confers acid properties 



CH 3 >. 

 upon the bodies containing it. From ethane, it is possible to 



CH 3 

 obtain either one or two carboxyl groups by oxidation : 



CH 3 CH 3 COOH 



CH 3 COOH COOH 



Ethane Acetic acid Oxalic acid 



(a monocarboxylic (a dicarboxylic 



acid ) acid ) 



A body containing one COOH, such as acetic acid above, is known 

 as a monocarboxylic acid; a body containing two such groups, like 

 oxalic acid, is known as a dicarboxylic acid. In general acids are 

 obtained by the oxidation of alcohols, aldehydes, ketones: 



+ 2 = CH 3 .COOH + H.,0 



Ethyl alcohol Acetic acid 



+O 2 = 2CH 3 .CH 2 .COOH 



Propaldehyde Propionic acid 



CH 3 .CO.CH 3 + 20 2 = CH 3 COOH + C0 2 + H,O 



Acetone Acetic acid 



In the group of bodies known as ammo-acids, one of the valencies 

 of the carbon atom is satisfied by the amino group NH 2 , instead of 

 with hydrogen: 



CH 3 CH 2 NH2 



COOH COOH 



Acetic acid Monamino-acetic acid 



(glycin) 

 I 



Just as there exist many acids of which acetic acid is the first of the 

 chain, so there exist many amino-acids of which glycin is the simplest. 

 By introducing two amino groups into the acid molecule, bodies 

 known as diamino-acids are obtained. 



4. If four molecules of oxygen be introduced into methane, two 



