54 BACTERIOLOGICAL AND ENZYME CHEMISTRY 

 With, phenyl hydrazine a compound of the following 



formula Cg Hg N — NH2 we have : — 



R=C=0 + H2N— NHCeHg = RCN— NHCeHg + H^O 



Acids. — ^Upon oxidation the CO group gives rise to an acid, 

 the exact composition of which depends on the elements or 

 groups attached to the carbon. Thus an aldehyde oxidises 

 as follows :^ 



/H /OH 



R + = R 



\ CO %C0 



A ketone gives a mixture of acids according to a rather 



more complex reaction. 



The group CO2H, which is a shortened form of the group 



^^OTT 

 — C^p. as written above, is known as the carboxyl group, 



and is characteristic of all organic acids which may be written 

 according to the general formula RCOOH ; thus in acetic acid 

 R is represented by the group CH3 or methyl, and the formula 

 of the acid is CH3COOH. The substance used as an illustra- 

 tion of the determination of a molecular formula on p. 44 was 

 acetic acid. On reduction with nascent hydrogen the group 



CO2H is reconverted to — C=0 and — CHjOH, i.e., acids 

 give on reduction aldehyde and alcohols. 



Esters. — ^Alcohols combine with organic acids to form what 

 are known as esters or ethereal salts ; thus ethyl alcohol com- 

 bines with acetic acid according to the following equation :— 



C2H5OH + CH3COOH = CH3COOC2H5 -f H2O 



which may be generalised as follows : — 



ROH + RCOOH = RCOOR -f H^O 



