8 2 Biological Chemistry. 



is readily produced when an aldehyde is warmed with an 

 ammoniacal solution of a silver salt. 



Aldehydes undergo, furthermore, certain characteristic 

 reactions. 



The chief of those reactions, to which repeated reference 

 will be made later, is that with phenylhydrazine, a deriva- 

 tive of benzene (see p. 150), a reagent which is employed 

 for the detection of aldehydes. A product known as a 

 phenylhydrazone or hydrazone is thereby produced. The 

 reaction may be represented by the following equation : 



R-CH;0 + H 2 iN.NHC 6 H 5 = H 2 + R-CH : N.NHC 6 H 5 



or, in the case of acetaldehydes 



CH 8 -CHO + H 2 N-NHC 6 H 5 * = H 2 + CH 3 -CH : N-NHC 6 H 5 



The phenylhydrazones are crystalline products which 

 can be readily purified by recrystallization from suitable 

 solvents, and are therefore well adapted for the identifica- 

 tion of various aldehydes. 



A second reagent, which acts in a similar way to 

 phenylhydrazine, is hydroxylamine, which forms with 

 aldehydes a series of compounds known as the oximes. 

 The character of the reaction is indicated by the 

 equation 



These substances are also usually crystalline, and like 

 the hydrazones can be employed for the identification of 

 aldehydes. 



A third reagent with similar action is semicarbazide, 

 NH 2 NH CO NH 2 . The general reaction takes place 

 according to the equation 



R.CHO + H 2 N.NH.CO-NH 2 = H 2 + R-CH : N-NH CO-NH 2 



* The reason for assigning this constitution to phenylhydrazine will be 

 given later. 



