REACTIONS 63 



employed for the identification of the corresponding aldehydes. 

 The use of phenylhydrazine for the identification of the sugars 

 has already been described. 



9. The aldehydes are able to react with alcohols with the 

 formation of condensation compounds known as acetals ; thus, 

 for example, acetic aldehyde reacts with ethyl alcohol as fol- 

 lows : — 



+ H,0 



By analogy, acetic aldehyde should also be able to react 

 with water as follows : — 



This substance does not, however, actually exist, since a 

 compound having two or more hydroxyl groups attached to 

 the same carbon atom is, as a rule, unstable, and at once loses 

 water. Exceptions to this rule are, however, occasionally met 

 with ; for example, chloral CCI3CHO forms a stable com- 

 pound, chloral hydrate, of the formula : — 



OH 

 CCI3— C^OH 

 ^h 



10. Aldehydes exhibit a tendency to polymerize, that is, 

 for two or more molecules to combine together to form new 

 compounds of higher molecular weight. 



Thus two molecules of formaldehyde will combine together, 

 forming a compound known as paraformaldehyde {C\l.fi)2 ', 

 this substance, which is a white solid, is obtained by evaporat- 

 ing an aqueous solution of formaldehyde. 



A second polymer formed from three molecules of formal- 

 dehyde is known as metaformaldehyde or trioxymethylene 



