REACTIOxNS OF ALDEHYDES 149 



vigorously, a rise in temperature takes place accompanied by 

 the formation of a white crystalline precipitate : — 



CH3CHO + HNaSOj = CH3CH0HS0.^Na 



7. Aldehydes also form additive compounds with hydrogen 

 cyanide ; these compounds are known as hydroxycyanides or 

 cyanohydrins : — 



CH3CHO + HCN = CH.,CHOHCN 



Acetic aldehyde cyanohydrin 



8. Aldehydes form crystalline compounds with hydro- 

 xylamine, phenylhydrazine, and semicarbazide ; in all cases 

 water is split off between the two reacting substances, 



CH3CHO + NH,OH = CH3CH : NOH + H^O 

 CH3CHO + CgHgNHNHa = CH3CH : N • NHCgHg + H2O 



The resulting compounds, which are known as oximes, hydra- 

 zones or semi-carbazones, are usually substances with a char- 

 acteristic crystalline form and melting point, which may be 

 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 : — 



CH3 CH3 



I HOC2H5 I OC^Hg 



O + = \ / + H2O 



t^' HOCjHg C— OC,H, 



\ \h 



Acetic Ethyl Acetal 



aldehyde alcohol 



By analogy, acetic aldehyde should also be able to react 

 with water as follows : — 



CH, CH, 



I HOH I OH 



I O + = \/ + HaO 



t/' HOH c— OH 



^H \h 



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

 compound having two or more hydroxyl groups attached to 



