REACTIONS 6 1 



{b) A little Fehling's solution is gently warmed with a few 



drops of dilute aldehyde solution ; a change in colour 



takes place, from blue to green and yellow ; finally 



the solution becomes colourless and a red precipitate 



of cuprous oxide (CuaO) comes down. 



The readiness with which aldehydes are oxidized to acids 



accounts for the fact that most samples of aldehydes, unless 



freshly prepared, contain varying amounts of free acid. 



2. Aldehydes are readily reduced by nascent hydrogen to 

 the corresponding primary alcohols, according to the equation 



CH3CH0 + 2H = CH3CH2OH 



Acetic aldehyde Ethyl alcohol 



3. Aldehydes restore the colour to Schiff's Reagent (a 

 solution of magenta decolorized by sulphurous acid). 



4. Aldehydes when warmed with caustic potash are con- 

 verted into resinous substances of unknown composition. This 

 can be readily shown with acetaldehyde ; formaldehyde, how- 

 ever, when treated with potash undergoes a different change, 

 being converted into a mixture of methyl alcohol and potassium 

 formate, according to the equation 



2HCHO + KOH = CH3OH + HCOOK 



Potassium formate 



5. Aldehydes react with ammonia to form additive 



compounds ; thus acetic aldehyde undergoes the following 



reaction : — - 



CH3CH0 + NH3 = CH3CHOHNH2 

 Acetic aldehyde Aldehyde ammonia 



Here again formaldehyde behaves differently ; if ammonia is 

 added to a formaldehyde solution, it is neutralized quantita- 

 tively according to the equation : — 



6CH2O + 4NH3 = (CHj^sN^ + 6H2O 

 Formaldehyde Hexamethylene tetramine 



with the formation of a crystalline solid which is used in 

 medicine under the name of urotropine. 



The reaction can be employed for estimating * the amount 



* For another method of estimating formaldehyde by weighing the 

 mercury produced by the reduction of an alkaline solution of mercuric 

 sulphite, see Feder : " Archiv. d. Pharm.," 1907, 245, 25. 



