54 ALDEHYDES 



(&) 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 (Cu 2 O) 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 



CH 3 CHO + 2H = CH 3 CH 2 OH 

 Acetic aldehyde Ethyl alcohol 



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

 tion of magenta decolorised 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 = CH 3 OH + HCOOK 



Potassium formate 



5. Aldehydes react with ammonia to form additive 

 compounds ; thus acetic aldehyde undergoes the following 

 reaction : 



CH 3 CHO + NH 3 = CH 3 CHOHNH 2 

 Acetic aldehyde Aldehyde ammonia 



Here again formaldehyde behaves differently ; if ammonia is 

 added to a formaldehyde solution, it is neutralized quantitatively 

 according to the equation 



6CH a O + 4 NH 3 = (CH 2 ) 6 N 4 + 6H 2 O 

 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 

 of formaldehyde in a solution by adding a known excess of 



* 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. 



