REACTIONS OF FORMALDEHYDE 147 



The reconversion of formaldehyde into the alcohol can be 

 effected by means of nascent hydrogen obtained by sodium 

 amalgam and water. 



Chemically, the aldehydes are very active, undergoing a 

 number of reactions, some ofwhich are of biological significance, 

 whilst others serve as valuable means of isolation or identi- 

 fication, 



1. Aldehydes are readily oxidized to the corresponding 

 acids by even such mild oxidizing agents as ammoniacal silver 

 hydroxide or Fehling's solution, or even atmospheric oxygen, 

 as is shown by the following experiments : — 



(a) A few drops of caustic potash are added to some silver 



nitrate solution in a test tube, ammonia is then care- 

 fully added, drop by drop, until the brown precipitate 

 has just redissolved. A little dilute acetaldehyde 

 solution is poured in and the mixture is warmed 

 gently ; if the solution be sufificiently dilute, a silver 

 mirror will be deposited on the side of the test tube ; 

 otherwise a black precipitate will be formed : — 



CHaCHO + AgjO = CH3COOH + 2Ag 



(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 (CUgO) 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 



CH3CHO + 2H = CH3CH2OH 



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 



10 "* 



