Reduction by addition of hydrogen

C-C bonds in any compound are not easily oxidised as a C=0 bond in an aldehyde or ketone is already oxidised. In aldehydes, but not ketones, there is a C-H bond from the C which is also joined by a double bond to an O, and it is these C-H bonds that can be oxidised to a 

So ketones cannot easily be oxidised but aldehydes can be. Indeed, aldehydes are so easily oxidised that they can be quite powerful reducing agents. 

In the list given in Table 7.2.1, methanal, ethanal and propanal are oxidisable, whereas the propanone is not oxidised with ordinary common chemical reagents. 

Tollen s reagent, a solution of silver nitrate in ammonia, oxidises aldehydes to carboxylic acids, and the silver ions are reduced to a deposit of silver metal which often collects as a thin silver mirror on the inside of the test tube  

Fehling s reagent contains complexed blue copper ions in the +2 oxidation state, and these are reduced when heated with aldehydes to a red precipitate of copper(I) oxide, CU2O. Ketones are not able to do these reactions. 

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