A2-Level Advanced Organic Chemistry

  • Aldehydes can be oxidised to carboxylic acids. It is not possible to oxidise a ketone.

  • The ability to further oxidise an alcohol and not a ketone can be used to help distinguish between aldehydes and ketones.

  • Aldehydes can be identified by using Tollens’ reagent (silver precipitate formed) or Fehling’s solution (red precipitate formed).  

  • Aldehydes and ketones can both be identified by using Brady’s reagent, producing an orange precipitate.

  • A methyl carbonyl group can be identified using a (warm) mixture containing iodine and sodium hydroxide, forming a yellow solid.

QUICK NOTES

Testing for an Aldehyde

 

It is easy to distinguish between an aldehyde and a ketone using Tollens’ reagent or Fehiling’s solution. In both cases, the aldehyde can be oxidised and cause a metal ion to be reduced.  This reduction of a metal ion is accompanied by a colour change in solution (silver precipitate in the case of Tollens’ reagent and a brick red precipitate for Fehiling’s solution).

 

 

 

 

 

 

 

 

 

 

 

 

Tollens’ reagent and Fehling’s solution both form carboxylate ions in the reaction, although these are easily converted into carboxylic acids by protonation, and at A-level it is often written as the formation of a carboxylic acid from aldehyde.

 

 

 

 

 

Testing for a carbonyl group

 

As beneficial as the above two tests are, they do not actually confirm the presence of a ketone, only an aldehyde. This means the tests cannot be used to identify a carbonyl group, only an aldehyde.

 

 

Brady’s reagent, a solution of 2,4-dinitrophenylhydrazine in dilute acid, reacts with both aldehydes and ketones to give a positive result (orange-yellow crystals).

Testing for methyl carbonyl

 

If a methyl carbonyl group is heated with a solution containing iodine and sodium hydroxide (aqueous), yellow crystals of triiodomethane are formed.