Unit 11:Ketones and aldehydes
It must be stressed that theketo and theenolforms are two distinct compounds,
not isomers. They are known as tautomers of each other. The presence of α-hydrogen is
necessary for this equilibrium: those compounds not possessing it are callednon-enolizable
ketones.
Figure 183 Mechanism of enol-keto tautomerism
86.5 Reactions of Aldehydes and Ketones
86.5.1 Reactions with the carbonyl carbon
Since aldehydes and ketones contain a polar carbonyl group, the partially positive carbon
atom can act as an electrophile. Strong and weak nucleophiles are able to attack this
carbonyl carbon, resulting in a net addition to the molecule.
Nucleophilic addition
With cyanide, nucleophilic addition occur to give a hydroxynitrile:
RR’C=O + CN-+ H+→RR’COHCN
e.g. propanone→2-hydroxymethylpropanonitrile
86.5.2 Reactions with the carbonyl oxygen
The partially negative oxygen can act as a nucleophile, or be attacked by electrophiles.
86.5.3 Oxidation
Using a strong oxidizing agent such as the Tollens’ Reagent (Ag 2 O in aqueous ammonia)
acidified dichromate, Benedict’s/Fehling’s reagent (essentially alkaline Cu+2); aldehydes
but not ketones may be oxidized into carboxylic acids. This is one way to test for the