Aldehydes are the compounds which have general formula RCHO, Ketones are compounds having general formula RR’CO. The groups R and R’ may be aliphatic or aromatic, similar or different alkyl groups.
Both aldehydes and ketones contain the carbonyl group, >C = O, and are often referred to collectively as carbonyl compounds.
It is the carbonyl group that governs mainly the chemistry of aldehydes and ketones.
Structure of Carbonyl group :
The carbonyl carbon atom is sp2 hybridized and bonded to three other atoms through three coplanar sigma bonds oriented about 120° apart. The unhybridized p orbital overlaps with a p orbital of oxygen to form a pi bond. The double bond between carbon and oxygen is similar to an alkene C = C double bond, except that the carbonyl double bond is shorter and stronger.
Another difference between the carbonyl and alkene double bonds is the large dipole moment of the carbonyl group. Oxygen is more electronegative than carbon, and the bonding electrons are not shared equally. In particular, the less tightly held pi electrons are pulled more strongly toward the oxygen atom, giving ketones and aldehydes larger dipole moments than most alkyl halides and ethers. We can use resonance structures to symbolize this unequal sharing of the pi electrons.
The first resonance structure is clearly more important, since it involves more bonds and less charge separation. The contribution of the second structure is evident by the large dipole moments of the ketones and aldehydes shown below.
This polarization of the carbonyl group contributes to the reactivity of ketones and aldehydes. The positively polarized carbon atom acts as an electrophile, and the negatively polarized oxygen acts as a nucleophile.