Carbonyl compounds mainly show nucleophilic addition. reactions. Why?

Explanation

Both aldehydes and ketones contain a polar carbonyl group. As oxygen is more electronegative than carbon, the oxygen atom of the carbonyl group carries a small negative charge while the carbon atom attached to it carries a small positive charge. This positively charged carbon atom is readly attacked by nucleophiles. resulting in the formation of an anion by the transference of the `pi`-electrons to the oxygen atom. The anion thus formed then takes up a proton either from the solvent (usually water ) or from the reagent to give the addition product. The reaction in general, may be represented as :

In case of weak nucleophiles likes ammonia or its derivatives the reaction is usually carried out in weakly acidic medium. In the presence of acids the oxygen atom of the carbonyl group gets protonated. This increases the positive charge on the carbon atom, thus facilitating the attack of the weak nucleophile.

For example : On addition HCN, cyanohydrin of aldehyde and ketones are formed.
`underset("Acetaldehyde")(CH_(3)-overset(H)overset(|)(C)=O) + HCN rarr underset("Acetaldehyde cyanohydrin")(CH_(3)-underset(CN)underset(|)overset(H)overset(|)(C)-OH)`
`underset("Acetone")(CH_(3)-overset(H)overset(||)(C)=CH_(3)) + HCN rarr underset("Acetone cyanohydrin")(CH_(3)-underset(CN)underset(|)overset(OH)overset(|)(C)-CH_(3))`