The attacks of a nucleophile $ X $ on a $ C $ atom

Frequent cases:

(1) $C$ is bound to a halogen atom or a hydroxyl group, and therefore bears a partial charge $\delta +$ due to the $ I^- $ effect of these substituents: < /span>

The observed nucleophilic attacks cause the halide ion to start:

(2) $C$ is a carbocation:

Nucleophile attacks are evident:

(3) $C$ is the carbon of a carbonyl group $C = O$ and therefore carries a partial $\delta +$ charge because of the $I^-$ effect of the $O$ atom (except case (1) ):

Frequently observed nucleophilic attacks move the π doublet on oxygen thus giving it a negative charge: