Nucleophilic Addition to the Carbonyl Group: Introduction

Carbonyl compounds are susceptible to a nucleophilic attack at the carbonyl carbon, which can be explained in more detail by looking at their structure.

The carbon and oxygen of a carbonyl compound are held together by σ and π bonds. The π electrons stay closer to the oxygen atom, due to the higher electronegativity of oxygen, whereas the unoccupied π* orbital lies in the close vicinity to the carbon atom. This distribution leads to a polarization of the carbon–oxygen bond, making the carbonyl carbon electrophilic. When a nucleophile attacks, the unoccupied π* orbital, which is predominantly present  on the electrophilic carbon, acts as a LUMO (Lowest Unoccupied Molecular Orbitals) and interacts with the filled HOMO (Highest Occupied Molecular Orbital) of the incoming nucleophile to form a σ bond. The addition of electrons to the π* antibonding orbitals weaken the C=O π bond, breaking it subsequently. The electrons present in the π orbitals move to the oxygen atom, making it negatively charged.