carbonyl polarity

Picture a tiny magnet hidden inside a molecule. The carbonyl group is a carbon atom double-bonded to an oxygen atom (written C=O), and it is one of the most important little magnets in all of organic chemistry. It sits at the heart of aldehydes, ketones, acids, esters and many other families. Understanding why it is lopsided in its electron sharing unlocks why it reacts the way it does.

Oxygen is much greedier for electrons than carbon (it is more electronegative). In the C=O double bond, oxygen pulls the shared electrons toward itself, so the oxygen end carries a partial negative charge (written delta-minus) and the carbon end carries a partial positive charge (delta-plus). A resonance picture helps: one contributor has a normal double bond, and a second contributor has a single bond with a full negative charge on oxygen and a full positive charge on carbon (C+-O-). The real molecule is a blend of these, but both pictures agree the carbon is electron-poor and the oxygen is electron-rich.

This single fact drives almost everything that follows. Because the carbon is electron-poor, electron-rich species (nucleophiles) are drawn to attack it. Because the oxygen is electron-rich, it can grab a proton or hold a developing negative charge. So the C=O bond is built to be attacked at carbon and protonated at oxygen, which is exactly the recipe for nucleophilic addition, the master reaction of this whole field.