The electrophilic character of the carbonyl carbon
A carbonyl group (C=O) consists of a strong double bond between carbon and oxygen. Oxygen is more electronegative than carbon, so it pulls electron density away, making the carbonyl carbon electrophilic and vulnerable to attack by nucleophiles. The C=O pi bond is also polarized, with oxygen carrying partial negative charge and carbon carrying partial positive charge. This polarity is visible in IR spectroscopy, where the C=O stretch appears around 1700 cm-1, and is fundamental to why carbonyl groups appear in so many reactions.
Addition of water, alcohols, and amines
Water attacks aldehydes and ketones to form hydrates (geminal diols), though these are usually unstable and revert to the carbonyl. Alcohols attack to form hemiacetals or acetals, depending on stoichiometry and conditions. Amines attack to form imines (C=N), which are more stable than the corresponding hydrate because nitrogen can donate electrons to stabilize the sp2 carbon. All these reactions proceed through a tetrahedral intermediate where the carbonyl carbon becomes sp3 hybridized. The intermediate is usually drawn with a lone pair on oxygen and a negative charge on one position.
Reducing agents and other carbon nucleophiles
Hydride donors like LiAlH4 or NaBH4 are powerful nucleophilic reductants that convert aldehydes and ketones to primary and secondary alcohols. Organometallic reagents like Grignard (R-MgX) or organolithium (R-Li) compounds add carbon nucleophiles to carbonyls, forming alcohols with a new C-C bond. Cyanide ion adds to give cyanohydrins, useful intermediates in synthesis. All these reactions are driven by the electrophilicity of the carbonyl carbon and the nucleophilicity of the attacking reagent. Changing the nucleophile changes the product in predictable ways.