The carbonyl group (C=O) is the single most important functional group in the back half of Orgo 2 — aldehydes, ketones, carboxylic acids, esters, amides, and more all contain it. Getting its fundamental reactivity right unlocks dozens of reactions, so this track is foundational.

An aldehyde has the carbonyl at the end of a chain (at least one H on the carbonyl carbon, –CHO); a ketone has it internal (two carbon groups flanking it). The defining feature of the carbonyl is its polarization: oxygen is far more electronegative than carbon, and the pi bond's electrons are pulled toward oxygen. This leaves the carbonyl carbon electrophilic (δ+) and the oxygen nucleophilic/basic (δ−). That polarity dictates everything: nucleophiles attack the carbonyl carbon, and acids/electrophiles interact with the oxygen.

Two reactivity trends to internalize:

Aldehydes are more reactive than ketones toward nucleophilic addition, for two reasons: aldehydes are less sterically hindered (an H is smaller than a second carbon group), and ketones' two electron-donating alkyl groups slightly stabilize the δ+ carbon, making it less electrophilic. So the general order is aldehyde > ketone.
The core reaction is nucleophilic addition. A nucleophile attacks the planar sp² carbonyl carbon; the pi electrons shift onto oxygen to give an alkoxide (under basic conditions) or, after protonation, an alcohol. The flat carbonyl can be attacked from either face — a point that matters for stereochemistry when a new stereocenter forms. This "attack the carbon, oxygen becomes O⁻/OH" pattern is the mechanistic template for the next track.

Preparation ties back to earlier chemistry: aldehydes and ketones come from oxidation of alcohols (Track 27) and ozonolysis of alkenes (Track 19), among others.

This track applies the "attack the electrophilic carbonyl carbon" template to the specific nucleophiles you must know, and the payoff is a large family of transformations that all follow one logic.

Hydride reduction. Hydride donors add H⁻ to the carbonyl carbon to give an alcohol. NaBH₄ is mild and selectively reduces aldehydes and ketones (→ 1° and 2° alcohols); LiAlH₄ is far stronger and reduces even carboxylic acids and esters down to alcohols. Reduction of an aldehyde gives a primary alcohol; a ketone gives a secondary alcohol.
Addition of oxygen and nitrogen nucleophiles. With alcohols, aldehydes/ketones form hemiacetals and then acetals (an acid-catalyzed, reversible equilibrium; excess alcohol and water removal drive acetal formation). Acetals are important as protecting groups — they mask a carbonyl so it survives conditions (like a Grignard) that would otherwise attack it, and are then removed by aqueous acid. With primary amines, the carbonyl forms an imine (C=N, a "Schiff base") after loss of water; with secondary amines it forms an enamine. These condensations are pH-sensitive (fastest at mildly acidic pH ~4–5).
Addition of HCN gives a cyanohydrin, adding both a carbon and a masked functional group.

The unifying theme: identify the nucleophile, add it to the δ+ carbon, protonate the oxygen, and — where a new C–C bond forms (Grignard, cyanohydrin, and later enolates) — recognize you are building the molecule.

"Attack the Carbon"