carbonyl group
/ KAR-buh-neel /
There is one little arrangement of atoms that shows up again and again across organic chemistry: a carbon double-bonded to an oxygen, written C=O. This is the carbonyl group, and it sits at the heart of a whole crowd of compound classes — aldehydes, ketones, carboxylic acids, esters, amides and more. If you understand the carbonyl, you hold the key to a huge fraction of organic reactions.
A carbonyl group is a carbon joined to an oxygen by a double bond. Oxygen is much more electronegative than carbon, so it hogs the shared electrons: the oxygen end becomes partially negative and the carbon end partially positive. That polarized, electron-poor carbon is an electrophile, an open invitation for electron-rich nucleophiles to attack. A typical reaction begins with a nucleophile adding to the carbonyl carbon, the C=O pi bond breaking to push a pair of electrons onto the oxygen, forming a tetrahedral intermediate that then collapses one way or another. What the carbonyl is attached to — two carbons, a hydrogen, an OH, an OR, an NH2 — determines whether you have a ketone, aldehyde, acid, ester or amide, and how it reacts.
The carbonyl group is arguably the single most important functional group in organic chemistry and biochemistry: sugars, fats, proteins and the molecules of metabolism are riddled with carbonyls. Its predictable polarity is the thread tying together a large middle section of any organic course. It also activates the carbon next to it (the alpha carbon), enabling another whole family of reactions, so the C=O earns its reputation as the workhorse of the field.
In acetone, (CH3)2C=O, the central carbon carries the carbonyl. The oxygen pulls electron density toward itself, leaving that carbon partially positive, which is exactly the spot a nucleophile such as a Grignard reagent or hydride will attack.
The C=O carbon is electron-poor and electrophilic — the universal target for nucleophiles.
Carbonyl names the C=O group itself; the compound class depends on what else is bonded to that carbon. Do not confuse the carbonyl (C=O) with the hydroxyl (O-H) or carboxyl (COOH, which contains a carbonyl plus a hydroxyl).