A LITTLE LIGHTBULB MOMENT
Aldehydes, ketones and carbonyl additions
Carbonyl carbon is electrophilic
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The carbonyl carbon wears a partial-positive hat
The idea behind the cue
In aldehydes and ketones, the C=O bond is strongly polarized. Oxygen is more electronegative, so the carbonyl carbon is electrophilic and is the site attacked by nucleophiles. This polarization explains most addition chemistry of carbonyl compounds.
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Short & simple
The carbonyl carbon wears a partial-positive hat
Acronym
OCN — Operatic Cupcakes Nod.
Story
A cartoon carbonyl carbon stands under a magnet labeled oxygen. Tiny nucleophiles rush in because the carbon looks electron-poor and exposed, while the oxygen hoards the electron cloud.
Connect the pieces
- O (Operatic) → Oxygen polarization: Oxygen pulls electron density toward itself, so the C=O bond is strongly polarized.
- C (Cupcakes) → Carbonyl electrophile: That polarization makes the carbonyl carbon electron-poor and ready for nucleophilic attack.
- N (Nod) → Nucleophilic attack: Nucleophiles attack the carbonyl carbon in aldehydes and ketones.
- Operatic · Oxygen polarization: Oxygen pulls electron density toward itself, so the C=O bond is strongly polarized.
- Cupcakes · Carbonyl electrophile: That polarization makes the carbonyl carbon electron-poor and ready for nucleophilic attack.
- Nod · Nucleophilic attack: Nucleophiles attack the carbonyl carbon in aldehydes and ketones.
Know the limits
Applies to aldehydes and ketones; carboxylic acid derivatives can react differently because a leaving group changes the pathway.
Further reading · OpenStax Organic Chemistry ↗Check your understanding
Why is the carbonyl carbon generally susceptible to nucleophilic attack?
A. Because the carbonyl carbon is always positively charged
B. Because nucleophiles prefer double bonds over single bonds
C. Because oxygen makes the C=O bond electron-rich at carbon
D. Because the C=O bond is polarized toward oxygen, leaving carbon electron-poor
Reveal explanation
Because the C=O bond is polarized toward oxygen, leaving carbon electron-poor
The oxygen withdraws electron density, so the carbonyl carbon is partially positive and electrophilic.