Organic Chemistry
Carbonyl Chemistry
The C=O group — heart of aldehydes, ketones, and the most reactive functional group
Carbonyl chemistry centers on the C=O group — found in aldehydes (R-CHO), ketones (R-CO-R'), and related compounds (acids, esters, amides). Highly polar (O δ⁻, C δ⁺) due to electronegativity. Carbon is electrophilic — accepts nucleophiles. Reactions: nucleophilic addition (aldehydes, ketones), nucleophilic acyl substitution (acids, esters, amides), enolization, aldol, etc. Carbonyls in: sugars (aldoses, ketoses), steroids, drugs, polymers (polyesters, polyamides). Most-studied functional group.
- C=O groupHighly polar (O δ⁻, C δ⁺)
- AldehydeR-CHO (one H attached to C=O)
- KetoneR-CO-R' (two carbons attached)
- Bond~1.20 Å length; ~733 kJ/mol energy
- ReactivityC electrophilic; attacked by nucleophiles
- Common reactionsAddition, substitution, aldol, reduction
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Why carbonyls matter
- Sugars. Aldoses, ketoses fundamental.
- Steroids. Many hormones contain C=O.
- Drugs. Many drugs have carbonyl groups.
- Polymers. Polyesters, polyamides via carbonyls.
- Synthesis. Versatile starting materials.
- Biology. Metabolism, enzymes.
- Smell. Aldehydes/ketones in flavors and perfumes.
Common misconceptions
- Aldehydes and ketones identical. Different reactivity.
- Carbonyl always reacts the same way. Many reactions possible.
- C=O is single bond. Double bond.
- Esters are aldehydes. Different (ester has -OR; aldehyde has -H).
- Tautomers are isomers. Yes — but interconvert rapidly.
- Carbonyl reactions are simple. Many mechanisms; conditions matter.
Frequently asked questions
What's the carbonyl group?
Carbon double-bonded to oxygen (C=O). Highly polar — O is much more electronegative. Carbon has partial positive charge; oxygen partial negative. Carbon is sp² hybridized — planar geometry around C=O (120° angles). Reactivity: O lone pairs are basic; C is electrophilic site. Foundation of much organic chemistry.
What's the difference between aldehyde and ketone?
Both have C=O. Aldehyde: at least one H attached to carbonyl C (R-CHO; e.g., formaldehyde HCHO, acetaldehyde CH₃CHO). Ketone: two carbons attached (R-CO-R'; e.g., acetone CH₃COCH₃). Aldehydes more reactive (less steric hindrance, less electron donation from H). Different reactions: Tollens', Fehling's identify aldehydes.
How does nucleophilic addition work?
Most basic carbonyl reaction. Nucleophile attacks electrophilic C; O takes electrons, becomes alkoxide; protonation gives alcohol. RCHO + Nu⁻ → R-CH(O⁻)-Nu → RCH(OH)Nu. Examples: hydration (Nu = OH⁻ → hydrate), addition of HCN (cyanohydrin), Grignard (RMgX → alcohol), reduction (NaBH₄ → alcohol).
What's an aldol reaction?
Carbonyl reacts with itself (or another carbonyl). Enolate (carbonyl with α-H) attacks another carbonyl; nucleophilic addition. Two molecules of acetaldehyde → 3-hydroxybutanal. Followed by dehydration (loss of water) gives α,β-unsaturated carbonyl. Aldol condensation = aldol + dehydration. Foundational C-C bond-forming reaction.
What's keto-enol tautomerism?
Carbonyls with α-H exist in equilibrium with enol form. R-CO-CH₂-R' ⇌ R-C(OH)=CH-R'. Keto form usually dominant (more stable). Acetaldehyde: ~99.999% keto. Some are enol-dominated (β-diketones, aromatic phenols). Tautomers: structural isomers; differ in H position. Important: enzyme reactions, aldol reactions go through enol.
How do carboxylic acids and esters fit?
Both have C=O attached to additional electronegative atoms. Carboxylic acid: -COOH (C=O + -OH). Ester: -CO-OR. Amide: -CO-NHR. These are "carboxylic acid derivatives" — undergo nucleophilic acyl substitution (different from addition). Reactivity order: acid chloride > anhydride > ester > amide. Ranks by leaving group ability.
What are biological carbonyls?
(1) Sugars: aldoses (with -CHO) and ketoses (with -C=O-). (2) Steroids: many have C=O (e.g., cortisol, testosterone). (3) Acetyl-CoA: thioester central to metabolism. (4) Pyridoxal phosphate (vitamin B6): aldehyde used for amino acid metabolism. (5) Aldehyde dehydrogenase: enzymes processing carbonyls. Critical biological functions.