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Two ketone carbonyls separated by one carbon. The intervening C–H bonds are relatively acidic, and the enol can be stabilized by pi conjugation and an intramolecular hydrogen bond.

The ketone $\mathrm{C}{\mathrm{H}}_3\mathrm{COCH}(\mathrm{Cl})\mathrm{C}{\mathrm{H}}_3$. Its methine and attached methyl show reciprocal vicinal coupling, while the acetyl methyl can appear as a singlet.

Acetone is the ketone $\mathrm{C}{\mathrm{H}}_3\mathrm{COC}{\mathrm{H}}_3$, with a carbonyl group between two methyl groups. Its polar carbonyl makes it a useful solvent and an electrophilic center for nucleophilic addition.

Hydrogens on its adjacent methyl groups can be removed to form an enolate. Thus one molecule illustrates both carbonyl electrophilicity and the carbon acidity used in carbon-carbon bond-forming reactions.

Acetophenone is the aromatic ketone ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{C}(=\mathrm{O})\mathrm{C}{\mathrm{H}}_3$. It contains a carbonyl attached directly to a phenyl group and an enolizable methyl group.

Reduction of the carbonyl gives an alcohol with a new stereogenic centre. A chiral reduction system can favour one enantiomer, making acetophenone a useful simple example of catalytic asymmetric synthesis.

An anion formed by removing the terminal proton of an alkyne, $\mathrm{RC}\equiv {\mathrm{C}}^{-}$. It can act as a carbon nucleophile, for example in addition to a ketone.

Addition of an enolate or enol to an aldehyde or ketone, giving a beta-hydroxy carbonyl compound. Subsequent dehydration is a distinct step and gives an aldol condensation.

The aldol reaction forms a carbon-carbon bond when an enol or enolate adds to an aldehyde or ketone. The initial addition product is a beta-hydroxy carbonyl compound, containing the combined carbon frameworks.

For example, acetaldehyde can add to another acetaldehyde molecule to give 3-hydroxybutanal. Subsequent loss of water gives an alpha,beta-unsaturated carbonyl compound, the condensation product. Keeping addition and dehydration distinct explains why an aldol reaction need not itself eliminate water. The reaction is useful for constructing longer carbon skeletons with predictable functional groups.

Alpha-ketoglutarate is another name for 2-Oxoglutarate, a five-carbon dicarboxylate containing a ketone adjacent to a carboxylate group. The names describe the same carbon skeleton using two positional naming conventions.

Alpha-ketoglutarate links the citric acid cycle with amino acid metabolism: transamination interconverts its carbon skeleton with that of glutamic acid. Its oxidative decarboxylation yields succinyl-CoA and carbon dioxide.^[ref]

Androstenedione is a 19-carbon steroid with ketone groups at carbon positions 3 and 17 and a double bond between positions 4 and 5. The structural name androst-4-ene-3,17-dione encodes these features. Its molecular formula is ${\mathrm{C}}_{19}{\mathrm{H}}_{26}{\mathrm{O}}_2$.^[ref]

Androstenedione is also a substrate in steroid hormone synthesis. Aromatase converts it to estrone, with formate among the products. The enzyme record lists a separate reaction converting testosterone to estradiol; the two starting steroids and their products should not be conflated.^[ref]

Carbon accounting gives a useful check on the first transformation: the substrate has 19 carbon atoms, whereas estrone has 18, and the remaining carbon appears in formate. That accounting describes only the carbon balance; the complete reaction also requires oxygen and reducing equivalents. Thus conversion to an estrogen involves a chemical rearrangement and oxidation process, rather than simply attaching a different biological label to the same steroid structure.^[ref]

Insertion of oxygen beside an aldehyde or ketone carbonyl, commonly with a peroxy acid. Cyclic ketones give ring-expanded lactones. The migrating group retains its configuration, but competing migrations can give different constitutional products.

Cleavage of a carbon–carbon bond beta to a metal, often from a metal alkoxide, coupled to carbonyl formation and transfer of a carbon fragment to the metal. For a tertiary benzylic alkoxide this can generate a ketone and a metal–aryl bond.

A compound with a ketone carbonyl at the beta position relative to an ester carbonyl.

Butan-2-one is the four-carbon ketone $\mathrm{C}{\mathrm{H}}_3\mathrm{C}(=\mathrm{O})\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$. Its two sides are constitutionally different, so its two alpha positions need not react equivalently.

Formation and trapping of different enolates can give distinct substitution products. The example connects a simple molecular structure with enolate regioselectivity.

Camphor is a bicyclic ketone with formula ${\mathrm{C}}_{10}{\mathrm{H}}_{16}\mathrm{O}$. Its rigid carbon framework and stereochemistry make it a useful example of how three-dimensional structure influences reactions at a carbonyl group.

It is also a familiar volatile natural product with a characteristic odor. The molecular formula alone does not describe its bridged skeleton or distinguish its enantiomers.

A compound containing the carbonyl functional group, $\mathrm{C}=\mathrm{O}$, such as a ketone or aldehyde.

Reduction at a carbonyl group, for example conversion of a ketone to an alcohol. In an enone, 1,2- and 1,4-reduction produce different functional-group patterns.

Reduction using hydrogen and a catalyst. Selectivity depends on the catalyst, pressure, solvent and functional groups: reducing an alkene while retaining a ketone is a distinct requirement from reducing a nitrile while retaining an ester.

carbon acylation of an ester enolate followed by deprotonation of the beta-dicarbonyl product. ketone-enolate acylation by an acyl chloride is a closely related acyl-substitution strategy, although it is not the classical ester self-condensation.

A six-membered cyclic ketone.

Cyclopentanone is a five-membered cyclic ketone with formula ${\mathrm{C}}_5{\mathrm{H}}_8\mathrm{O}$. Its carbonyl carbon is part of the ring, rather than attached to a separate cyclic substituent.

The neighboring carbons can form enolates, so the molecule is a useful substrate for carbonyl addition, reduction, and alpha-carbon functionalization. Ring geometry constrains the conformations available to those reacting groups.