Nomenclature
Both aldehydes and ketones have a C=O group, but they differ in what is attached to the carbonyl carbon. An aldehyde has at least one hydrogen on the carbonyl carbon (R-CHO). A ketone has two non-H substituents on the carbonyl carbon (R-CO-R’). The naming rules reflect this structural distinction with different suffixes.
Aldehyde Naming: -al Suffix
Aldehydes use the suffix “-al” replacing the final “-e” of the parent alkane name. Because the carbonyl carbon is always at the end of the chain (it has an H attached, so it must be terminal), the carbonyl carbon is automatically numbered 1.
- CH₃CHO = ethanal (formerly acetaldehyde, a common name still used).
- CH₃CH₂CHO = propanal.
- CH₃CH₂CH₂CHO = butanal.
- HCHO = methanal (common name: formaldehyde).
Since the aldehyde carbon is always C1, you do not need a locant number for the carbonyl position. Other substituents get numbered starting from C1 as usual.
Example: 3-methylbutanal is (CH₃)₂CHCH₂CHO. The aldehyde carbon is C1, the methyl branch is at C3.
Ketone Naming: -one Suffix
Ketones use the suffix “-one” replacing the final “-e” of the parent alkane name. Unlike aldehydes, ketones need a locant to specify where the C=O sits, because the carbonyl can be at various positions within the chain (though never at the end - that would be an aldehyde).
- CH₃COCH₃ = propan-2-one (common name: acetone).
- CH₃CH₂COCH₃ = butan-2-one (common name: methyl ethyl ketone, MEK).
- CH₃CH₂COCH₂CH₃ = pentan-3-one.
Numbering rules: give the carbonyl the lowest locant possible. If there are ties, use substituent priorities as usual.
Common Names You Should Know
A handful of carbonyl common names appear regularly:
| Common name | IUPAC |
|---|---|
| Formaldehyde | methanal (HCHO) |
| Acetaldehyde | ethanal (CH₃CHO) |
| Benzaldehyde | benzaldehyde (C₆H₅CHO) - retained in IUPAC |
| Propionaldehyde | propanal |
| Acetone | propan-2-one |
| Methyl ethyl ketone (MEK) | butan-2-one |
| Acetophenone | 1-phenylethanone or phenyl methyl ketone (C₆H₅COCH₃) |
| Benzophenone | diphenylmethanone (C₆H₅COC₆H₅) |
Formaldehyde is especially common as the 37% aqueous solution called formalin - used as a tissue preservative and in histology.
When Carbonyl Coexists with Higher-Priority Groups
If a molecule has a functional group higher in priority (carboxylic acid, ester, amide), the aldehyde or ketone becomes a prefix rather than a suffix:
- Aldehyde as prefix: -CHO in the middle of a chain is called “oxo-” (for the =O substituent) or “formyl-” when attached at the terminal position.
- Ketone as prefix: =O in a chain becomes “oxo-” followed by the locant.
Example: HOOC-CH₂-CO-CH₃ is 3-oxobutanoic acid (carboxylic acid is the highest priority group, so -oic acid is the suffix; the ketone becomes 3-oxo-).
Cyclic Ketones
Cyclic ketones are named by placing the carbonyl carbon at C1 of the ring:
- Cyclohexan-1-one: cyclohexane with a ketone at C1. Usually written “cyclohexanone.”
- 2-methylcyclopentan-1-one: cyclopentanone with a methyl at C2.
Dialdehydes and Diketones
Multiple carbonyls get multiplier prefixes:
- Propanedial = OHC-CH₂-CHO.
- Hexane-2,5-dione = CH₃-CO-CH₂-CH₂-CO-CH₃.
- Pentane-2,4-dione (a classic 1,3-diketone, key substrate for Ch 7 enolate chemistry).
Stereo Prefixes for Conjugated Carbonyls
An alpha-beta unsaturated aldehyde or ketone (C=C-C=O) is called an enone. Example: but-3-en-2-one (methyl vinyl ketone, MVK) is CH₂=CH-CO-CH₃. This is the workhorse Michael acceptor in Ch 7.
Geometric (E/Z) isomerism of the C=C bond can be specified the usual way:
- (E)-4-methylpent-3-en-2-one = (E)-mesityl oxide.