Nomenclature and Structure of Aldehydes and Ketones: MDCAT Chemistry notes
Nomenclature and Structure of Aldehydes and Ketones for MDCAT: carbonyl group, CnH2nO formula, common and IUPAC names, oxo prefix, formalin composition.
The carbonyl group
Aldehydes and ketones both contain the carbonyl group, C=O.
- Aldehyde: the carbonyl carbon carries at least one hydrogen, $\mathrm{R{-}CHO}$. Formaldehyde, HCHO, carries two.
- Ketone: the carbonyl carbon carries two alkyl or aryl groups, $\mathrm{R{-}CO{-}R'}$.
Saturated open-chain aldehydes and ketones share the general formula $\mathrm{C_nH_{2n}O}$, so an aldehyde and a ketone with the same number of carbons are functional isomers (propanal and propanone are both $\mathrm{C_3H_6O}$). Ethers and alcohols are $\mathrm{C_nH_{2n+2}O}$; carboxylic acids are $\mathrm{C_nH_{2n}O_2}$.
Structure
The carbonyl carbon is sp$^2$ hybridized. Its three $\sigma$ bonds lie in one plane at about 120$^\circ$, so the region around C=O is planar. The C=O double bond is one $\sigma$ bond plus one $\pi$ bond formed by sideways overlap of p orbitals. Oxygen is more electronegative, so the bond is polar, $\mathrm{C^{\delta+}{=}O^{\delta-}}$, and the mobile $\pi$ electrons are pulled most strongly towards oxygen.
Common names
Common names of aldehydes come from the carboxylic acid with the same number of carbon atoms, replacing "-ic acid" by "-aldehyde".
| Formula | Common name | IUPAC name |
|---|---|---|
| HCHO | Formaldehyde (from formic acid) | Methanal |
| $\mathrm{CH_3CHO}$ | Acetaldehyde (from acetic acid) | Ethanal |
| $\mathrm{CH_3CH_2CHO}$ | Propionaldehyde | Propanal |
| $\mathrm{CH_3CH_2CH_2CHO}$ | Butyraldehyde | Butanal |
| $\mathrm{C_6H_5CHO}$ | Benzaldehyde | Benzenecarbaldehyde |
| $\mathrm{CH_3COCH_3}$ | Acetone, dimethyl ketone | Propanone |
| $\mathrm{CH_3COCH_2CH_3}$ | Ethyl methyl ketone | Butanone |
Common names of ketones list the two alkyl groups followed by "ketone".
IUPAC rules
- Aldehydes: replace the final -e of the alkane with -al. The CHO carbon is always C1, so no locant is needed.
- Ketones: replace -e with -one and give the carbonyl carbon the lowest possible number.
- When a higher-priority group is present, the lower one becomes a prefix. Priority: carboxylic acid > aldehyde > ketone > alcohol. A ketone C=O then becomes the prefix oxo-.
Worked examples:
- $\mathrm{CH_3CH_2CH_2COCH_2CHO}$: six carbons; the aldehyde is C1 and the ketone is at C3 → 3-oxohexanal.
- $\mathrm{CH_3COCH_2COOH}$: acid takes the suffix, ketone is oxo → 3-oxobutanoic acid (pattern: oxo…oic acid).
- Diisopropyl ketone, $\mathrm{(CH_3)_2CH{-}CO{-}CH(CH_3)_2}$: longest chain five carbons, C=O at C3, methyls at C2 and C4 → 2,4-dimethylpentan-3-one.
Formalin
Formalin is an aqueous solution of formaldehyde: about 40% HCHO, 8% methanol and 52% water. The methanol prevents polymerization. It is used to preserve biological specimens and in embalming.
Common MDCAT traps
- "3-onehexanal" and "3-oxohexanone" are invalid; when the aldehyde is present the ketone is named oxo.
- $\mathrm{C_nH_{2n}O}$ fits aldehydes and ketones, not ethers or carboxylic acids.
- Formalin contains formaldehyde and methanol, not acetaldehyde or ethanol.
- A carbonyl compound with at least one H on the carbonyl carbon is an aldehyde, not a ketone.
Quick revision
- Carbonyl carbon: sp$^2$, planar, about 120$^\circ$.
- Aldehyde suffix -al; ketone suffix -one; prefix form oxo-.
- Acid outranks aldehyde, which outranks ketone.
- Formalin: 40% HCHO, 8% $\mathrm{CH_3OH}$, 52% water.