Aldehydes and ketones: naming, structure, physical properties and uses
Aldehydes (-CHO) and ketones (C=O between two carbons) share the carbonyl group. Its polar C=O bond explains their boiling points and solubility.
The carbonyl group
The carbonyl group is a carbon joined to an oxygen by a double bond, >C=O. In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen (-CHO). In a ketone, it is bonded to two carbon groups. The carbonyl carbon is sp² hybridised. It forms three σ (sigma) bonds that lie in one plane at about 120°. Its leftover p orbital overlaps with a p orbital of oxygen to form a π bond. Oxygen is more electronegative than carbon, so the electrons are pulled towards oxygen. The C=O bond is therefore polar: carbon carries a partial positive charge (δ+) and oxygen a partial negative charge (δ−). We show this with resonance: >C=O ↔ >C⁺-O⁻. So the carbonyl carbon is an electrophile (a Lewis acid, it accepts electrons), and the oxygen is a nucleophile (a Lewis base). Aldehydes and ketones have large dipole moments, larger than ethers.
Common and IUPAC names
| Formula | Common name | IUPAC name |
|---|---|---|
| HCHO | Formaldehyde | Methanal |
| CH₃CHO | Acetaldehyde | Ethanal |
| (CH₃)₂CHCHO | Isobutyraldehyde (α-methylpropionaldehyde) | 2-Methylpropanal |
| (CH₃)₂CHCH₂CHO | Isovaleraldehyde (β-methylbutyraldehyde) | 3-Methylbutanal |
| CH₃CH=CHCHO | Crotonaldehyde | But-2-enal |
| C₆H₅CHO | Benzaldehyde | Benzenecarbaldehyde (benzaldehyde is also accepted) |
| C₆H₁₁CHO (CHO on a cyclohexane ring) | No common name | Cyclohexanecarbaldehyde |
| CH₃COCH₃ | Acetone | Propanone |
| CH₃COCH₂CH₃ | Ethyl methyl ketone | Butan-2-one |
| CH₃COCH₂CH(CH₃)₂ | Isobutyl methyl ketone | 4-Methylpentan-2-one |
| C₆H₅COCH₃ | Acetophenone | 1-Phenylethanone |
| C₆H₅COC₆H₅ | Benzophenone | Diphenylmethanone |
How to write the IUPAC name
- 1Choose the longest chain that contains the carbonyl carbon. In an aldehyde, the CHO carbon is always C1.
- 2★Replace the final 'e' of the alkane name with -al (aldehyde) or -one (ketone).Propane becomes propanal or propanone
- 3For a ketone, number the chain so the C=O carbon gets the lowest number, and write that number before -one: butan-2-one.
- 4Name and number the substituents and write them as prefixes in alphabetical order: 6-chloro-4-ethylhexan-3-one.
- 5If the CHO group is attached to a ring, use the suffix -carbaldehyde; the CHO carbon is not counted in the ring.Cyclohexanecarbaldehyde, benzenecarbaldehyde
- 6In common names, the carbon next to C=O is α, the next is β, and so on. IUPAC names use numbers instead.β-Methylbutyraldehyde = 3-methylbutanal
★ key step
Worked example: name these compounds
Give the IUPAC names of: (i) CH₃CH₂CH(CH₃)CH₂CHO (ii) CH₃COCH₂CH₂COCH₃ (iii) CH₃CH=CHCH₂CHO (iv) CH₃CH₂COCH(CH₃)CH₂CH₂Br
- (i) The chain has 5 carbons, counting the CHO carbon as C1. The methyl group is on C3. Name: 3-methylpentanal.
- (ii) The chain has 6 carbons with C=O at C2 and C5. Two -one groups make -dione, and the 'e' of hexane stays before a consonant. Name: hexane-2,5-dione.
- (iii) The chain has 5 carbons: CHO is C1, and the double bond is between C3 and C4. Name: pent-3-enal.
- (iv) The longest chain runs through the CH₂CH₂Br end: 6 carbons. Numbering from the CH₃CH₂ end puts C=O at C3 (from the other end it would be C4). The methyl is on C4 and Br on C6. Alphabetical order: bromo before methyl. Name: 6-bromo-4-methylhexan-3-one.
3-Methylpentanal; hexane-2,5-dione; pent-3-enal; 6-bromo-4-methylhexan-3-one
Physical properties and the reasons
| Property | What you see | Why |
|---|---|---|
| State | Methanal is a gas; ethanal is a volatile liquid; most others are liquids or solids at room temperature | Larger molecules attract each other more strongly |
| Boiling point compared with hydrocarbons and ethers | Higher | The polar C=O gives dipole-dipole attraction between molecules |
| Boiling point compared with alcohols | Lower | No O-H bond, so no intermolecular hydrogen bonding |
| Example data (molar mass 58 to 60) | n-Butane 273 K, methoxyethane 281 K, propanal 322 K, propanone 329 K, propan-1-ol 370 K | Order: alkane < ether < aldehyde, ketone < alcohol |
| Solubility in water | Methanal, ethanal and propanone mix with water in all proportions; solubility falls as the chain grows | The C=O oxygen forms hydrogen bonds with water; a long alkyl chain is non-polar |
| Smell | Lower aldehydes smell sharp and pungent; larger aldehydes and many ketones smell pleasant | So many are used in perfumes and flavours |
Uses of aldehydes and ketones
- Formalin, about a 40% solution of methanal in water, preserves biological specimens.
- Methanal is used to make bakelite (a phenol-formaldehyde resin), urea-formaldehyde glues and other polymers.
- Ethanal is a starting material for ethanoic acid, ethyl ethanoate, vinyl ethanoate, polymers and drugs.
- Benzaldehyde is used in perfumes and in making dyes.
- Propanone (acetone) and butan-2-one (ethyl methyl ketone) are common industrial solvents. Acetone is in nail polish remover.
- Many natural smells and flavours are carbonyl compounds: vanillin (vanilla beans), cinnamaldehyde (cinnamon), salicylaldehyde (meadowsweet), and camphor, a ketone.
The warm smell of dalchini (cinnamon) in a biryani comes from cinnamaldehyde, and vanilla ice cream gets its flavour from vanillin. The acetone in nail polish remover is a ketone used as a solvent.
2 marks: “Write the IUPAC names of (i) CH₃COCH₂CH(CH₃)₂ (ii) C₆H₅COCH₃.” (i) 4-Methylpentan-2-one: a 5-carbon chain, C=O at C2, methyl at C4. (ii) 1-Phenylethanone (acetophenone).
2 marks: “Arrange in increasing order of boiling point and give a reason: CH₃CH₂CH₂OH, CH₃COCH₃, CH₃CH₂CH₂CH₃, CH₃OCH₂CH₃.” CH₃CH₂CH₂CH₃ (273 K) < CH₃OCH₂CH₃ (281 K) < CH₃COCH₃ (329 K) < CH₃CH₂CH₂OH (370 K). Butane has only weak dispersion forces; the ether is weakly polar; propanone has strong dipole-dipole attraction; propan-1-ol forms intermolecular hydrogen bonds.
3 marks: “Why is the carbonyl group polar? Why do the lower aldehydes and ketones dissolve in water while the higher ones do not?” O is more electronegative, so C=O is polar: C is δ+, O is δ−. Lower members form hydrogen bonds with water through the C=O oxygen. In higher members, the large non-polar alkyl part outweighs this, so solubility falls.
-al for aldehyde, -one for ketone. Boiling points climb with attraction: alkane < ether < carbonyl < alcohol.