Chapter 8 · Chemistry
Same Weight, 0 °C vs 97 °C: Physical Properties of Aldehydes and Ketones
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Five compounds of the same mass boil from 0 °C to 97 °C. Aldehydes and ketones sit in the middle: their polar C=O gives dipole-dipole attraction, but with no O–H they cannot hydrogen-bond with each other. Water can hydrogen-bond to the C=O oxygen, so small aldehydes and ketones mix with water while long chains do not.
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Nucleophilic Addition to the Carbonyl GroupMost reactions of aldehydes and ketones begin with one move. How a nucleophile attacks the polar C=O, and why aldehydes react faster than ketones.Chemistry
Key points
- Same molar mass (58–60): butane 0 °C < methoxyethane 8 °C < propanal 49 °C < acetone 56 °C < propan-1-ol 97 °C (NCERT 8.3).
- The C=O is polar (C δ+, O δ−): neighbouring molecules attract dipole to dipole, so aldehydes and ketones boil above hydrocarbons and ethers.
- No O–H, so no hydrogen bonds between their own molecules: they boil below alcohols of similar mass.
- Water's H hydrogen-bonds to the carbonyl O: methanal, ethanal and propanone mix in all proportions; longer chains are less soluble.
Boiling points at similar mass
| Compound | Family | Boiling point | Attraction |
|---|---|---|---|
| n-butane | alkane | 0 °C (273 K) | weak dispersion only |
| methoxyethane | ether | 8 °C (281 K) | weak dipole |
| propanal | aldehyde | 49 °C (322 K) | dipole-dipole |
| acetone | ketone | 56 °C (329 K) | dipole-dipole |
| propan-1-ol | alcohol | 97 °C (370 K) | hydrogen bonds |
State, smell and solubility
| Property | What to remember |
|---|---|
| State | methanal gas; ethanal volatile liquid; others liquids or solids |
| Smell | lower aldehydes pungent; larger ones fragrant (perfumes, flavours) |
| In water | small ones mix in all proportions (H-bonds to C=O); solubility falls as the chain grows |
| Organic solvents | all dissolve in benzene, ether, methanol, chloroform |
Quick quiz
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