BRAINCAKEPhysical Properties of Carbonyls
The C=O bond in aldehydes and ketones is polar, because oxygen is much more electronegative than carbon. This permanent dipole controls their physical properties.
Boiling points
Between carbonyl molecules there are London forces and permanent dipole-dipole forces. Carbonyl compounds cannot form hydrogen bonds with each other, because they have no H atom bonded to an oxygen atom. So:
• their boiling points are higher than those of alkanes of similar Mr, which have only London forces;
• their boiling points are lower than those of alcohols of similar Mr, which also form hydrogen bonds.
Worked comparison (similar Mr, so similar London forces):
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Key terms in this lesson
- london forces
- Weak intermolecular attractions caused by temporary, instantaneous dipoles induced by the random movement of electrons.
- dipole-dipole force
- An intermolecular attraction between the permanent dipoles of neighbouring polar molecules.
More in Carbonyl Compounds
- Aldehydes and Ketones
- Naming Carbonyl Compounds
- Oxidation of Aldehydes
- Tollens' Reagent
- Fehling's and Benedict's Solutions
- Iodoform Reaction
- 2,4-DNPH Test
- Reduction with LiAlH4
All 11 lessons in Carbonyl Compounds · All Edexcel A-level Chemistry topics