Physical Properties

Full Notes

Boiling Point

Aldehydes and ketones boil at higher temperatures than many hydrocarbons because of their polarity. The carbon–oxygen double bond (C=O) introduces dipole–dipole interactions between molecules, which require more energy (heat) to overcome.

However, unlike alcohols, these compounds cannot form intermolecular hydrogen bonds. This means they don’t reach the same boiling point levels as alcohols of similar molar mass.

Example Comparison:
Ethanol (alcohol) > Propanal (aldehyde) > Propane (alkane)

Solubility

Shorter chain aldehydes and ketones dissolve well in water. This is because the C=O group enables hydrogen bonding between aldehyde or ketone molecules and water molecules.

• Compounds with up to four carbon atoms are usually miscible with water.
• Larger molecules become less soluble as their hydrocarbon portion dominates the interaction.

Odour and Appearance

Many small aldehydes and ketones have distinct smells – some sweet, some sharp.

• Formaldehyde and acetaldehyde are gases or volatile liquids with a strong, pungent odour.
• Acetone, a common ketone, is a colourless liquid with a sweet, recognisable smell.

Summary

• Aldehydes and ketones are polar due to the C=O bond and show dipole–dipole interactions.
• They boil at higher temperatures than alkanes and ethers but lower than alcohols.
• Low-mass members dissolve readily in water by hydrogen bonding with water molecules.
• Volatile aldehydes and ketones often have characteristic odours useful for identification.