Physical Properties of Aldehydes and Ketones

Physical Properties of Aldehydes and Ketones Physical Properties of Aldehydes and Ketones

Aldehydes and ketones are the class of organic compounds that have a carbonyl group i.e. carbon-oxygen double bond (-C=O). As they do not have any other reactive groups like –OH or -Cl attached to the carbon atom in the carbonyl group they are very simple compounds.  The chemistry of aldehydes and ketones is greatly influenced by the presence of carbonyl group.

Physical Properties of Aldehydes and Ketones:

Boiling point: The boiling point of methanal is -19o C and for ethanal it is +21o C. From this we can say that the boiling point of ethanal is close to room temperature. Generally the boiling point of aldehydes and ketones increases with increase in molecular weight. Boiling point depends upon the strength of the intermolecular forces.

  • Vander Waals dispersion forces: As the molecules get longer and the number of electrons increases, the attraction between them also increases. For both aldehydes and ketones the boiling point increases with the increase in number of carbon atoms.
  • Vander Waals dipole-dipole attraction: Because of the presence of carbon-oxygen double bond both aldehydes and ketones are polar in nature. There will be attraction between permanent dipoles as well as the molecules which are near to it. This is the reason for aldehydes and ketones having boiling point higher than the similar sized hydrocarbons.

Solubility: Aldehydes and ketones are soluble in water but their solubility decreases with increase in the length of the chain. Methanal, ethanal and propanone are those aldehydes and ketones which are of small size and are miscible with water in almost all proportions.

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Aldehydes and ketones cannot form hydrogen bonds with themselves, but they can have hydrogen bonds with  water molecules and this forms the basis for good solubility of aldehydes and ketones in water. This is also because of dispersion forces and dipole-dipole interactions. Aldehydes and ketones find many applications in different industries. They are extensively used in the manufacturing of polymers, blending of perfumes and also as flavouring agents.

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