Physical Properties of Aldehydes and Ketones

In aldehydes and ketones, the greater electronegativity of the oxygen atom attached to the carbon atom induces polarity in the “C=O” bond. The carbonyl carbon becomes partially positive, and the carbonyl oxygen becomes partially negative. This induction of polarity dictates physical properties like boiling point and solubility in aldehydes and ketones.

Unlike the weak dispersion forces that operate between hydrocarbon molecules, stronger dipole-dipole forces exist between aldehydes and ketones, tightly holding them together in pure liquids. Hence aldehydes and ketones boil at much higher temperatures than their corresponding hydrocarbons of comparable molecular weights.

Aldehydes and ketones do not possess hydrogen bond donors; therefore, they do not exhibit any hydrogen bonding interactions in their pure liquids. Thus, compared to alcohols and acids that display strong hydrogen bonding interactions and hence require more heat to boil, aldehydes and ketones boil at much lower temperatures.

The electronegative oxygen atom in aldehydes and ketones can act as a good hydrogen bond acceptor and form hydrogen bonds with donors such as alcohol and water, as shown below. This is the reason why aldehydes and ketones are miscible in donor solvents like alcohol and water.

The solubility of aldehydes and ketones in water depends on the length of the hydrophobic hydrocarbon chain. Aldehydes and ketones with up to four carbon atoms are readily soluble in water, but the solubility decreases with an increase in molecular weight.