Aldehydes Ketones Properties

1. Physical state: Most of the aldehydes (except formaldehyde which is a gas) are liquids at room temperature. The lower ketones are colourless liquids and have pleasant smell. The higher members are colourless solids. Aromatic ketones are usually solids with a pleasant smell.

2. Boiling points: Aldehydes and ketones have relatively high boiling points as compared to hydrocarbons of comparable molecular masses. It is due to the reason that aldehydes and ketones contain polar carbonyl group and, therefore, they have stronger intermolecular dipole-dipole interactions between the opposite ends of C = O dipoles.

The dipole-dipole interactions are however weaker than intermolecular H-bonding in alcohols. Consequently, boiling points aldehydes and ketones are relatively lower than the alcohols of comparable molecular masses.



Ketones are relatively more polar than their corresponding isomeric aldehydes due to the presence of two electron repelling alkyl groups around the carbonyl carbon. Thus, boiling points of ketones are slightly higher than those of isomeric aldehydes.



3. Solubility: The lower members of aldehydes and ketones (up to four carbon atoms) are soluble in water. It is due to their capability of forming hydrogen bonds with water molecules. The solubility of these compounds in water decreases with the increase in the size of alkyl group. It is because of the increase in the magnitude of non-polar part in the molecule. However, higher homologues are soluble in organic solvents.

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