Physical Properties of Aldehydes and ketones Preparation and Properties
→ Physcal State : Most aldehyde (Except formaldehyde which is gas) are liquid at room temperature whereas higher members of ketones are colourless liquid and have a sweet smell but higher members of serie are colourless solid. Aromatic ketones are mostly sweet smelling solids.
→ Boiling Points : The boiling point of aldehyde and ketones are higher than hydrocarbon and ethers of comparable molecular mass. This is due to weak molecular association produced by dipole-dipole interaction in aldehyde and ketones.
→ Aldehyde and ketones have lower boiling point than those of alcohols of similar molecular masses due to absence of intermolecular hydrogen bonding. The boiling point of some compounds whose molecular mass is between 58 and 60, are given below:
Tabl 12.3 Boiling Points of Some Compounds
|Boiling Point (K)
→ In carbonyl compounds the boiling point of ketone is higher as compare to aldehyde this is because of the presence of two alkyl group. It increases dipole moment therefore boiling point also increases.
→ Solubility : The lower members of aldehydes and ketones such as methanal, ethanal and propanone are miscible with water in all proportions, because they form hydrogen bonds with water molecules. However, the solubility decreases with increase in length of alkyl chain, due to hydrophobic nature of alkyl group
→ All aldehyde and ketones are also soluble in organic solvents like benzene, ether, methanol, chloroform etc.
→ Smell : Lower aldehyde have strong unpleasent smell. As the size of the molecules increases, the odour becomes less pungent and more fragrant. ¡n fact many naturally occuring aldehydes and ketones have been used in blending of perfumes and flavouring agents.
Example : Vaneline (obtained from Vanellia bean), salicylaldehyde (obtain from medosweet) and
Cinnamaldehyde (obtain from Daichini) gives pleasent smell.
These are used in various eatable product and medicines for pleasent smell.