Physical Properties of Aldehydes and ketones Preparation and Properties

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.

Physical Properties of Aldehydes and ketones Preparation and Properties

→ 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

Compound Boiling Point (K) Molecular Mass
n-butane 273 58
Methoxy ethane 281 60
Propanal 322 58
Acetone 329 58
Propane-l-ol 370 60

→ 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.

Physical Properties of Aldehydes and ketones Preparation and Properties 1

→ 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

Physical Properties of Aldehydes and ketones Preparation and Properties 2

→ 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.

Physical Properties of Aldehydes and ketones Preparation and Properties

Example : Vaneline (obtained from Vanellia bean), salicylaldehyde (obtain from medosweet) and

Cinnamaldehyde (obtain from Daichini) gives pleasent smell.

Physical Properties of Aldehydes and ketones Preparation and Properties 3

These are used in various eatable product and medicines for pleasent smell.

Chemistry Notes