Physical Properties of Aldehydes and Ketones
The physical properties of Aldehydes and Ketones are largely governed by the presence of the polar Carbonyl group ($>C=O$). This group creates significant dipole moments and influences boiling points and solubility.
1. Physical State and Odor
- Methanal (Formaldehyde): Gas at room temperature. Has a pungent, irritating odor.
- Ethanal (Acetaldehyde): Volatile liquid.
- Higher Members: Liquids or solids at room temperature.
- Odor: Lower aldehydes have sharp, pungent odors. As the molecular mass increases, the odor becomes less pungent and more fragrant. Many ketones and higher aldehydes are used in perfumes and flavoring agents (e.g., Cinnamaldehyde, Vanillin, Acetophenone).
2. Boiling Points
The boiling points of aldehydes and ketones follow a specific trend when compared to other organic compounds of similar molecular mass.
A. Comparison with Non-polar Compounds
Aldehydes and ketones have higher boiling points than hydrocarbons (alkanes) and ethers of comparable molecular masses.
Reason: The carbonyl group is polar, leading to weak intermolecular attraction (Dipole-Dipole interactions), which are stronger than the weak Van der Waals forces in alkanes.
B. Comparison with Alcohols
Aldehydes and ketones have lower boiling points than their corresponding alcohols.
Why?
Alcohols can form strong intermolecular Hydrogen Bonds because they have a hydrogen atom directly bonded to oxygen ($O-H$). Carbonyl compounds have oxygen but no hydrogen attached to it, so they cannot form intermolecular hydrogen bonds with themselves.
Example (Mass ~58-60):
Propan-1-ol ($97^\circ C$) > Acetone ($56^\circ C$) > Propanal ($49^\circ C$) > Butane ($-0.5^\circ C$)
3. Solubility in Water
Lower members (like Methanal, Ethanal, and Propanone) are miscible with water in all proportions.
Reason
Although carbonyls cannot H-bond with themselves, they can form Hydrogen bonds with water molecules. The oxygen of the carbonyl group attracts the hydrogen of water.
Effect of Chain Length
As the length of the alkyl chain (hydrophobic part) increases, the solubility decreases rapidly. Higher aldehydes and ketones are practically insoluble in water but soluble in organic solvents like benzene, ether, and chloroform.
4. Dipole Moment
The carbonyl group is highly polar due to the electronegativity difference between Carbon and Oxygen.
This high dipole moment affects their chemical reactivity (nucleophilic addition) and boiling points.
5. Density
Aldehydes and ketones are generally lighter than water ($< 1 \ g/cm^3$). Density increases with molecular mass.
Properties Quiz
Test your knowledge on physical trends. 10 MCQs with explanations.
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