Answer the following questions. Arrange the following compounds in the increasing order of their boiling points: CH4, CH3CHO, CH3CH2OH, CH3COOH. Write simple chemical tests to distinguish between the following pair of compounds: Propanal and 2-propanone. Draw the structure of the following: p-Methylbenzaldehyde (ii) 4-Methylpent-3-en-2-one. Find out the IUPAC names and structural formula for two esters present in perfumes.
The Correct Answer and Explanation is:
1. Increasing Order of Boiling Points:
The compounds given are:
- CH₄ (Methane)
- CH₃CHO (Acetaldehyde)
- CH₃CH₂OH (Ethanol)
- CH₃COOH (Acetic acid)
Order:
CH₄ < CH₃CHO < CH₃CH₂OH < CH₃COOH
Explanation:
Boiling point depends on molecular mass and intermolecular forces. CH₄ has only weak London dispersion forces. CH₃CHO has dipole-dipole interactions. CH₃CH₂OH shows hydrogen bonding, which is stronger. CH₃COOH forms dimers via hydrogen bonding, raising its boiling point further.
2. Chemical Tests to Distinguish Propanal and 2-Propanone:
Propanal (an aldehyde) and 2-propanone (a ketone) can be distinguished by the following tests:
a) Tollen’s Test:
- Propanal gives a positive result (silver mirror forms).
- 2-propanone gives a negative result (no reaction).
b) Fehling’s Test:
- Propanal reduces the blue Fehling’s solution to red precipitate.
- 2-propanone does not react.
These tests work because aldehydes are easily oxidized, while ketones are not.
3. Structure Drawing:
i) p-Methylbenzaldehyde:
This is benzaldehyde with a methyl group at the para position.
markdownCopyEdit CHO
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C6H4—CH3 (methyl at para to CHO)
ii) 4-Methylpent-3-en-2-one:
Five-carbon chain, double bond between C-3 and C-4, ketone at C-2, methyl at C-4.
Structure:
CH₃–CO–CH=CH–CH₃
(with a CH₃ branch at C-4)
4. IUPAC Names and Structures of Two Esters in Perfumes:
a) Ethyl ethanoate (ethyl acetate):
Structure: CH₃COOCH₂CH₃
IUPAC: Ethyl ethanoate
Smells fruity and is widely used in perfumes.
b) Methyl salicylate:
Structure: C₆H₄(OH)COOCH₃
IUPAC: Methyl 2-hydroxybenzoate
Has a wintergreen scent and is also used in perfumes.
These esters are chosen for their pleasant aromas and volatility.
