Question 3

1. Natural rubber is $cis$-1,4-polyisoprene. Its repeating unit is $-CH_2-C(CH_3)=CH-CH_2-$.
2. Complete ozonolysis ($O_3/Zn-H_2O$) of natural rubber involves the cleavage of the double bonds:

$[-CH_2-C(CH_3)=CH-CH_2-]_n \xrightarrow{O_3/Zn-H_2O} n \cdot O=C(CH_3)-CH_2-CH_2-CH=O$

The major product $X$ is 4-oxopentanal (also known as levulinaldehyde). 3. Characteristics of $X$:

• It gives a positive iodoform test because it contains a methyl keto group ($-COCH_3$).

• It gives a positive Tollen's test because it contains an aldehyde group ($-CHO$).

4. Reaction of $X$ with aqueous $NaOH$ and heating leads to an intramolecular Aldol condensation:

• Deprotonation occurs at the methyl group (C5) to form a stable enolate: $CH_2^--CO-CH_2-CH_2-CHO$.

• This enolate undergoes nucleophilic attack on the aldehyde carbonyl (C1) to form a five-membered ring intermediate.

• Dehydration of the resulting $\beta$-hydroxy ketone gives the $\alpha,\beta$-unsaturated ketone, which is cyclopent-2-en-1-one.

5. Therefore, the major product $Y$ is cyclopent-2-en-1-one, which matches the structure in option (A).