Step-by-step Derivation

The problem asks for the structure of an intermediate organic compound Q in a two-step reaction sequence starting from an acyclic hydrocarbon P (C₆H₁₀).

The sequence of reactions is:

$P~(C_6H_{10}) \xrightarrow{(i)~conc.~H_2SO_4~(aq), H_2O} Q \xrightarrow{(ii)~O_3~(iii)~Zn, H_2O} \text{Acetone (only)}$

1. Analyze the second reaction (Ozonolysis of Q): The second step is the reductive ozonolysis of the intermediate Q. Reductive ozonolysis (using O₃ followed by Zn/H₂O) cleaves carbon-carbon double bonds (C=C). Each carbon atom of the double bond is converted into a carbonyl group (C=O).

2. Determine the structure of Q from the ozonolysis product: The problem states that the only organic product of this reaction is acetone. The structure of acetone is: $\underset{\text{Acetone}}{CH_3-CO-CH_3} \quad \text{or} \quad (CH_3)_2C=O$ For the ozonolysis of an alkene to yield a single product, the alkene must be symmetrical with respect to its double bond. To produce only acetone, both carbons of the double bond in Q must be bonded to two methyl groups each. We can deduce the structure of Q by conceptually reversing the ozonolysis reaction with two molecules of acetone: $(CH_3)_2C=O + O=C(CH_3)_2 \longrightarrow (CH_3)_2C=C(CH_3)_2 + [O_2]$ Thus, the structure of the intermediate Q must be 2,3-dimethylbut-2-ene. The reaction is: $(CH_3)_2C=C(CH_3)_2 \xrightarrow{(i)~O_3~(ii)~Zn, H_2O} 2~(CH_3)_2C=O$

3. Compare the deduced structure of Q with the given options: Let's examine the provided options:

   • A: (CH3)2CH-C≡C-CH3 (4-methylpent-2-yne)
   • B: (CH3)2C=C(CH3)2 (2,3-dimethylbut-2-ene)
   • C: (CH3)3C-C≡CH (3,3-dimethylbut-1-yne)
   • D: (CH3)2CH-CH=CH-CH3 (4-methylpent-2-ene)

   Our deduced structure for Q, 2,3-dimethylbut-2-ene, perfectly matches Option B.

4. Verification and analysis of the first step (optional but good practice): The first step describes the conversion of P (C₆H₁₀) to Q (C₆H₁₂) using conc. H₂SO₄ (aq), H₂O. The molecular formula changes from C₆H₁₀ to C₆H₁₂. This corresponds to the addition of H₂ (a reduction), not H₂O (hydration). The given reagents (aqueous sulfuric acid) do not typically perform this reduction. This suggests a likely error in the problem statement for the first step. However, the information from the second step is unambiguous and sufficient to determine the structure of Q.

   If we were to perform ozonolysis on option D (4-methylpent-2-ene), the products would be isobutyraldehyde and acetaldehyde, not just acetone. $(CH_3)_2CH-CH=CH-CH_3 \xrightarrow{O_3, Zn/H_2O} (CH_3)_2CH-CHO + CH_3-CHO$

   Therefore, based on the definitive result of the ozonolysis reaction, Q must be 2,3-dimethylbut-2-ene.

Conclusion

The intermediate Q, upon ozonolysis, gives only acetone. This uniquely identifies Q as 2,3-dimethylbut-2-ene. This structure corresponds to option B.