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Question #44483

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The Question

In the Friedel-Crafts alkylation of benzene (used as solvent in large excess) with 1-chloropropane and anhydrous \ce{AlCl3} at 300 K300\text{ K}, the initial Lewis acid-base complex undergoes two competing pseudo-first-order processes:

\textbf{Path A:} Direct electrophilic aromatic substitution to form n-propylbenzene, with an activation energy Ea,directE_{a,\text{direct}}. \textbf{Path B:} A 1,2-hydride shift to form a secondary carbocation complex, which rapidly reacts with benzene to form isopropylbenzene (Product PP). The activation energy for the hydride shift is Ea,shiftE_{a,\text{shift}}.

Given Ea,direct−Ea,shift=11.5 kJ mol−1E_{a,\text{direct}} - E_{a,\text{shift}} = 11.5\text{ kJ mol}^{-1} and assuming identical pre-exponential factors for both pathways, PP is isolated as the major product.

PP is oxidized with \ce{O2} at 400 K400\text{ K} followed by acid hydrolysis to yield an aromatic compound QQ and an aliphatic compound RR. QQ is then treated with \ce{CHCl3} and aqueous \ce{NaOH} at 340 K340\text{ K}, followed by acidification, to yield SS as the major product.

Identify the structure of SS and the approximate initial molar ratio of PP to n-propylbenzene. (Given: R=8.314 J K−1 mol−1R = 8.314\text{ J K}^{-1}\text{ mol}^{-1}, ln⁡(10)=2.30\ln(10) = 2.30)

Options

  • A.

    \text{2-Hydroxybenzaldehyde ; } 100:1

  • B.

    \text{4-Hydroxybenzaldehyde ; } 10:1

  • C.

    \text{2-Hydroxybenzoic acid ; } 100:1

  • D.

    \text{2-Hydroxybenzaldehyde ; } 10:1

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Correct Answer

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Explanation

How the Examiner Thinks

Numerical_Precision

Improve calculation stamina and avoid rounding errors in high-stakes competitive examinations.

Concepts Behind This Question

Friedel-Crafts Alkylation Limitations|Reaction Progress Coordinate|Resonance Energy and Stability

Why This Question Is Tricky

Common Mistakes

The question tests the intersection of physical organic chemistry and industrial synthesis.

The 'hidden variable' is the quantitative kinetic competition between direct electrophilic attack and carbocation rearrangement (1,2-hydride shift) in Friedel-Crafts alkylation.

Students often memorize that 'rearrangement occurs' without understanding the exponential dependence of the product ratio on the activation energy difference (ΔEa\Delta E_a).

By calculating the Arrhenius rate ratio, students prove why the primary product is a minor trace.

Furthermore, the problem chains this into the industrial Cumene process and the Reimer-Tiemann reaction, testing regioselectivity (ortho-formylation via dichlorocarbene) and reagent specificity (\ce{CHCl3} vs \ce{CCl4}).

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Question Information

SubjectChemistry
ChapterHydrocarbons - Class 11 Part 2
ConceptFriedel-Crafts Alkylation Limitations
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree