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Question #44483
The Question
In the Friedel-Crafts alkylation of benzene (used as solvent in large excess) with 1-chloropropane and anhydrous \ce{AlCl3} at , 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 . \textbf{Path B:} A 1,2-hydride shift to form a secondary carbocation complex, which rapidly reacts with benzene to form isopropylbenzene (Product ). The activation energy for the hydride shift is .
Given and assuming identical pre-exponential factors for both pathways, is isolated as the major product.
is oxidized with \ce{O2} at followed by acid hydrolysis to yield an aromatic compound and an aliphatic compound . is then treated with \ce{CHCl3} and aqueous \ce{NaOH} at , followed by acidification, to yield as the major product.
Identify the structure of and the approximate initial molar ratio of to n-propylbenzene. (Given: , )
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
Step-by-Step Solution
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 ().
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}).
Quick Facts About This Question
Question Information
| Subject | Chemistry |
| Chapter | Hydrocarbons - Class 11 Part 2 |
| Concept | Friedel-Crafts Alkylation Limitations |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |