Q4
(a) (i) Consider the following reaction : Name the product X and outline the mechanism indicating the rate-determining step. (10 marks) (ii) Indicate the major products of the following reactions and point out the mechanism as S_N1, S_N2, E1 or E2 : (1) (CH_3)_3CBr + C_2H_5OH Heat/60°C (2) CH_3CH = CHCl + NaNH_2 (5 marks) (b) (i) Consider the following reaction : How would you confirm that the above reaction is intramolecular by crossover experiment? (10 marks) (ii) Draw the energy profile diagram for the conversion of benzene to chlorobenzene giving structures of transition states. (5 marks) (c) (i) Write the structure of the major product(s) formed in the following reactions : (A) (B) (C) (10 marks) (ii) Write the structure of the product in the following reaction and describe the steps involved : (10 marks)
हिंदी में प्रश्न पढ़ें
(a) (i) निम्नलिखित अभिक्रिया पर विचार कीजिए : उत्पाद X का नाम लिखिए और दर-निर्धारक चरण दर्शाते हुए क्रियाविधि की रूपरेखा लिखिए। (10 अंक) (ii) निम्नलिखित अभिक्रियाओं में मुख्य उत्पादों को दर्शाइए व क्रियाविधि को S_N1, S_N2, E1 या E2 के रूप में इंगित कीजिए : (1) (CH_3)_3CBr + C_2H_5OH ताप/60°C (2) CH_3CH = CHCl + NaNH_2 (5 अंक) (b) (i) निम्नलिखित अभिक्रिया पर विचार कीजिए : पारगमन प्रयोग द्वारा आप कैसे पुष्टि करेंगे कि उपर्युक्त अभिक्रिया अंतर-आणविक है? (10 अंक) (ii) बेंजीन से क्लोरोबेंजीन के रूपांतरण में संक्रमण अवस्था की संरचना लिखते हुए ऊर्जा प्रोफाइल आरेख बनाइए। (5 अंक) (c) (i) निम्नलिखित अभिक्रियाओं में बनने वाले मुख्य उत्पाद/उत्पादों की संरचना लिखिए : (A) (B) (C) (10 अंक) (ii) निम्नलिखित अभिक्रिया में उत्पाद की संरचना लिखिए और इसमें सम्मिलित चरणों का वर्णन कीजिए : (10 अंक)
Directive word: Outline
This question asks you to outline. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.
See our UPSC directive words guide for a full breakdown of how to respond to each command word.
How this answer will be evaluated
Approach
The directive 'outline' demands a systematic presentation of mechanisms with clear stepwise progression. Structure your answer by addressing each sub-part sequentially: spend ~40% time on (a)(i) mechanism with rate-determining step identification, ~20% on (a)(ii) SN1/SN2/E1/E2 classification, ~25% on (b)(i) crossover experiment design, and ~15% on (b)(ii) energy profile with transition states. Use clear arrow-pushing diagrams throughout and explicitly label rate-determining steps.
Key points expected
- For (a)(i): Correct identification of product X with complete mechanism showing electron flow, intermediates, and explicit labeling of rate-determining step with reasoning
- For (a)(ii): Major product identification for t-butyl bromide-ethanol reaction (SN1/E1 competition favoring substitution at 60°C) and chloropropene-NaNH2 reaction (elimination-addition via benzyne for aryl halide)
- For (b)(i): Design of crossover experiment using isotopically labeled substrates (e.g., deuterated or 13C-labeled) to distinguish intramolecular vs intermolecular pathways by analyzing product distribution
- For (b)(ii): Energy profile diagram showing benzene → σ-complex (Wheland intermediate) → chlorobenzene with proper transition state structures (sp3 hybridized carbon in TS1, TS2) and relative energy levels
- For (c)(i): Structure determination for three reactions considering stereochemistry, regioselectivity, and rearrangements where applicable
- For (c)(ii): Multi-step mechanism with curved arrows showing electron movement, intermediate formation, and final product structure with stereochemical outcome
Evaluation rubric
| Dimension | Weight | Max marks | Excellent | Average | Poor |
|---|---|---|---|---|---|
| Concept correctness | 25% | 12.5 | Demonstrates mastery of carbocation stability, benzyne mechanism, and aromatic electrophilic substitution; correctly predicts SN1/E1 competition outcome for (a)(ii) and explains why aryl halides undergo elimination-addition; for (b)(i) understands isotopic labeling strategy | Identifies most mechanisms correctly but confuses SN1/SN2 conditions or misapplies benzyne pathway; crossover experiment explanation lacks clarity on isotope choice; minor errors in predicting major products | Fundamental confusion between substitution and elimination mechanisms; fails to recognize benzyne intermediate; crossover experiment described incorrectly or confused with other techniques; major conceptual errors in aromatic substitution |
| Mechanism / equation | 25% | 12.5 | All mechanisms show proper curved-arrow notation, clearly identified intermediates, and explicit rate-determining step labeling; for (a)(i) includes complete kinetic reasoning; for (c)(ii) presents logical stepwise progression with correct reagents | Mechanisms generally correct but arrow-pushing inconsistent or incomplete; rate-determining step identified without justification; some intermediates missing or incorrectly drawn; steps in (c)(ii) present but logical flow unclear | Mechanisms fundamentally flawed with incorrect arrow directions; missing or wrong intermediates; no identification of rate-determining step; (c)(ii) shows disconnected steps without mechanistic logic |
| Numerical accuracy | 10% | 5 | Correctly applies kinetic isotope effect reasoning in crossover experiment; proper energy values relative ordering in profile diagram; quantitative understanding of activation energy differences between pathways | Qualitative understanding of energy relationships but no numerical values; recognizes relative stability trends without quantitative expression | No numerical or quantitative reasoning; energy profile shows incorrect relative heights; completely ignores kinetic aspects of rate-determining steps |
| Diagram / structure | 25% | 12.5 | Clear, labeled structural drawings for all products with correct stereochemistry; energy profile diagram properly scaled with labeled axes, transition states showing partial bonds, and Wheland intermediate structure; crossover experiment schematic understandable | Structures generally correct but stereochemistry ambiguous or missing; energy profile present but poorly labeled or missing one transition state; diagrams understandable but lack professional presentation | Structures incorrectly drawn (wrong connectivity, missing hydrogens); no energy profile or completely wrong diagram; illegible or confusing presentation; missing essential structural features |
| Application context | 15% | 7.5 | Relates mechanisms to synthetic utility (e.g., benzyne route for inaccessible substitution patterns); discusses temperature effects on SN1/E1 competition; connects crossover methodology to broader mechanistic proof strategies in organic chemistry | Brief mention of practical relevance without elaboration; recognizes conditions affect outcome but limited discussion of why; some connection to real-world applications | No discussion of context or applications; purely theoretical treatment; fails to explain why specific conditions (heat, strong base) are chosen |
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