Chemistry 2023 Paper II 50 marks Explain

Q2

2.(a)(i) Based on Hückel rule, predict the above compounds as aromatic, antiaromatic, and nonaromatic. 5 marks 2.(a)(ii) Identify the above reactions that show primary kinetic isotope effect and secondary kinetic isotope effect. 5 marks 2.(a)(iii) Identify (A) and (B) in the above reactions and explain the mechanism. 5 marks 2.(b)(i) The rate of hydrolysis (k₁ and k₂) of the reaction (A) is much faster than that of (B). Explain. 5 marks 2.(b)(ii) The reaction of the compounds (A) and (B) with AgClO₄ in MeOH gives the same product (C). Explain. 5 marks 2.(b)(iii) Write the major product of the above reaction showing proper stereochemistry and explain the mechanism. 10 marks 2.(c)(i) Write the structure of the product of the above reaction and provide suitable mechanism. 10 marks 2.(c)(ii) Write the preferred position (C-2 or C-3) in electrophilic substitution of indole. Explain your observation with the help of resonance structures. 5 marks

हिंदी में प्रश्न पढ़ें

2.(a)(i) हकल नियम के अनुप्रयोग से निम्न यौगिकों का उनके ऐरोमैटिक, एंटी-ऐरोमैटिक, तथा नॉन-ऐरोमैटिक आधार पर पहचान करें । 5 2.(a)(ii) निम्नलिखित अभिक्रियाओं की प्राथमिक गतिक समस्थानिक प्रभाव व द्वितीयक गतिक समस्थानिक प्रभाव दर्शाने के आधार पर पहचान करें । 5 2.(a)(iii) निम्नलिखित अभिक्रियाओं में उत्पाद (A) तथा (B) को पहचानिए एवं संमिलित क्रियाविधि की व्याख्या कीजिए । 5 2.(b)(i) अभिक्रिया (A) की जल अपघटन दर (k₁ व k₂) अभिक्रिया (B) से बहुत द्रुत है । व्याख्या कीजिए । 5 2.(b)(ii) MeOH में AgClO₄ के साथ यौगिक (A) और (B) की अभिक्रियाएँ समान उत्पाद (C) देती हैं । व्याख्या कीजिए । 5 2.(b)(iii) विभिन्न रसायन दर्शाते हुए निम्नलिखित अभिक्रिया के मुख्य उत्पाद की संरचना लिखिए एवं क्रियाविधि की व्याख्या कीजिए । 10 2.(c)(i) निम्नलिखित अभिक्रिया के उत्पाद की संरचना लिखिए व क्रियाविधि की व्याख्या कीजिए । 10 2.(c)(ii) इंडोल के इलेक्ट्रॉनरागी प्रतिस्थापन अभिक्रिया में अधिमान्य स्थान (C-2 या C-3) की पहचान कीजिए । अनुनादी संरचनाओं की सहायता से अपने प्रेक्षण की व्याख्या कीजिए । 5

Evaluate your answer to this question → See all 2023 Chemistry questions

Directive word: Explain

This question asks you to explain. 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

Explain requires systematic demonstration of reasoning with evidence. Structure: brief introduction stating Hückel's rule and kinetic isotope effect principles; body addressing each sub-part sequentially with clear mechanistic diagrams, rate comparisons, and stereochemical analysis; conclusion summarizing key electronic effects governing aromaticity and reaction outcomes.

Key points expected

  • Correct application of Hückel (4n+2)π rule to classify rings as aromatic, antiaromatic (4nπ, planar), or nonaromatic (non-planar/insufficient conjugation)
  • Distinction between primary KIE (C-H/D bond cleavage in rate-determining step) and secondary KIE (hybridization change at adjacent carbon)
  • Identification of intermediates (A) and (B) with arrow-pushing mechanisms showing carbocation, carbanion, or radical pathways
  • Explanation of rate differences via anchimeric assistance, neighboring group participation, or ring strain effects in hydrolysis
  • Stereochemical analysis using chair conformations, anti-periplanar geometry, and orbital overlap requirements for elimination/substitution
  • Resonance stabilization of indole electrophilic substitution intermediates at C-2 vs C-3 positions

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Precise statements of Hückel criteria (4n+2 π electrons, cyclic, planar, fully conjugated); accurate KIE definitions with energy diagram implications; correct identification of aromaticity exceptions (pyrrole vs pyridine lone pairs)Basic Hückel rule stated but misapplied to heterocycles or confused 4n/4n+2; KIE types named without mechanistic context; minor errors in counting π electronsFundamental misunderstanding of aromaticity criteria; conflation of primary/secondary KIE; incorrect electron counting leading to wrong classifications
Mechanism / equation25%12.5Complete curved-arrow mechanisms for all reactions with proper electron flow, intermediate structures, and rate-determining step identification; clear distinction between SN1/SN2/E1/E2 pathwaysMechanisms shown with some arrow errors or missing intermediates; correct overall transformation but ambiguous rate-determining step; partial stereochemical rationaleMissing mechanisms or incorrect arrow formalism; wrong reaction pathway proposed; no identification of intermediates or transition states
Numerical accuracy10%5Correct π electron counts for all rings; accurate rate ratio interpretations (k₁/k₂ magnitude significance); proper application of kinetic equations where relevantCorrect electron counts for simple rings but errors in fused or heterocyclic systems; qualitative rate discussion without quantitative reasoningIncorrect electron counts throughout; no numerical reasoning for rate comparisons; arithmetic errors in any calculations
Diagram / structure25%12.5Clear 2D/3D structures with proper stereochemistry (wedge-dash, chair conformations); resonance hybrid drawings for indole substitution; energy profile diagrams for KIE; all aromatic rings drawn with correct delocalizationStructures drawn but stereochemistry ambiguous or missing; flat representations where 3D required; resonance forms incomplete or improperly connectedMissing essential structures; incorrect connectivity; no stereochemical indication; illegible or chemically impossible drawings
Application context15%7.5Connection to pharmaceutical relevance (indole in tryptophan, serotonin); industrial catalysis implications; reference to Indian research (CSIR-IICB indole chemistry, IIT Bombay mechanistic studies)Brief mention of biological or industrial relevance without specific examples; generic statements about aromatic compound importanceNo application context provided; purely theoretical treatment without real-world significance

Practice this exact question

Write your answer, then get a detailed evaluation from our AI trained on UPSC's answer-writing standards. Free first evaluation — no signup needed to start.

Evaluate my answer →

More from Chemistry 2023 Paper II