Chemistry 2025 Paper II 50 marks Compulsory Explain

Q1

(a) (i) Tropolone is aromatic, but fulvene is non-aromatic. Why? (ii) Explain with example pseudo-aromaticity. (b) Identify the missing reagent and intermediates in the following chemical conversion : (c) Write the structure of the major product when neomenthyl chloride is reacted with sodium ethoxide in ethanol. Justify your answer : (d) (i) Discuss in detail how the reaction of a carbene with cis-2-butene can be used to define the spin state (S/T) of carbene. (ii) para-Bromophenol on reaction with NaNH₂/NH₃ (l) followed by acidic workup yields one major product. Explain the reaction by writing the steps involved. (e) With example, elucidate the permanent and temporary denaturation of a protein.

हिंदी में प्रश्न पढ़ें

(a) (i) ट्रोपोलोन ऐरोमैटिक है, लेकिन फल्वीन नॉन-ऐरोमैटिक है। ऐसा क्यों है? (ii) छद-ऐरोमैटिकता को उदाहरण देकर समझाइए। (b) निम्नलिखित रासायनिक रूपांतरण में लुप्त अभिकर्मक तथा मध्यवर्तियों की पहचान कीजिए : (c) निओमेथिल क्लोराइड की सोडियम एथॉक्साइड के साथ एथेनॉल में अभिक्रिया करने पर बनने वाले मुख्य उत्पाद की संरचना लिखिए। अपने उत्तर का औचित्य सिद्ध कीजिए : (d) (i) विस्तार से विवेचना कीजिए कि एक कार्बिन की सिस-2-ब्यूटीन के साथ हुई अभिक्रिया से कार्बिन की प्रचक्रण अवस्था (S/T) कैसे परिभाषित की जा सकती है। (ii) पैरा-ब्रोमोफिनोल की NaNH₂/NH₃ (l) के साथ हुई अभिक्रिया व तत्पश्चात् अम्लीय विवेचन से एक मुख्य उत्पाद प्राप्त होता है। इस अभिक्रिया की सम्मिलित चरणों का वर्णन करते हुए व्याख्या कीजिए। (e) एक प्रोटीन के स्थायी व अस्थायी विकृतीकरण को उदाहरण देते हुए स्पष्ट कीजिए।

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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

The directive 'explain' demands clear reasoning with cause-effect relationships across all sub-parts. Structure your answer as: brief definitions for (a)(i-ii) on aromaticity concepts; stepwise mechanisms with curved arrows for (b) and (d)(ii); stereochemical analysis with chair conformations for (c); orbital diagrams for (d)(i); and biochemical examples for (e). Allocate approximately 25% time to (a), 20% each to (b), (c), and (d), and 15% to (e), ensuring each sub-part receives proportional depth.

Key points expected

  • (a)(i) Tropolone: 10π electrons, planar, follows Hückel's rule (4n+2), dipolar resonance stabilization; Fulvene: 6π electrons but non-planar, lacks cyclic delocalization, dipolar contributor dominates
  • (a)(ii) Pseudo-aromaticity: systems with 4n π-electrons showing temporary aromatic stabilization in transition states or excited states; example: cyclobutadiene rectangular distortion or Dewar benzene
  • (b) Identification of reagents and intermediates for the given conversion (likely involving Birch reduction, ozonolysis, or similar transformation with clear electron-pushing)
  • (c) Neomenthyl chloride: E2 elimination, anti-periplanar requirement, menth-2-ene (more substituted, Zaitsev) vs menth-3-ene; neomenthyl allows better H-C(4) alignment, yielding predominantly 3-menthene
  • (d)(i) Singlet vs triplet carbene: stereospecificity test—singlet gives cis-cyclopropane (concerted), triplet gives mixture (stepwise via diradical); orbital diagrams showing spin correlation
  • (d)(ii) Benzyne mechanism: elimination-addition, formation of benzyne intermediate, regioselectivity of nucleophilic attack, final product m-bromophenol or aniline derivative depending on substitution pattern

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness22%11Precisely distinguishes aromatic (tropolone), non-aromatic (fulvene), and pseudo-aromatic systems with correct electron counting; accurately defines singlet/triplet carbene spin states; correctly identifies neomenthyl chloride stereochemistry; explains benzyne mechanism and protein denaturation types with correct biochemical principlesBasic correct identification of aromaticity in tropolone but vague on fulvene; superficial mention of carbene spin states without orbital explanation; generic E2 discussion without stereochemical detail; incomplete benzyne or protein denaturation explanationConfuses aromaticity criteria (cites wrong electron count), misidentifies spin states, incorrect regioselectivity for neomenthyl elimination, omits benzyne intermediate, or conflates permanent/temporary denaturation
Mechanism / equation24%12Complete curved-arrow mechanisms for (b) showing all intermediates; detailed stepwise benzyne formation and nucleophilic capture in (d)(ii); clear orbital diagrams for carbene spin states; proper anti-periplanar elimination geometry for (c); mechanistic basis for protein denaturationMechanisms present but missing key intermediates or incorrect arrow pushing; benzyne mentioned but steps unclear; generic elimination without conformational analysis; superficial mechanistic detail for denaturationMissing mechanisms for (b) or (d); incorrect arrow pushing; no conformational analysis for (c); fails to show benzyne intermediate; no mechanistic explanation for denaturation
Numerical accuracy10%5Correct π-electron counts (10 for tropolone, 6 for fulvene), proper Hückel number assignments, accurate stereochemical descriptors (R/S, E/Z), correct oxidation states in intermediatesGenerally correct electron counts with minor errors; inconsistent stereochemical notation; some numerical inconsistencies in oxidation statesMajor errors in electron counting (e.g., 8 for tropolone), incorrect application of 4n+2 rule, wrong stereochemical assignments, inconsistent oxidation state tracking
Diagram / structure24%12Clear structural drawings of tropolone resonance forms, fulvene dipolar structure, chair conformations of neomenthyl/menthyl systems with axial/equatorial labels, orbital diagrams for singlet/triplet carbenes, benzyne structure with strained triple bond, and protein structural levelsStructures drawn but lacking stereochemical detail or conformational clarity; missing orbital diagrams; generic protein structures without level-specific featuresNo structures or incorrect drawings; missing chair conformations; no orbital representations; absent or wrong benzyne depiction; no protein structural illustrations
Application context20%10Connects tropolone to natural products (colchicine, stipitatic acid) and medicinal chemistry; relates carbene stereochemistry to synthetic applications (Simmons-Smith); links benzyne to pharmaceutical synthesis; cites Indian research (e.g., protein denaturation studies at IISc); practical relevance of protein stability in biotechnologyBrief mention of natural product relevance for tropolone; generic synthetic mention for carbenes; limited context for benzyne chemistry; basic protein example without research connectionNo real-world applications cited; purely theoretical treatment; missing Indian context; no mention of synthetic utility or biochemical significance

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