Chemistry 2021 Paper II 50 marks Compulsory Explain

Q1

(a) In the reaction R—COO⁻ + Br—CN → R—CN + Br⁻ + CO₂↑ What is the origin of —CN group in the product ? Explain by using isotopic labelling technique. 10 marks (b) Discuss the product(s) formation when above quaternary ammonium salt is treated with sodium amide at low temperature and at high temperature. 10 marks (c) Write the products A and B in the above reaction. Also give the mechanism of their formation. Which one of these is the major product and why ? [Diagram: Cyclohexane ring with C(CH₃)₂OH substituent] BF₃ : OEt₂ → A + B 10 marks (d) Discuss the reactivity of following compounds towards nucleophile in the presence of BF₃ : OEt₂ : (i) p-trifluoromethyl benzaldehyde (ii) p-tolualdehyde 10 marks (e) Complete the above transformations. (i) (ii) 10 marks

हिंदी में प्रश्न पढ़ें

(a) इस अभिक्रिया में R—COO⁻ + Br—CN → R—CN + Br⁻ + CO₂↑ सायनाइड (—CN) समूह उत्पाद में कहाँ से उत्पन्न होता है ? समस्थानिक लेबलिंग प्रविधि के प्रयोग द्वारा स्पष्ट करें । 10 अंक (b) निम्नलिखित चतुष्क अमोनियम लवण को जब सोडियम एमाइड के साथ निम्न तापमान और उच्च तापमान पर अभिक्रियत करते हैं तो उत्पादों के बनने का विवरण दें । (CH₃)₃N⁺—CH₂—C₆H₅ X⁻ 10 अंक (c) निम्नलिखित अभिक्रिया के उत्पाद A और B लिखें । उनके बनने की क्रियाविधि भी दें । इनमें से कौन सा मुख्य उत्पाद है और क्यों ? [आरेख: साइक्लोहेक्सेन वलय C(CH₃)₂OH प्रतिस्थापक के साथ] BF₃ : OEt₂ → A + B 10 अंक (d) निम्नलिखित यौगिकों की नाभिकस्नेही के प्रति BF₃ : OEt₂ की उपस्थिति में अभिक्रियता की विवेचना कीजिये : (i) पैरा(p)-ट्राइफ्लुओरोमेथिल बेंज़ैल्डिहाइड (ii) पैरा(p)-टॉलुएल्डिहाइड 10 अंक (e) निम्नलिखित रूपान्तरणों को पूर्ण करें : (i) (ii) 10 अंक

Evaluate your answer to this question → See all 2021 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

This question requires you to explain, discuss, and solve across five organic chemistry sub-parts. Begin with (a) explaining the isotopic labelling experiment to trace cyanide origin; for (b) discuss Hofmann elimination at different temperatures; for (c) draw clear chair conformations showing carbocation rearrangement; for (d) analyse electronic effects on carbonyl reactivity; and for (e) complete the transformation sequences. Allocate approximately 20% time to each part, ensuring mechanisms are drawn with curved arrows and stereochemistry is explicitly shown.

Key points expected

  • (a) Design and interpretation of isotopic labelling experiment using ¹³C or ¹⁴C in either R—COO⁻ or Br—CN to prove cyanide originates from cyanogen bromide, not decarboxylation
  • (b) Low temperature: Sommelet-Hauser rearrangement (benzylic rearrangement via ylide); High temperature: Hofmann elimination (E2 with least substituted alkene); stereochemistry of elimination
  • (c) BF₃·OEt₂ promotes dehydration of tertiary alcohol; formation of 1,2-hydride/alkyl shift in cyclohexyl carbocation; chair conformations showing axial/equatorial preferences; Saytzeff vs Hofmann product distribution
  • (d) BF₃ Lewis acid activation of aldehyde carbonyl; -CF₃ strong -I effect deactivates toward nucleophilic attack vs -CH₃ weak +I effect; relative rates and resonance structures
  • (e) Completion of two transformation sequences with correct reagents, intermediates, and stereochemical outcomes

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%10Demonstrates precise understanding that (a) cyanide comes from BrCN not CO₂ loss; (b) distinguishes Sommelet-Hauser vs Hofmann conditions correctly; (c) identifies carbocation rearrangement driving force; (d) correctly assigns -I vs +I effects on carbonyl electrophilicity; (e) applies correct synthetic logicIdentifies most concepts correctly but confuses (a) cyanide origin or (b) temperature conditions; minor errors in electronic effect assignmentsFundamental misconceptions: claims cyanide from decarboxylation, reverses temperature effects, or misassigns electronic effects of substituents
Mechanism / equation25%12.5All mechanisms with correct curved arrows: (a) nucleophilic attack on BrCN; (b) ylide formation and [2,3]-sigmatropic rearrangement vs E2 elimination; (c) stepwise dehydration with 1,2-shift; (d) BF₃ coordination and nucleophilic addition; clear transition states where relevantMechanisms mostly correct but missing key arrows (e.g., ylide rearrangement arrows) or incomplete steps; minor errors in intermediate structuresMissing mechanisms, incorrect arrow pushing, wrong intermediates, or failure to show Lewis acid coordination in (c) and (d)
Numerical accuracy10%5Correct isotope positions in (a); accurate product ratios based on statistical factors in (b); correct stereochemical ratios in (c) based on conformational analysisMinor numerical errors in isotope counting or product distribution estimatesNo numerical data provided where required or significant calculation errors
Diagram / structure25%12.5Clear chair conformations for cyclohexane in (c) showing axial leaving group; proper 3D representation of ylide in (b); resonance structures for (d); isotopic labelling clearly marked; clean skeletal structures throughoutDiagrams present but poor stereochemical representation; flat structures instead of chairs; missing isotope labelsAbsent or illegible diagrams; wrong stereochemistry; no attempt to show conformational preferences or transition state geometries
Application context20%10Connects (a) to general isotopic tracer methods in organic synthesis; (b) to pharmaceutical applications of selective alkene synthesis; (c) to terpene biosynthesis rearrangements; (d) to design of activated carbonyl compounds in drug synthesis; Indian examples like use of Hofmann elimination in alkaloid synthesisBrief mention of applications without specific examples or connections to Indian chemical industry/researchNo application context provided; purely theoretical treatment without real-world relevance

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 2021 Paper II