Chemistry 2021 Paper II 50 marks Explain

Q3

(a) Complete the following reactions with mechanisms: (i) Conc. H₂SO₄ (10 marks) (ii) EtO⁻ (10 marks) (b) (i) Write the structure of the reaction product between 3-chlorocyclopropene and SbCl₅. What is the unique feature of this product in ¹H NMR spectrum? (ii) Which one of the above compounds is more acidic and why? (Structures A and B shown) (iii) Predict the aromaticity of tropolones and sydnones. (c) Complete the following reactions along with mechanisms: (i) meso-2,3-dibromobutane →(I⁻) A →(OEt⁻) B (ii) CH₃—C—CHI—CH₃ →(AgNO₃, EtOH, Δ) ? | CH₃

हिंदी में प्रश्न पढ़ें

(a) निम्नलिखित अभिक्रियाओं को उनकी क्रियाविधि दर्शाते हुए पूर्ण करें : (i) Conc. H₂SO₄ (10) (ii) EtO⁻ (10) (b) (i) 3-क्लोरोसाइक्लोप्रोपीन और SbCl₅ की परस्पर अभिक्रिया के उत्पाद की संरचना लिखें। इस उत्पाद की ¹H NMR स्पेक्ट्रम में अनूठी विशेषता क्या है? (ii) निम्नलिखित यौगिकों में कौन सा ज्यादा अम्लीय है और क्यों? (A और B की संरचनाएं दिखाई गई हैं) (iii) ट्रोपोलोन्स और सिड्नोन्स की ऐरोमैटिकता का अनुमान लगाएं। (c) निम्नलिखित अभिक्रियाओं को उनकी क्रियाविधि के साथ पूर्ण करें : (i) मेसो-2,3-डाइब्रोमोब्यूटेन →(I⁻) A →(OEt⁻) B (ii) CH₃—C—CHI—CH₃ →(AgNO₃, EtOH, Δ) ? | CH₃

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Directive word: Explain

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How this answer will be evaluated

Approach

Explain each reaction mechanism with clear arrow-pushing and stereochemical outcomes. Allocate ~35% time to part (a) mechanisms, ~35% to part (c) stereochemistry-heavy transformations, and ~30% to part (b) covering aromaticity, NMR features, and acidity comparisons. Begin with brief identification of reaction types, proceed with stepwise mechanisms using curved arrows, and conclude with stereochemical assignments where applicable.

Key points expected

  • (a)(i) E1 or E2 dehydration mechanism with carbocation intermediate and Zaitsev product formation
  • (a)(ii) E2 elimination with anti-periplanar geometry requirement, stereospecific product
  • (b)(i) Formation of cyclopropenyl cation (aromatic 2π-electron system) with SbCl₅; NMR shows single peak due to ring current and equivalent protons
  • (b)(ii) Comparative acidity based on aromaticity of conjugate base (cyclopropenyl anion vs cation stability)
  • (b)(iii) Tropolone (10π-electron aromatic) and sydnone (6π-electron aromatic including N-oxide contribution)
  • (c)(i) Double inversion via SN2 with I⁻ (meso → enantiomer), then E2 elimination with OEt⁻ giving trans-alkene
  • (c)(ii) Ag⁺-promoted SN1/SN2 with rearrangement; neopentyl-type substrate favors elimination or rearranged substitution

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%2Correctly identifies all reaction types (E1/E2/SN2), recognizes aromaticity criteria (Hückel's rule) for tropolone and sydnone, and accurately explains why cyclopropenyl cation is aromatic (2π electrons, planar)Identifies most reaction types correctly but confuses E1/E2 conditions or misapplies aromaticity rules (e.g., counts wrong number of π electrons)Fundamental errors like calling cyclopropenyl cation antiaromatic, confusing SN1/SN2 for neopentyl systems, or violating Hückel's rule
Mechanism / equation25%2.5Complete curved-arrow mechanisms for all transformations: carbocation formation/rearrangement in (a)(i), anti-periplanar E2 in (a)(ii), double inversion in (c)(i), and Ag⁺-assisted ionization with possible Wagner-Meerwein shift in (c)(ii)Shows mechanisms with some arrow-pushing but misses key steps like carbocation rearrangement or stereoelectronic requirementsIncomplete mechanisms, missing intermediates, or incorrect arrow directions showing fundamental misunderstanding
Numerical accuracy10%1Correct π-electron counts: 2 for cyclopropenyl cation, 10 for tropolone, 6 for sydnone; correct number of signals and coupling constants in NMR analysisCorrect basic counts but inconsistent application or minor errors in electron counting for heteroatomsWrong electron counts leading to incorrect aromaticity conclusions; no understanding of how N-oxide contributes to sydnone aromaticity
Diagram / structure25%2.5Clear structural drawings with stereochemistry: anti-periplanar geometry in E2, meso vs racemic relationships, chair-like transition states where relevant, and proper representation of aromatic resonance formsStructures drawn but stereochemistry ambiguous or missing; inadequate representation of three-dimensional requirementsPoorly drawn structures, missing stereochemical wedges/dashes, or incorrect connectivity in products
Application context20%2Relates to Indian chemical research context (e.g., sydnone chemistry developed significantly at IISc Bangalore; tropolone natural products from Indian medicinal plants like neem/cinchona); explains why strong Lewis acid SbCl₅ generates stable carbocation for studyMentions general synthetic utility without specific Indian context; basic understanding of why certain reagents are chosenNo contextual awareness; fails to explain why AgNO₃ is used (halophilic precipitation) or why SbCl₅ is superior to AlCl₃ for cyclopropenyl cation generation

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