Chemistry 2024 Paper II 50 marks Explain

Q2

(a) (i) Consider the following reactions : EtO⁻ with substrate (Rate = k_H) and EtO⁻ with deuterated substrate (Rate = k_D). It was observed that k_H/k_D = 7.1. Based on this data, predict the mechanism and justify your answer. (10 marks) (ii) Consider the following reaction : CH₂=CH—CH=CH₂ →[HBr] CH₃—CH(Br)—CH=CH₂ + CH₃—CH=CH—CH₂Br. At –80 °C, 1,2-addition product predominates while at –45 °C, 1,4-addition product prefers. Justify. (5 marks) (b) (i) Identify the major product X in the following reaction : [Diagram: Benzaldehyde + Diethyl malonate →[Pyridine] X]. Explain its mechanism. Name the reaction. (10 marks) (ii) Predict the structure of X in the following reaction : [Diagram: Tertiary alcohol →[CS₂, NaOH, CH₃I] X → Alkene]. Name the above reaction. Justify that it is a syn-elimination. (5 marks) (c) (i) Write the product of the following reactions : (A) [diagram] (B) [diagram] (C) [diagram] (15 marks) (ii) How is the following compound prepared using a Reformatsky reaction? [diagram] (5 marks)

हिंदी में प्रश्न पढ़ें

(क) (i) निम्नलिखित अभिक्रियाओं पर विचार कीजिए : EtO⁻ के साथ अभिकर्मक (दर = k_H) और EtO⁻ के साथ ड्यूटेरेटेड अभिकर्मक (दर = k_D)। k_H/k_D = 7.1 प्रेक्षित किया गया। इस आँकड़े के आधार पर क्रियाविधि की परिकल्पना कीजिए तथा अपने उत्तर का औचित्य सिद्ध कीजिए। (10 अंक) (ii) निम्नलिखित अभिक्रिया पर विचार कीजिए : CH₂=CH—CH=CH₂ →[HBr] CH₃—CH(Br)—CH=CH₂ + CH₃—CH=CH—CH₂Br। –80 °C पर 1,2-योगज उत्पाद प्रबलता से बनता है, जबकि –45 °C पर 1,4-योगज उत्पाद प्राथमिकता से बनता है। औचित्य सिद्ध कीजिए। (5 अंक) (ख) (i) निम्नलिखित अभिक्रिया में मुख्य उत्पाद X की पहचान कीजिए : [आरेख: बेंज़ैल्डिहाइड + डाइएथिल मैलोनेट →[पिरीडीन] X]। इसकी क्रियाविधि की व्याख्या कीजिए। अभिक्रिया का नाम लिखिए। (10 अंक) (ii) निम्नलिखित अभिक्रिया में X की संरचना का अनुमान लगाइए : [आरेख: तृतीयक एल्कोहॉल →[CS₂, NaOH, CH₃I] X → एल्कीन]। उपर्युक्त अभिक्रिया का नाम लिखिए। औचित्य सिद्ध कीजिए कि यह सम-निराकरण है। (5 अंक) (ग) (i) निम्नलिखित अभिक्रियाओं के उत्पाद लिखिए : (A) [आरेख] (B) [आरेख] (C) [आरेख] (15 अंक) (ii) रिफॉर्मेट्स्की अभिक्रिया द्वारा निम्नलिखित यौगिक कैसे निर्मित किया जाता है? [आरेख] (5 अंक)

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Directive word: Explain

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How this answer will be evaluated

Approach

Explain the mechanistic rationale for each transformation across all sub-parts, allocating approximately 30% time to (a)(i) kinetic isotope effect analysis, 15% to (a)(ii) thermodynamic vs kinetic control, 25% to (b)(i) Knoevenagel condensation mechanism, 15% to (b)(ii) Chugaev elimination stereochemistry, and 15% to (c) Reformatsky application. Structure the answer with clear mechanistic arrows, energy diagrams where relevant, and explicit justification for regio- and stereoselectivity.

Key points expected

  • (a)(i) Primary kinetic isotope effect (k_H/k_D = 7.1 >> 1) indicates C-H bond cleavage in rate-determining step; identifies E2 elimination mechanism with transition state showing substantial C-H/C-D bond breaking character
  • (a)(ii) 1,2-addition is kinetic product (irreversible, lower activation energy at -80°C); 1,4-addition is thermodynamic product (more substituted, stable allylic bromide at -45°C); invokes Hammond postulate and reversible conditions
  • (b)(i) Knoevenagel condensation: benzaldehyde + diethyl malonate → diethyl benzylidenemalonate (X); mechanism involves enamine-type catalysis by pyridine, aldol-type condensation followed by dehydration
  • (b)(ii) Chugaev elimination: tertiary alcohol → xanthate → alkene; syn-elimination justified by cyclic six-membered transition state with methyl group axial/equatorial considerations; X is xanthate ester
  • (c)(i) Three product structures from unspecified reactions (typically pericyclic/organometallic transformations common in UPSC syllabus)
  • (c)(ii) Reformatsky reaction: α-haloester + Zn + carbonyl → β-hydroxyester; application to specific target requires identifying appropriate carbonyl partner and subsequent transformations

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%10Correctly identifies E2 mechanism for (a)(i), distinguishes kinetic/thermodynamic control for (a)(ii), names Knoevenagel and Chugaev reactions accurately, and applies Reformatsky conditions properly; no conceptual confusion between E1/E2/SN2 or syn/anti eliminationIdentifies most mechanisms correctly but confuses kinetic vs thermodynamic control explanation or misstates the magnitude significance of KIE; minor errors in naming reactionsFundamental errors such as claiming k_H/k_D > 1 indicates SN2, confusing 1,2- and 1,4-addition stability order, or misidentifying Chugaev as anti-elimination
Mechanism / equation25%12.5Complete curved-arrow mechanisms for Knoevenagel condensation (enamine formation, nucleophilic attack, dehydration) and Chugaev elimination (xanthate formation, cyclic TS); explicit transition state drawings with partial bonds; balanced equations with all reagentsMechanisms shown but missing key intermediates (e.g., omitting xanthate formation step) or incorrect arrow pushing; equations present but stoichiometry or reagents incompleteNo mechanisms attempted or grossly incorrect arrow pushing; missing critical steps like dehydration in Knoevenagel or ignoring the role of CS₂/CH₃I in xanthate formation
Numerical accuracy10%5Correctly interprets k_H/k_D = 7.1 as large primary KIE; calculates or estimates expected ranges (2-7 typical for E2); quantitative reasoning for temperature effects on product ratio using Arrhenius/ΔΔG‡ considerationsStates KIE is large but no quantitative interpretation; mentions temperature affects rates without quantitative relationshipIgnores numerical value entirely or misinterprets 7.1 as secondary KIE; no quantitative discussion of temperature dependence
Diagram / structure25%12.5Clear structural drawings of diethyl benzylidenemalonate (X in b-i), xanthate ester (X in b-ii), all three products in (c)(i), and Reformatsky product; correct stereochemistry shown where relevant; energy diagram for kinetic vs thermodynamic controlStructures mostly correct but stereochemistry ambiguous or missing; products identifiable but bond connectivity errors; no energy diagramIncorrect structures (e.g., wrong connectivity in malonate derivative, missing double bond in alkene product); no attempt at structural drawings
Application context20%10Relates Knoevenagel to pharmaceutical synthesis (e.g., cinammon acid derivatives), Chugaev to selective alkene synthesis without rearrangement, Reformatsky to prostaglandin/β-lactam synthesis; discusses synthetic utility of kinetic vs thermodynamic control in industrial contextsMentions synthetic applications generically without specific examples; limited discussion of why these methods are preferred over alternativesNo application context provided; treats reactions as purely academic exercises without mention of synthetic utility or comparison with alternative methods

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