Chemistry 2023 Paper II 50 marks Compulsory Explain

Q1

1.(a)(i) pKₐ value of cyclopentadiene is almost similar to water. Explain. 5 marks 1.(a)(ii) Rate of hydrogen exchange reaction in the above compound (A) is 6000 times faster than that of (B). Explain. 5 marks 1.(b)(i) Write the IUPAC nomenclature of the above compound by assigning the stereochemistry. 5 marks 1.(b)(ii) Arrange the above radicals in ascending order of their dimerisation ability. 5 marks 1.(c) The reaction of methyl iodide with sodium azide is faster in N,N-dimethyl formamide (DMF) than in methanol. Explain. 10 marks 1.(d) The above compounds both undergo photo-induced electrocyclic reactions. What are the structures and stereochemistry of the products? 10 marks 1.(e)(i) Identify the major product of the above reaction. 5 marks 1.(e)(ii) Identify the name reaction which produces nitrogen as a byproduct. (A) Fischer Indole synthesis (B) von Richter reaction (C) Stobbe reaction (D) Bischler-Napieralski reaction 5 marks

हिंदी में प्रश्न पढ़ें

1.(a)(i) साइक्लोपेन्टाडाइन का pKₐ मान लगभग पानी के समान है । व्याख्या कीजिए । 5 1.(a)(ii) निम्नलिखित यौगिक (A) में हाइड्रोजन विनिमय अभिक्रिया की दर यौगिक (B) की तुलना में 6000 गुना द्रुत होती है । व्याख्या कीजिए । 5 1.(b)(i) निम्नलिखित यौगिक की त्रिविम रसायन निर्दिष्ट करते हुए IUPAC नाम लिखिए । 5 1.(b)(ii) निम्नलिखित मूलकों को उनके द्वितीयन क्षमता के आरोही क्रम में व्यवस्थित करें । 5 1.(c) सोडियम एजाइड की मेथिल आयोडाइड के साथ अभिक्रिया मेथनॉल की तुलना में डीएमएफ (DMF) में द्रुत होती है । व्याख्या कीजिए । 10 1.(d) निम्नलिखित दोनों यौगिकों के प्रकाश प्रेरित विद्युतचक्रीय अभिक्रिया से बने उत्पादों की विभिन्न रासायनिक संरचना लिखिए । 10 1.(e)(i) निम्नलिखित अभिक्रिया के प्रमुख उत्पाद की पहचान करें । 5 1.(e)(ii) निम्न अभिक्रियाओं में उस अभिक्रिया की पहचान कीजिए जो नाइट्रोजन को उपोत्पाद के रूप में उत्पन्न करता है । (A) फिशर इंडोल संश्लेषण (B) वॉन रिच्टर अभिक्रिया (C) स्टोब अभिक्रिया (D) बिश्लर-नापीयराल्सकी अभिक्रिया 5

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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

Explain the underlying chemical principles for each sub-part, allocating time proportionally: ~15% on (a)(i)-(ii) aromaticity and kinetic acidity, ~15% on (b)(i)-(ii) stereochemical nomenclature and radical stability, ~20% on (c) solvent effects in SN2 reactions, ~25% on (d) Woodward-Hoffmann rules for electrocyclic reactions, and ~25% on (e)(i)-(ii) reaction identification and named reactions. Begin with clear structural representations, develop mechanistic reasoning with orbital diagrams where relevant, and conclude with comparative summaries.

Key points expected

  • For 1(a)(i): Explanation of cyclopentadiene's enhanced acidity (pKa ~16) due to aromatic stabilization of cyclopentadienyl anion (6π-electron Hückel system) making it comparable to water (pKa ~15.7)
  • For 1(a)(ii): Kinetic vs thermodynamic acidity distinction; compound A (cyclopentadiene) undergoes rapid H/D exchange via aromatic transition state, while compound B (e.g., acetone or similar non-aromatic enolizable compound) lacks this stabilization
  • For 1(b)(i): Correct IUPAC name with E/Z or R/S stereochemical descriptors based on Cahn-Ingold-Prelog priority rules; proper numbering and identification of principal functional group
  • For 1(b)(ii): Radical dimerization ability correlates inversely with stability; order reflects degree of conjugation, steric hindrance, and resonance stabilization (tertiary < secondary < primary < methyl, or specific order based on given structures)
  • For 1(c): Explanation of SN2 rate enhancement in polar aprotic solvents (DMF) vs polar protic (methanol); hard-soft acid-base considerations, nucleophile solvation effects, and transition state stabilization
  • For 1(d): Application of Woodward-Hoffmann rules for 4n and 4n+2 π-electron systems; conrotatory vs disrotatory ring closure under photochemical conditions with stereochemical outcome prediction
  • For 1(e)(i): Identification of major product based on named reaction mechanism (likely Fischer indole synthesis or related transformation)
  • For 1(e)(ii): Recognition that Fischer indole synthesis produces N2 as byproduct via hydrazone intermediate and [3,3]-sigmatropic rearrangement with subsequent elimination

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%10Demonstrates flawless understanding across all sub-parts: correctly identifies aromaticity in cyclopentadienyl anion, distinguishes kinetic/thermodynamic control, applies Cahn-Ingold-Prelog rules accurately, explains hard-soft acid-base theory for solvent effects, and correctly applies Woodward-Hoffmann rules for photochemical reactionsShows basic understanding of most concepts but with minor errors in aromaticity criteria, stereochemical priority assignment, or confusion between thermal and photochemical electrocyclic selection rulesFundamental misconceptions: confuses acidity with basicity, misapplies aromaticity rules, incorrect stereochemical descriptors, or completely wrong solvent effect explanation
Mechanism / equation20%10Provides complete, arrow-pushing mechanisms for (a)(ii) H/D exchange via aromatic intermediate, (c) SN2 transition state with proper orbital representation, (d) electrocyclic ring closure with orbital symmetry analysis, and (e) Fischer indole mechanism with [3,3]-sigmatropic rearrangementShows partial mechanisms with correct intermediates but missing arrow details, or correct final products without clear mechanistic pathway; some confusion in orbital overlap descriptionsMissing or incorrect mechanisms; arrows showing impossible electron movements, wrong intermediates, or no mechanistic insight provided for any sub-part
Numerical accuracy15%7.5Correctly cites pKa values (cyclopentadiene ~16, water ~15.7), accurately interprets the 6000-fold rate ratio in terms of activation energy difference (ΔΔG‡ ≈ RT ln 6000 ≈ 5.2 kcal/mol at 298K), and correctly applies quantum mechanical calculations for orbital coefficientsApproximate pKa values given without precision, qualitative mention of rate ratio without quantitative interpretation, or minor calculation errors in energy differencesNo numerical data provided, grossly incorrect pKa values, or complete absence of quantitative reasoning where explicitly required
Diagram / structure25%12.5Clear, well-drawn structures for all compounds: cyclopentadiene with aromatic anion resonance forms, proper wedge-dash stereochemistry in (b)(i), orbital diagrams for HOMO in electrocyclic reactions showing conrotatory/disrotatory modes, and complete Fischer indole mechanism with all intermediatesAdequate structures but poor stereochemical representation, missing resonance forms, or incomplete orbital diagrams; hand-drawn quality acceptable but clarity compromisedMissing essential structures, incorrect connectivity, no stereochemical indicators, or diagrams that contradict written explanations; structures too small or poorly labeled
Application context20%10Connects concepts to practical applications: cyclopentadienyl anion as ligand in ferrocene (Indian connection: CSIR-NCL Pune work on metallocenes), DMF as solvent in pharmaceutical synthesis (Indian API industry), electrocyclic reactions in vitamin D biosynthesis, and Fischer indole in drug discovery (indomethacin synthesis)Brief mention of applications without specific examples, or generic statements about industrial importance without Indian or contemporary relevanceNo application context provided; purely theoretical treatment without recognition of real-world significance or contemporary chemical research

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