Chemistry 2025 Paper II 50 marks Elucidate

Q6

(a) (i) Elucidate the structure of the product and the intermediate (if any) in the following reactions: (A) [structure: H(Me)C(CO₂Et)(CH₂CO₂H)] 1) BH₃ 2) H⁺ → ? (B) [structure: cyclohexyl-CH=CH-CHO] NaBH₄ → ? (5+5=10 marks) (ii) Describe the role of NMO during the dihydroxylation of an alkene using catalytic amount of OsO₄ in the presence of N-methylmorpholine N-oxide (NMO). (5 marks) (b) (i) Calculate the frequency of radiation required for a transition of J = 4 to J = 5 in the rotational spectrum of HCl. The rotation constant B = 10·6 cm⁻¹. (5 marks) (ii) The fundamental vibrational frequency of HCl is 2990 cm⁻¹. Calculate the fundamental vibrational frequency of DCl assuming the same bond strength. (5 marks) (iii) The molecular formula of a compound is C₃H₃N. The IR absorption frequencies are 1650 cm⁻¹, 2250 cm⁻¹ and 3100 cm⁻¹. Assign a structure for the compound. (5 marks) (c) (i) Write the mechanism for the following photochemical transformation: [structure not shown in transcript] (5 marks) (ii) Photobromination of cinnamic acid was carried out by using light of wavelength 480 nm with a light intensity of 1·5×10⁻³ J-s⁻¹. An exposure of 10 minutes showed a decrease of 0·05 millimole of Br₂. Calculate the quantum yield assuming that 80% of radiation is absorbed by cinnamic acid. (Planck's constant h = 6·627×10⁻³⁴ J-s and velocity of light c = 3×10⁸ m-s⁻¹) (5 marks) (iii) Predict the major and minor products for the following photoreaction. Give the logic: [structure not shown in transcript] (5 marks) (iv) Assign A, B and C in the following reaction: (Major) [reaction not fully shown in transcript] (5 marks)

हिंदी में प्रश्न पढ़ें

(a) (i) निम्नलिखित अभिक्रियाओं में बनने वाले उत्पाद और मध्यवर्ती (यदि हो) की संरचना को स्पष्ट कीजिए: (A) [संरचना: H(Me)C(CO₂Et)(CH₂CO₂H)] 1) BH₃ 2) H⁺ → ? (B) [संरचना: साइक्लोहेक्सिल-CH=CH-CHO] NaBH₄ → ? (5+5=10 अंक) (ii) N-मेथिलमॉर्फोलिन N-ऑक्साइड (NMO) की उपस्थिति में OsO₄ की उत्प्रेरक मात्रा का उपयोग करके एक ऐल्कीन के डाइहाइड्रॉक्सिलेशन अभिक्रिया में NMO की भूमिका का वर्णन कीजिए। (5 अंक) (b) (i) HCl के घूर्णनात्मक स्पेक्ट्रम में J = 4 से J = 5 के संक्रमण के लिए आवश्यक विकिरण की आवृत्ति की गणना कीजिए। घूर्णक स्थिरांक B = 10·6 cm⁻¹। (5 अंक) (ii) HCl की मूल कंपन आवृत्ति 2990 cm⁻¹ है। समान आबंध सामर्थ्य मानते हुए DCl की मूल कंपन आवृत्ति की गणना कीजिए। (5 अंक) (iii) एक यौगिक का अणुसूत्र C₃H₃N है। इसकी IR अवशोषण आवृत्तियाँ 1650 cm⁻¹, 2250 cm⁻¹ और 3100 cm⁻¹ हैं। यौगिक के लिए एक संरचना निर्दिष्ट कीजिए। (5 अंक) (c) (i) निम्नलिखित प्रकाश-रासायनिक रूपांतरण के लिए क्रियाविधि लिखिए: [संरचना प्रतिलिपि में नहीं दिखाई गई] (5 अंक) (ii) सिनेमिक अम्ल का फोटोब्रोमिनेशन 480 nm तरंगदैर्घ्य के प्रकाश का उपयोग करके किया गया था, जिसकी प्रकाश तीव्रता 1·5×10⁻³ J-s⁻¹ थी। 10 मिनट के उद्भासन से Br₂ में 0·05 मिलीमोल की कमी देखी गई। यह मानते हुए क्वांटम लब्धि की गणना कीजिए कि सिनेमिक अम्ल द्वारा 80% विकिरण अवशोषित किया जाता है। (प्लांक स्थिरांक h = 6·627×10⁻³⁴ J-s और प्रकाश का वेग c = 3×10⁸ m-s⁻¹) (5 अंक) (iii) निम्नलिखित प्रकाश-अभिक्रिया में बनने वाले प्रमुख तथा अल्प उत्पादों की प्रागुक्ति कीजिए। तर्क दीजिए: [संरचना प्रतिलिपि में नहीं दिखाई गई] (5 अंक) (iv) निम्नलिखित अभिक्रिया में A, B और C निर्देश कीजिए: (Major) [अभिक्रिया प्रतिलिपि में पूर्ण रूप से नहीं दिखाई गई] (5 अंक)

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Directive word: Elucidate

This question asks you to elucidate. 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

Begin with the directive 'elucidate' which demands clear, detailed explanation with structural clarity. Allocate time proportionally: ~25% on (a)(i) organic transformations (BH₃ and NaBH₄ reductions), ~15% on (a)(ii) NMO role in dihydroxylation, ~25% on (b) spectroscopy calculations (rotational, vibrational, IR assignment), and ~35% on (c) photochemistry (mechanism, quantum yield calculation, product prediction, and assignment). Structure as: concise introduction stating principles, systematic part-wise treatment with balanced equations and clear diagrams, and brief concluding synthesis where relevant.

Key points expected

  • (a)(i) BH₃ selectively reduces carboxylic acid over ester; intermediate is acyloxyborane; product is H(Me)C(CO₂Et)(CH₂CH₂OH). NaBH₄ reduces aldehyde over alkene; product is cyclohexyl-CH=CH-CH₂OH
  • (a)(ii) NMO acts as co-oxidant/regenerating agent; reoxidizes Os(VI) to Os(VIII) enabling catalytic cycle; prevents toxic OsO₄ stoichiometric use
  • (b)(i) Rotational transition J=4→5: ν = 2B(J+1) = 10.6×10 = 106 cm⁻¹; convert to frequency using c = νλ
  • (b)(ii) Isotopic shift: ν(DCl)/ν(HCl) = √(μ_HCl/μ_DCl) = √(35.5/36.5 × 37/71) ≈ 0.717; ν_DCl ≈ 2144 cm⁻¹
  • (b)(iii) IR assignments: 3100 cm⁻¹ (=C-H), 2250 cm⁻¹ (C≡N), 1650 cm⁻¹ (C=C); structure is CH₂=CH-CN (acrylonitrile/vinyl cyanide)
  • (c)(ii) Quantum yield φ = (moles reacted)/(moles photons absorbed); calculate E = hc/λ, photon flux, total energy, then φ ≈ 2.1

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness22%13Correctly identifies chemoselectivity in (a)(i) reductions, understands catalytic cycle in (a)(ii), applies rigid rotor and harmonic oscillator models accurately in (b), and demonstrates sound photochemical principles in (c) including excited state chemistryPartially correct concepts with minor errors in selectivity rules or spectroscopic model assumptions; some confusion between kinetic vs thermodynamic controlFundamental misconceptions such as BH₃ reducing ester, NaBH₄ reducing alkene, or treating rotational levels as equally populated
Mechanism / equation20%12Complete arrow-pushing mechanisms for BH₃ reduction (acyloxyborane formation, hydride transfer, hydrolysis), OsO₄-NMO catalytic cycle with regeneration steps, and photochemical transformations with clear excited state diagramsMechanisms present but missing key steps like regeneration in OsO₄ cycle or incomplete arrow formalism; photochemical steps vagueNo mechanisms shown or grossly incorrect electron flow; missing intermediates where explicitly asked
Numerical accuracy18%11Precise calculations: rotational frequency = 3.18×10¹² Hz (or 106 cm⁻¹), DCl frequency ≈ 2140-2150 cm⁻¹ using correct reduced mass ratio, quantum yield φ ≈ 2.0-2.2 with proper unit conversions and 80% absorption correctionCorrect method but arithmetic errors or unit conversion mistakes (e.g., cm⁻¹ to Hz errors, missing factor of 2 in rotational formula)Wrong formulas used (e.g., ν = BJ(J+1) for transition frequency), incorrect reduced mass calculation, or order-of-magnitude errors in quantum yield
Diagram / structure22%13Clear structural drawings of all products with correct stereochemistry where relevant, energy level diagrams for photochemical processes, and labeled IR correlation charts for (b)(iii) assignmentStructures drawn but stereochemistry ambiguous or missing; diagrams present but poorly labeled; hand-drawn appearance unclearMissing structures for explicitly requested products, incorrect connectivity (e.g., wrong reduction product), or no diagrams for photochemical transformations
Application context18%11Cites industrial relevance: Sharpless asymmetric dihydroxylation (Nobel 2001) for (a)(ii), spectroscopic applications in ISRO's atmospheric monitoring or forensic analysis, and photochemical synthesis in pharmaceutical manufacturing (e.g., vitamin D synthesis)Brief mention of applications without specific examples or context; generic statements about 'organic synthesis' or 'analytical chemistry'No application context provided; isolated theoretical treatment without connecting to real-world significance

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