Chemistry 2021 Paper II 50 marks Compulsory Explain

Q5

(a) 2,4-Pentadione exhibits five signal in ¹H NMR spectrum. Account for the observation. Also write the approximate chemical shift. (10 marks) (b) (i) Arrange the above monomers in order of decreasing ability to undergo anionic polymerization. (5 marks) (ii) Draw the structures of synthetic rubber and natural rubber and discuss their configurations. (5 marks) (c) Predict the product(s) and suggest mechanism in each of the following chemical transformations: I. [Structure: Acetophenone] SeO₂ → ? II. [Structure: Benzoic acid] Na, NH₃/C₂H₅OH → ? (10 marks) (d) Arrange the compounds in the above groups for ease of Norrish type-II H-abstraction in decreasing order giving the plausible explanations. (i) Ph — CO — CH₂CH₂CH₃ (I), Ph — CO — CH₂CH₂CH₂CH₃ (II), Ph — CO — CH₂CH₂ — CH(CH₃)₂ (III) (ii) Ph — CO — CH₂CH₂CH₃ (I) (e) 2-Pentanone and 3-Pentanone are structural isomers. Only one of the isomers exhibits McLafferty rearrangement. Identify and show the McLafferty fragmentation for this isomer. Also propose base peak for the other isomer, which does not exhibit the McLafferty rearrangement. (10 marks)

हिंदी में प्रश्न पढ़ें

(a) ¹H NMR स्पेक्ट्रम में 2,4-पेंटाडाइओन पाँच सिग्नल दर्शाता है। इस प्रेक्षण का लेखा दो व अनुमानित रासायनिक विस्थापन (केमिकल शिफ्ट) भी लिखे। (10 अंक) (b) (i) निम्नलिखित एकलकों में ऋणायनी बहुलकन की घटती हुई क्षमता का क्रम बताएं। (5 अंक) (ii) कृत्रिम रबड़ और प्राकृतिक रबड़ की संरचना लिखें और उनके विन्यासों की विवेचना करें। (5 अंक) (c) निम्नलिखित रासायनिक रूपांतरणों में उत्पाद/उत्पादों का अनुमान लगाएं और प्रत्येक की क्रियाविधि का सुझाव दें। I. [संरचना: एसीटोफीनोन] SeO₂ → ? II. [संरचना: बेंजोइक अम्ल] Na, NH₃/C₂H₅OH → ? (10 अंक) (d) निम्नलिखित समूहों में यौगिकों को उनके नॉरिस टाइप-II हाइड्रोजन प्रत्याकरण की सरलता के घटते हुए क्रमानुसार लिखिए व उपयुक्त व्याख्या दीजिए। (i) Ph — CO — CH₂CH₂CH₃ (I), Ph — CO — CH₂CH₂CH₂CH₃ (II), Ph — CO — CH₂CH₂ — CH(CH₃)₂ (III) (ii) Ph — CO — CH₂CH₂CH₃ (I) (e) 2-पेंटेनोन और 3-पेंटेनोन संरचनात्मक समावयवी हैं। इनमें से केवल एक समावयवी मैक्लैफर्टी पुनर्विन्यास दर्शाता है। इस समावयवी की मैक्लैफर्टी खंडन को चिह्नित करें और दर्शाएं। दूसरे समावयवी, जो मैक्लैफर्टी खंडन नहीं दर्शाता है उसके आधार शिखर की प्रस्तावना करें। (10 अंक)

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Directive word: Explain

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How this answer will be evaluated

Approach

Explain the spectroscopic observations and reaction mechanisms with clarity, allocating approximately 20% time to part (a) on ¹H NMR of 2,4-pentadione, 20% to part (b) on polymerization and rubber structures, 20% to part (c) on SeO₂ oxidation and Birch reduction mechanisms, 20% to part (d) on Norrish type-II photochemistry with stereoelectronic reasoning, and 20% to part (e) on McLafferty rearrangement in mass spectrometry. Begin with clear structural diagrams, proceed through mechanistic arrows and electron flow, and conclude with comparative analyses where requested.

Key points expected

  • Part (a): Explanation of keto-enol tautomerism in 2,4-pentadione creating two distinct species; identification of five non-equivalent proton environments (two methyls in keto form, one methyl in enol form, enolic OH, and CH) with approximate δ values (enol OH ~15 ppm, enol CH ~5.5 ppm, keto CH₂ ~3.5 ppm, methyls ~2.0-2.2 ppm)
  • Part (b)(i): Arrangement of monomers (isoprene, butadiene, styrene derivatives) by anionic polymerization ability based on electron-withdrawing/donating effects and carbanion stability; nitrile-substituted > carbonyl > simple alkene
  • Part (b)(ii): Structures of cis-1,4-polyisoprene (natural rubber, all-cis) vs trans-1,4-polyisoprene (gutta-percha) and synthetic polybutadiene/styrene-butadiene rubber; discussion of stereoregularity and conformational properties
  • Part (c): SeO₂ oxidation of acetophenone via ene mechanism to phenylglyoxal; Birch reduction of benzoic acid to 1,4-cyclohexadiene-1-carboxylic acid with electron/proton transfer steps
  • Part (d): Norrish type-II ordering based on γ-hydrogen accessibility and transition state stability: III (branched, more stable 6-membered TS) > II (straight chain) > I; stereoelectronic requirement for coplanar γ-C-H with carbonyl n→π* orbital
  • Part (e): 2-Pentanone shows McLafferty rearrangement (γ-hydrogen transfer to carbonyl oxygen, cleavage to m/z 58 and 42); 3-pentanone cannot form 6-membered cyclic TS, gives base peak at m/z 57 (C₃H₅O⁺ or C₄H₉⁺) via α-cleavage
  • Integration of spectroscopic data with mechanistic reasoning across all parts, demonstrating mastery of organic structure determination methods

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Demonstrates flawless understanding of tautomeric equilibrium, anionic polymerization electronics, photochemical stereoelectronics, and mass spectral fragmentation rules; correctly identifies that 2,4-pentadione's five signals arise from slow exchange between keto and enol tautomers on the NMR timescaleShows basic understanding of most concepts but confuses tautomeric exchange rates, misidentifies polymerization order, or makes minor errors in photochemical regioselectivityFundamental misconceptions such as attributing five signals to a single static structure, confusing anionic with cationic polymerization, or incorrect McLafferty eligibility criteria
Mechanism / equation25%12.5Provides complete curved-arrow mechanisms for SeO₂ ene reaction (including [2,3]-sigmatropic rearrangement), Birch reduction with explicit electron transfers and protonation steps, and clear orbital diagrams for Norrish type-II hydrogen abstraction; shows McLafferty rearrangement with proper 6-membered cyclic transition stateShows mechanisms with correct overall connectivity but missing electron flow details, or presents mechanisms without proper arrow notationIncorrect mechanisms, missing key intermediates, or failure to show electron movement; confuses oxidation states or reaction conditions
Numerical accuracy15%7.5Precise chemical shifts for 2,4-pentadione: enol OH ~14-16 ppm, vinyl CH ~5.5 ppm, keto CH₂ ~3.5 ppm, enol methyl ~2.0 ppm, keto methyl ~2.2 ppm; correct m/z values for all fragments (58, 42 for 2-pentanone; 57 base peak for 3-pentanone)Approximate chemical shifts within ±1 ppm, correct m/z values but missing some fragmentsWildly incorrect chemical shift values or m/z calculations, confusion between molecular and fragment ions
Diagram / structure20%10Clear, properly drawn structures for all compounds including stereochemistry of natural rubber (Z-configuration at C2-C3 double bonds) vs gutta-percha (E-configuration); accurate depiction of chair-like 6-membered transition states for Norrish type-II and McLafferty rearrangements; labeled NMR spectrum schematicAcceptable structures but missing stereochemical detail, poor transition state geometry, or cramped presentationMissing or incorrect structures, failure to show stereochemistry where critical, or confusing line-angle drawings
Application context15%7.5Connects 2,4-pentadione NMR to analytical chemistry applications; relates rubber stereochemistry to Indian rubber industry (Kerala natural rubber production); discusses synthetic rubber importance for tire manufacturing in India; explains photochemical reactions in environmental and materials degradation contextsBrief mention of industrial relevance without specific Indian context, or generic statements about spectroscopy utilityNo application context provided, or irrelevant connections that demonstrate misunderstanding of practical significance

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