Chemistry 2023 Paper II 50 marks Draw

Q7

(a) (i) Draw the structure of 2'-deoxycytidine-3'-monophosphate. (ii) Why nucleotides and nucleic acids are acidic ? Mention the factor(s) that stabilize the DNA duplex. (iii) Show the hydrogen bonding in between cytosine and guanine base pair. 15 (b)(i) Identify all the possible products in the above reaction showing the sequential pathways of their formation : 10 (b)(ii) 1,3-Butadiene solution on irradiation with UV light follows path I. However, irradiating 1,3-butadiene in the presence of benzophenone at 366 nm follows path II. Give a suitable explanation. 5 (c)(i) Write the structure of the compounds (A), (B), (C), (D) and (E). 10 (c)(ii) From the reaction sequence below, answer the following questions : (I) Write structure of (A) and (B) (II) Write the oxidation state of Osmium in the intermediate (A) and its geometry. 5 (c)(iii) Write the structure of (A) and (B) in the above reactions. 5

हिंदी में प्रश्न पढ़ें

(क) (i) 2'-डिऑक्सीसाइटिडिन-3'-मोनोफॉस्फेट की संरचना लिखिए। (ii) न्यूक्लिओटाइड तथा न्यूक्लिक अम्ल अम्लीय क्यों होते हैं ? DNA डबल हेलिक्स संरचना को स्थिर करने वाले कारकों का उल्लेख कीजिए। (iii) साइटोसीन व गुआनिन शार युग्म के मध्य हाइड्रोजन आबंधन को दिखाइए। 15 (ख)(i) निम्नलिखित अभिक्रिया में सभी संभावित उत्पादों की पहचान करें जो उनके गठन का अनुक्रमिक मार्ग दिखाते हैं। 10 (ख)(ii) 1,3-ब्यूटाडाइन विलयन पराबैंगनी किरण पर पथ I का अनुसरण करता है। हालांकि बेंजोफेनॉन की उपस्थिति में 366 nm पर 1,3-ब्यूटाडाइन का पराबैंगनी किरण पथ II का अनुसरण करता है। उपयुक्त स्पष्टीकरण दीजिए। 5 (ग)(i) यौगिकों (A), (B), (C), (D) तथा (E) की संरचना लिखिए। 10 (ग)(ii) नीचे दिए गए अभिक्रिया क्रम से, निम्नलिखित प्रश्नों का उत्तर दें : (I) (A) तथा (B) की संरचना लिखिए । (II) मध्यवर्ती (A) में ऑस्मियम की ऑक्सीकरण अवस्था तथा ज्यामिति लिखिए । 5 (ग)(iii) निम्नलिखित अभिक्रियाओं में (A) तथा (B) की संरचना लिखिए । 5

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How this answer will be evaluated

Approach

Begin with (a) parts (i)-(iii) on nucleic acids (15 marks), spending ~30% time on accurate structural drawings and explanations of acidity/stabilization. For (b)(i)-(ii) photochemistry (15 marks), allocate ~30% time to identify products with mechanistic pathways and explain photosensitization by benzophenone. Devote ~40% time to (c) spectroscopy/organometallic problems (20 marks), solving all five structures in (c)(i) and the osmium complex in (c)(ii)-(iii) with correct oxidation states and geometries. Use clear sequential numbering for all structures.

Key points expected

  • For (a)(i): Correct Haworth projection of 2'-deoxycytidine-3'-monophosphate showing β-configuration at C-1', 2'-deoxy (no OH), phosphate at 3'-position, and cytosine base
  • For (a)(ii): Explanation that phosphate groups (pKa ~1-2) make nucleotides acidic; DNA duplex stabilization via Watson-Crick H-bonding, base stacking (van der Waals), and hydrophobic effects in aqueous medium
  • For (a)(iii): Accurate depiction of three hydrogen bonds between cytosine (N-3, O-2, N-4) and guanine (N-1, N-2, O-6) with correct donor-acceptor geometry
  • For (b)(i)-(ii): Identification of electrocyclic ring closure products (4π conrotatory thermal vs photochemical); explanation of benzophenone as triplet sensitizer enabling intersystem crossing to T1 state of butadiene for disrotatory closure
  • For (c)(i): Spectroscopic deduction of five compounds using IR, NMR, and MS data with correct functional group identification and structural elucidation
  • For (c)(ii)-(iii): Osmium tetroxide dihydroxylation mechanism showing cyclic osmate ester intermediate with Os(VI) oxidation state and trigonal bipyramidal/octahedral geometry; correct diol products

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%10Precise knowledge of nucleotide chemistry (phosphate acidity, pKa values), DNA stabilization forces (quantitative mention of ~10 kJ/mol per H-bond, stacking energy), and photochemical selection rules (Woodward-Hoffmann); correct oxidation state assignment for Os(VI) in osmate esterBasic understanding of nucleotide acidity and DNA stability; general awareness of pericyclic reactions but confused about conrotatory/disrotatory selection; approximate oxidation state for osmiumFundamental errors like calling nucleotides basic, confusing DNA/RNA stability, or assigning wrong oxidation states; no understanding of photochemical sensitization mechanism
Mechanism / equation20%10Clear stepwise mechanisms: (b)(i) shows thermal conrotatory and photochemical disrotatory pathways with orbital symmetry diagrams; (b)(ii) explains energy transfer from benzophenone triplet (ET ~69 kcal/mol) to butadiene; (c)(ii) depicts concerted [3+2] cycloaddition of OsO4 to alkeneGeneral description of ring closure without orbital diagrams; mentions photosensitization without energy transfer details; sketchy osmate ester formation without stereochemical implicationsNo mechanistic insight; confused thermal/photochemical pathways; incorrect mechanism for dihydroxylation or missing osmate ester intermediate entirely
Numerical accuracy10%5Correct wavelength-energy calculation for 366 nm (82 kcal/mol); accurate oxidation state +6 for Os in osmate ester; proper molecular formula deductions from spectroscopic data in (c)(i)Approximate values for energy calculations; correct final oxidation state but wrong calculation steps; minor errors in degree of unsaturation calculationsMajor calculation errors; wrong oxidation states (+8 or +4 for Os); incorrect interpretation of spectroscopic integration or chemical shift values
Diagram / structure35%17.5All seven required structures drawn with stereochemical precision: (a)(i) β-2'-deoxycytidine-3'-MP with correct furanose pucker; (a)(iii) three H-bonds with 2.9 Å distances shown; (b)(i) both cyclobutene stereoisomers; (c)(i) five compounds with stereochemistry; (c)(ii)-(iii) cyclic osmate ester with correct geometryCorrect connectivity but missing stereochemical details (α/β, R/S); approximate H-bond representation; acceptable but not optimal structural representationsWrong structures (ribose instead of deoxyribose, wrong base pairing, incorrect ring size in products); missing or illegible diagrams; no attempt at stereochemistry
Application context15%7.5Connects (a) to Indian biotech applications (DNA fingerprinting at CDFD Hyderabad); (b) to industrial photochemistry; (c) to asymmetric dihydroxylation (Sharpless, Nobel 2001) and pharmaceutical analysis; mentions Upjohn dihydroxylation for cis-diolsBrief mention of real-world relevance without specific Indian or contemporary examples; generic statements about importance of spectroscopyNo application context; isolated academic treatment without connecting to analytical chemistry, drug design, or Indian research contributions

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