Q7
(a) Discuss the different types of secondary structures of proteins and compare these structures with tertiary structure of proteins. 15 marks (b) Complete the following reactions and give suitable mechanisms for the formation of products: (i) CH₃—CH₂—CH₂—C(=O)—Cl (1) 2NaBH₄ → ? (2) H₃O⁺ (ii) CH₃—C≡C—CH₃ Na in liq. NH₃ → ? 10+5=15 marks (c) (i) A compound (A) containing C, H and O has molecular weight 102 and displays two signals in the ¹H NMR spectrum at δ 1·1 (d) and 3·55 (septet) in the integral ratio of 6:1. Treatment of A with 1 mole of HI gives rise to B and C. In the IR spectrum, B gives a strong absorption band at 3330 cm⁻¹, whereas its ¹H NMR spectrum shows signals at δ 1·05(d, 6H), 3·6 (septet, 1H) and 4·4 (s, 1H, disappeared with D₂O). The ¹H NMR spectrum of C gives signals at δ 1·9(d, 6H) and 4·25 (septet, 1H). Reaction of A with excess of HI gives only C. Identify A, B and C. Write all the reactions involved. 10 marks (ii) Three isomeric compounds having MF C₅H₁₀O give positive 2,4-DNP test and display the following NMR spectral characteristics. Identify the compounds. Among them, one isomeric compound on treatment with KOH (concentrated) gives two products. Write their structures also: (1) A triplet at δ1.05 and a quartet at δ2.47 (2) Two singlets (3) A doublet at δ1.0, a singlet at δ2.1 and a septet at δ2.2 10 marks
हिंदी में प्रश्न पढ़ें
(a) प्रोटीन के भिन्न प्रकारों की द्वितीयक संरचनाओं की विवेचना कीजिए तथा इन संरचनाओं की तुलना प्रोटीन के तृतीयक संरचना से कीजिए। 15 अंक (b) निम्नलिखित अभिक्रियाओं को पूर्ण कीजिए तथा उत्पादों के बनने की उपयुक्त क्रियाविधि दीजिए : (i) CH₃—CH₂—CH₂—C(=O)—Cl (1) 2NaBH₄ → ? (2) H₃O⁺ (ii) CH₃—C≡C—CH₃ तरल NH₃ में Na → ? 10+5=15 अंक (c) (i) C, H तथा O के एक यौगिक (A) का आणविक भार 102 है और ¹H NMR स्पेक्ट्रम में दो सिग्नल δ 1·1 (d) तथा 3·55 (सेप्टेट) पर अभिन्न अनुपात 6:1 में देता है। A का विवेचन जब 1 मोल HI के साथ किया जाता है, तो B तथा C उत्पाद बनते हैं। B के IR स्पेक्ट्रम में 3330 cm⁻¹ पर एक प्रबल अवशोषण बैंड है, जबकि ¹H NMR स्पेक्ट्रम में δ 1·05(d, 6H), 3·6 (सेप्टेट, 1H) और 4·4 (s, 1H, D₂O में लुप्त) पर सिग्नल दिखाता है। C के ¹H NMR स्पेक्ट्रम में δ 1·9(d, 6H) तथा 4·25 (सेप्टेट, 1H) पर सिग्नल देता है। यदि A की अभिक्रिया अतिरिक्त HI से की जाती है, तो केवल C बनता है। A, B तथा C को पहचानिए। सभी संबंधित अभिक्रियाओं को लिखिए। 10 अंक (ii) तीन समावयवी यौगिकों, जिनका आणविक सूत्र (MF) C₅H₁₀O है, 2,4-DNP परीक्षण सकारात्मक देते हैं तथा निम्नलिखित NMR स्पेक्ट्रमी अभिलक्षण देते हैं। यौगिकों को पहचानिए। इनमें एक समावयवी यौगिक KOH (संद्रित) के साथ विवेचन करने पर दो उत्पाद देता है। उत्पादों की संरचना भी बताइए : (1) δ1.05 पर एक ट्रिप्लेट और δ2.47 पर एक क्वार्टेट (2) दो सिंगलेट (3) δ1.0 पर एक डबलेट, δ2.1 पर एक सिंगलेट और δ2.2 पर एक सेप्टेट 10 अंक
Directive word: Discuss
This question asks you to discuss. 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
The directive 'discuss' for part (a) requires comprehensive coverage with critical comparison, while parts (b) and (c) demand mechanistic and analytical problem-solving. Allocate approximately 30% time/words to part (a) (15 marks), 30% to part (b) (15 marks), and 40% to part (c) (20 marks). Structure: begin with protein secondary structures and their comparison with tertiary structure; proceed to complete reaction mechanisms with clear electron-flow arrows; conclude with systematic spectral analysis for structural elucidation of unknowns.
Key points expected
- Part (a): Description of α-helix (right-handed, 3.6 residues/turn, 0.54 nm pitch, intrachain H-bonds) and β-pleated sheet (parallel and antiparallel, interchain H-bonds); comparison with tertiary structure (3D folding, hydrophobic interactions, disulfide bridges, ionic bonds, hydrogen bonds)
- Part (b)(i): Recognition that NaBH₄ reduces acid chloride to aldehyde then to primary alcohol; mechanism involves hydride attack on carbonyl carbon followed by protonation; final product CH₃CH₂CH₂CH₂OH
- Part (b)(ii): Birch reduction of internal alkyne; trans-alkene formation via radical anion mechanism; product is trans-2-butene
- Part (c)(i): Structure determination of A as isopropyl isopropyl ether (diisopropyl ether), B as isopropanol, C as isopropyl iodide; interpretation of NMR splitting patterns (d and septet indicating isopropyl group), IR OH stretch at 3330 cm⁻¹, and D₂O exchange
- Part (c)(ii): Identification of three isomers—(1) pentan-2-one, (2) pentan-3-one, (3) 3-methylbutan-2-one; recognition that pentan-3-one undergoes self-condensation with concentrated KOH to give diacetone alcohol and mesityl oxide
Evaluation rubric
| Dimension | Weight | Max marks | Excellent | Average | Poor |
|---|---|---|---|---|---|
| Concept correctness | 25% | 12.5 | Demonstrates precise understanding of protein structural hierarchy (α-helix parameters: 3.6 residues/turn, 0.54 nm rise, φ/ψ angles; β-sheet types; tertiary structure stabilizing forces); correctly identifies all unknown structures in (c)(i) and (c)(ii) with proper reasoning from spectral data | Describes basic features of secondary structures but confuses parallel/antiparallel β-sheets or misstates helix parameters; identifies some products correctly but with incomplete reasoning for spectral assignments | Confuses primary/secondary/tertiary structures; fundamental errors in interpreting NMR multiplicities (e.g., misassigning septet as quartet); incorrect identification of carbonyl compounds in (c)(ii) |
| Mechanism / equation | 25% | 12.5 | Complete balanced equations for both (b)(i) and (b)(ii); detailed mechanisms with curved arrows showing hydride delivery to acid chloride, tetrahedral intermediate formation, and Birch reduction radical anion pathway with electron transfer steps and protonation at trans positions | Correct products shown but mechanisms lack electron-flow detail; missing key intermediates or incorrect stereochemical outcome for alkyne reduction | Incorrect products or no mechanisms attempted; shows NaBH₄ reducing ester/carboxylic acid or gives cis-alkene from Birch reduction |
| Numerical accuracy | 15% | 7.5 | Correct molecular formula determination (C₆H₁₄O for A, MW 102); accurate integration ratios (6:1 for A; 6:1:1 for B); correct calculation of unsaturation index; proper chemical shift assignments consistent with spectral data | Correct molecular formula but minor errors in integration interpretation; chemical shift values quoted without proper structural correlation | Incorrect molecular formula calculation; fundamental errors in interpreting integration ratios or chemical shift values; inconsistent with given spectral data |
| Diagram / structure | 20% | 10 | Clear structural drawings of α-helix (helical wheel or ribbon diagram), β-sheet (showing H-bonding pattern); all organic structures drawn with correct connectivity; Newman projections or Fischer projections where stereochemistry relevant; labeled IR correlation chart for functional groups | Basic structural drawings present but lacking stereochemical detail; helix diagram incomplete or β-sheet H-bonds not shown; organic structures correct but hand-drawn quality poor | Missing structural drawings; incorrect connectivity in organic structures; no attempt to illustrate protein conformations |
| Application context | 15% | 7.5 | Connects protein structures to Indian context (e.g., silk fibroin β-sheet structure in Indian silk industry; enzyme stability in extremophiles from Indian hot springs); relates spectral analysis to pharmaceutical quality control (Indian API manufacturing); mentions applications of Birch reduction in natural product synthesis | Brief mention of biological significance without specific Indian examples; generic statements about spectroscopy applications | No application context provided; purely theoretical treatment without real-world relevance |
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