Chemistry 2025 Paper II 50 marks Calculate

Q4

(a) (i) After polymerization of p-hydroxybenzoic acid, IR analysis shows 0·2% unreacted —COOH. Calculate the molecular weight of the polymer and categorize it. (7 marks) (ii) Write the structure of the polymer formed when p-hydroxybenzoic acid is polymerized. (3 marks) (iii) What is the precursor of the main component of natural rubber? Draw the structures of both monomer and polymer. (5 marks) (b) (i) Compare the salient properties of Nylon 6 and Nylon 6,6. (5 marks) (ii) How would you prepare syndiotactic polystyrene? (5 marks) (iii) What are the salient features of alpha helix in regards to handedness, residues per turn and elongation per residue? What are the principal stabilizing factors for an alpha helix? (5 marks) (c) (i) Compounds 1 and 2 on reaction with NaN₃ yield same product but compound 1 reacts at room temperature while compound 2 reacts at 200 °C. Identify the product formed and explain the reasons for different reactivities of compounds 1 and 2: (10 marks) (ii) Identify the major product in the following reactions and justify your answer: (10 marks)

हिंदी में प्रश्न पढ़ें

(a) (i) p-हाइड्रॉक्सीबेंजोइक अम्ल के बहुलकन के बाद IR विश्लेषण 0·2% बचा हुआ —COOH दर्शाता है। बहुलक के आण्विक भार की गणना कीजिए और उसे वर्गीकृत कीजिए। (7) (ii) p-हाइड्रॉक्सीबेंजोइक अम्ल के बहुलकन में बने बहुलक की संरचना लिखिए। (3) (iii) प्राकृतिक रबर के मुख्य घटक का पूर्वगामी क्या है? एकलक तथा बहुलक, दोनों की संरचनाएं बनाइए। (5) (b) (i) नाइलॉन 6 तथा नाइलॉन 6,6 के मुख्य गुणधर्मों की तुलना कीजिए। (5) (ii) सिंडियोटैक्टिक पॉलिस्टाइरीन का निर्माण कैसे करेंगे? (5) (iii) ऐल्फा हेलिक्स के वर्तिता, प्रति मोड अवशिष्ट तथा प्रति अवशिष्ट दीर्घन के संबंध में मुख्य लक्षण क्या हैं? ऐल्फा हेलिक्स के प्रमुख स्थिरीकरण कारक क्या हैं? (5) (c) (i) यौगिक 1 तथा 2 की NaN₃ के साथ हुई अभिक्रिया में समान उत्पाद बनता है जबकि यौगिक 1 कमरे के तापमान पर अभिक्रिया करता है लेकिन यौगिक 2 की अभिक्रिया 200 °C पर होती है। बनने वाले उत्पाद की पहचान कीजिए तथा यौगिक 1 एवं 2 की विभिन्न अभिक्रियाशीलता के कारणों की व्याख्या कीजिए : (10) (ii) निम्नलिखित अभिक्रिया में मुख्य उत्पाद की पहचान कीजिए तथा अपने उत्तर का औचित्य सिद्ध कीजिए : (10)

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How this answer will be evaluated

Approach

Calculate the molecular weight of poly(p-hydroxybenzoic acid) using Carothers' equation for (a)(i), then draw structures for (a)(ii) and (a)(iii). Compare Nylon 6 and Nylon 6,6 properties for (b)(i), explain syndiotactic polystyrene preparation for (b)(ii), and describe alpha helix features for (b)(iii). For (c), identify products and explain reactivity differences with mechanistic reasoning. Allocate ~25% time to numerical calculations in (a), ~35% to comparative and descriptive parts in (b), and ~40% to mechanistic explanations in (c).

Key points expected

  • For (a)(i): Apply Carothers' equation using 0.2% unreacted —COOH to calculate DP and molecular weight of poly(p-hydroxybenzoic acid); categorize as liquid crystalline aromatic polyester (Vectra-type)
  • For (a)(ii): Draw the linear aromatic polyester structure with ester linkages and para-substituted benzene rings
  • For (a)(iii): Identify isopentenyl pyrophosphate (IPP) as precursor; draw isoprene (2-methyl-1,3-butadiene) and cis-1,4-polyisoprene structures with correct stereochemistry
  • For (b)(i): Compare Nylon 6 (caprolactam, single monomer, 6 carbons) vs Nylon 6,6 (hexamethylenediamine + adipic acid, two monomers, 6+6 carbons) on properties like mp, tensile strength, moisture absorption
  • For (b)(ii): Explain syndiotactic polystyrene preparation using metallocene catalysts (e.g., zirconocene with MAO) or Ziegler-Natta catalysts with specific conditions for syndiotacticity
  • For (b)(iii): State alpha helix is right-handed, 3.6 residues/turn, 1.5 Å rise per residue; identify hydrogen bonds between C=O of residue i and N—H of residue i+4 as principal stabilizing factor
  • For (c)(i): Identify product as azide or amine; explain compound 1 is allylic/benzylic halide (SN2, room temp) vs compound 2 is vinyl/aryl halide (requires high temp, addition-elimination or radical mechanism)
  • For (c)(ii): Predict major products based on regioselectivity and stereochemistry; justify using Baldwin's rules, neighboring group participation, or pericyclic selection rules as applicable

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Correctly identifies liquid crystalline polyester for (a), distinguishes Nylon 6 vs 6,6 by monomer origin, recognizes metallocene catalysis for syndiotactic control, accurately describes alpha helix parameters, and correctly identifies mechanistic pathways for (c) with proper reasoning for reactivity differencesIdentifies basic polymer types but confuses Nylon 6/6,6 distinction or makes minor errors in alpha helix parameters; provides plausible but incomplete mechanistic rationale for (c)Fundamental misconceptions such as calling poly(p-hydroxybenzoic acid) an addition polymer, confusing nylon types completely, wrong helix handedness, or incorrect mechanistic identification for (c)
Mechanism / equation20%10Writes complete step-growth polymerization mechanism for (a); clearly shows anionic/cationic ring-opening for Nylon 6 vs condensation for Nylon 6,6; details metallocene coordination-insertion mechanism for syndiotactic control; illustrates SN2 vs addition-elimination mechanisms for (c)(i) with proper arrow pushingShows basic polymerization steps but misses catalytic cycle details or has incomplete mechanistic arrows for (c); recognizes reaction types without full detailMissing or incorrect mechanisms; confuses step-growth with chain-growth polymerization; no mechanistic explanation for reactivity differences in (c)
Numerical accuracy15%7.5Correctly applies Carothers' equation: DP = 1/(1-p) where p = 0.998, calculates DP = 500, and molecular weight = 500 × 120 = 60,000 g/mol (or using exact repeat unit mass); shows clear working with proper significant figuresCorrect formula but arithmetic error in calculation; or correct final answer with unclear working; understands relationship between extent of reaction and DPIncorrect formula application, confuses % unreacted with p value, or completely omits calculation; order of magnitude errors in final answer
Diagram / structure25%12.5Clear, accurate structures: poly(p-hydroxybenzoic acid) with correct ester connectivity; isoprene and cis-1,4-polyisoprene with proper stereochemistry; syndiotactic polystyrene showing alternating stereochemistry; detailed alpha helix diagram with hydrogen bonds; complete reaction schemes for (c)Basically correct structures but poor stereochemistry representation, missing hydrogen atoms, or unclear polymer connectivity; adequate but not precise diagramsIncorrect functional groups, wrong stereochemistry (trans instead of cis for rubber), missing double bonds, or no structures provided where required
Application context15%7.5Cites Indian context: Nylon 6,6 production (Gujarat State Fertilizers, Chennai Petroleum); mentions Vectra/Zenite applications in aerospace; connects natural rubber to Indian economy (Kerala, Rubber Board); relates alpha helix to protein structure determination (Indian contributions to crystallography); discusses industrial importance of stereoregular polymersMentions general industrial applications without specific Indian context; or provides context for some parts but not othersNo application context provided; purely theoretical treatment without connecting to real-world relevance or Indian industry

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