Q6
(a) (i) Explain the various steps involved in benzoyl peroxide-initiated polymerization of ethylene to give polyethylene. (10 marks) (ii) Predict the physical properties of atactic, syndiotactic and isotactic polystyrenes based on their structure. (5 marks) (b) Write down the mechanism for the formation of compounds B and C from compound A on photoirradiation : (15 marks) (c) Identify A, B, C and D in the following reaction sequence : A $\xrightarrow[\text{MeOH}]{\text{NaBH}_4}$ [cyclohexanol structure] $\xrightarrow{\text{H}^+}$ B $\xrightarrow{\text{C}}$ [3-bromocyclohexene structure] $\xrightarrow{\text{mCPBA}}$ D Write the mechanism of the first step of the above reaction sequence. (20 marks)
हिंदी में प्रश्न पढ़ें
(a) (i) बेंज़ॉयल परॉक्साइड द्वारा प्रारंभ किए गए एथिलीन के बहुलकन, जिसमें पॉलीएथिलीन बनता है, में सम्मिलित विभिन्न चरणों की व्याख्या कीजिए। (10 अंक) (ii) अव्यवस्थ (एटैक्टिक), एकांतर व्यवस्थ (सिन्डियोटैक्टिक) तथा समव्यवस्थ (आइसोटैक्टिक) पॉलिस्टाइरीनों के भौतिक गुणधर्मों का उनकी संरचना के आधार पर अनुमान लगाइए। (5 अंक) (b) यौगिक A के प्रकाशिक किरणन पर यौगिकों B तथा C के बनने की क्रियाविधि लिखिए : (15 अंक) (c) निम्नलिखित अभिक्रिया क्रम में A, B, C तथा D की पहचान कीजिए : उपर्युक्त अभिक्रिया क्रम में प्रथम चरण की क्रियाविधि लिखिए। (20 अंक)
Directive word: Explain
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How this answer will be evaluated
Approach
Begin with a brief introduction defining radical polymerization and stereoregularity. For part (a)(i), explain the three-step mechanism (initiation, propagation, termination) with clear radical structures; for (a)(ii), compare tacticity effects on crystallinity and Tg using diagrams. Part (b) requires detailed photochemical mechanisms with curved arrows showing excited state chemistry. Part (c) demands identification of all four compounds and a complete mechanism for NaBH4 reduction. Allocate approximately 25% time to (a)(i), 10% to (a)(ii), 30% to (b), and 35% to (c) based on marks distribution. Conclude with industrial relevance of polyethylene and polystyrene in Indian manufacturing context.
Key points expected
- For (a)(i): Homolytic cleavage of benzoyl peroxide to benzoyloxy radicals, then phenyl radicals; initiation by addition to ethylene; propagation with radical chain growth; termination by coupling or disproportionation
- For (a)(ii): Isotactic PS has regular packing, high crystallinity, higher Tm and mechanical strength; syndiotactic has alternating stereochemistry with moderate crystallinity; atactic is amorphous, transparent, lower Tg, used in disposable cups
- For (b): Photoirradiation of compound A (typically a carbonyl or alkene) generates excited singlet/triplet states; Norrish Type I or II cleavage, or [2+2] cycloaddition leads to products B and C with complete arrow-pushing mechanisms
- For (c): A is cyclohexanone; B is cyclohexene; C is HBr or NBS/hν; D is 3-bromocyclohexene epoxide; mechanism shows NaBH4 delivering hydride to carbonyl, alkoxide protonation, then acid-catalyzed dehydration
- For (c) mechanism: Concerted or stepwise hydride transfer, axial/equatorial stereochemistry consideration, formation of tetrahedral intermediate
- Industrial context: LDPE/HDPE production in India (Reliance, GAIL); polystyrene applications in packaging and insulation
Evaluation rubric
| Dimension | Weight | Max marks | Excellent | Average | Poor |
|---|---|---|---|---|---|
| Concept correctness | 20% | 10 | Accurately defines all key concepts: radical stability, tacticity terminology, photochemical excited states, regioselectivity in epoxidation; correctly identifies all compounds A-D in the sequence with proper reasoning | Defines most concepts correctly but confuses syndiotactic with isotactic or misidentifies one compound; minor errors in stereochemical terminology | Fundamental misconceptions: describes ionic rather than radical polymerization, confuses atactic with isotactic properties, or fails to identify more than two compounds correctly |
| Mechanism / equation | 25% | 12.5 | Complete arrow-pushing for all three mechanisms: benzoyl peroxide decomposition with radical structures, photochemical transformation with excited state notation, and NaBH4 reduction with tetrahedral intermediate; includes all termination steps and side reactions | Shows most mechanistic steps but misses radical coupling termination, omits excited state multiplicity labels, or shows incomplete alkoxide protonation step | Major mechanistic errors: incorrect radical generation, missing propagation steps, no curved arrows, or completely wrong reduction mechanism |
| Numerical accuracy | 10% | 5 | Correctly calculates or states relevant quantitative data: approximate bond dissociation energies for peroxide (~30 kcal/mol), degree of polymerization relations, or stereochemical ratios where applicable | States some numerical values but with minor errors or omits quantitative aspects entirely while remaining qualitatively correct | No numerical data provided where relevant, or seriously incorrect values that contradict chemical principles |
| Diagram / structure | 25% | 12.5 | Clear structural drawings: benzoyl peroxide and phenyl radical structures, Fischer or zig-zag projections for all three polystyrene tacticities, excited state orbital diagrams for photochemistry, chair conformations for cyclohexane derivatives with stereochemistry indicated | Draws most structures but with poor stereochemical representation, missing wedge/dash bonds, or unclear radical dot notation; one missing structure | No structures drawn or seriously incorrect representations; inability to show tacticity differences or missing multiple compound structures |
| Application context | 20% | 10 | Connects to Indian industrial context: Reliance Industries' polyethylene production, difference between LDPE (high-pressure, branched) and HDPE (Ziegler-Natta, linear); discusses environmental concerns of single-use polystyrene and India's plastic waste management rules | Mentions general industrial applications without specific Indian examples or provides generic context without linking to reaction mechanisms | No application context provided; purely academic treatment without relevance to polymer industry or photochemical applications |
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