Chemistry 2021 Paper II 50 marks Explain

Q4

(a) (i) Heating of 3-deuteroindene causes scrambling of the deuterium. Explain with mechanism. (20 marks) (ii) Write the structure of product(s) formed during the thermal reaction of maleic anhydride with cyclopentadiene. Explain with mechanism. (20 marks) (b) Write the product(s) of the following reactions: (i) (CH₃)₃C—CH=CH₂ →(HCl) ? (5 marks) (ii) CH₂=CH—CH₂—Br →(HBr, benzoyl peroxide) ? (10 marks) (c) (i) Predict the product in the above chemical conversions and also identify the name reaction involved. (10 marks) I. PhCH₂Cl →(1) (C₆H₅)₃P (2) RLi (3) [cyclohexanone structure] ? II. PhNHNH₂ + [cyclohexanone structure] →(Glacial acetic acid) ? (ii) Which of the following pair(s) gives α,β-unsaturated carbonyl compound in presence of base? Justify the answer. I. HCHO and PhCHO II. PhCHO and Ph—CH—CHO | CH₃ III. PhCHO and CH₃CH₂CHO IV. HCHO and PhCOPh

हिंदी में प्रश्न पढ़ें

(a) (i) 3-ड्यूटेरोइंडीन को गर्म करने पर ड्यूटीरियम का व्यामिश्रण होता है। क्रियाविधि देते हुए समझाइए। (20) (ii) मेलेइक ऐनहाइड्राइड की साइक्लोपेंटाडाइन के साथ उष्मीय अभिक्रिया में बने उत्पादों की संरचना लिखें। क्रियाविधि के साथ समझाइए। (20) (b) निम्नलिखित अभिक्रियाओं के उत्पाद/उत्पादों को लिखें : (i) (CH₃)₃C—CH=CH₂ →(HCl) ? (5) (ii) CH₂=CH—CH₂—Br →(HBr, बेंजॉयल परॉक्साइड) ? (10) (c) (i) निम्नलिखित रासायनिक रूपांतरणों का उत्पाद लिखें और संलग्न अभिक्रिया को पहचानते हुए नाम लिखिए। (10) I. PhCH₂Cl →(1) (C₆H₅)₃P (2) RLi (3) [साइक्लोहेक्सनोन संरचना] ? II. PhNHNH₂ + [साइक्लोहेक्सनोन संरचना] →(मैशल, Glacial acetic acid) ? (ii) निम्नलिखित में से कौन सा युगल क्षार की उपस्थिति में, α,β-असंतृप्त कार्बोनिल यौगिक देता है? उत्तर को सिद्ध करें : I. HCHO और PhCHO II. PhCHO और Ph—CH—CHO | CH₃ III. PhCHO और CH₃CH₂CHO IV. HCHO और PhCOPh

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Directive word: Explain

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How this answer will be evaluated

Approach

Explain the pericyclic mechanisms for (a)(i) indene deuterium scrambling via [1,5]-hydrogen shifts and (a)(ii) Diels-Alder reaction with correct stereochemistry; for (b) apply Markovnikov vs anti-Markovnikov rules with radical mechanism for (ii); for (c)(i) identify Wittig and Wolff-Kishner/reduction reactions with products, and for (c)(ii) analyze crossed aldol condensations selecting pairs with α-hydrogens for α,β-unsaturated carbonyl formation. Allocate ~40% time to combined (a) parts, ~35% to (b), and ~25% to (c).

Key points expected

  • (a)(i) Recognition of 3-deuteroindene undergoing thermal [1,5]-sigmatropic hydrogen/deuterium shifts with suprafacial migration on the indene π-system, leading to scrambling at C1 and C3 positions
  • (a)(ii) Identification of Diels-Alder [4+2] cycloaddition between cyclopentadiene (diene) and maleic anhydride (dienophile), giving endo-norbornene-type adduct with correct stereochemistry
  • (b)(i) Markovnikov addition of HCl to 3,3-dimethyl-1-butene with carbocation rearrangement via 1,2-methyl shift to give 2-chloro-2,3-dimethylbutane
  • (b)(ii) Anti-Markovnikov addition via radical mechanism (peroxide effect/Kharasch effect) giving 1,3-dibromopropane or equivalent product with correct radical chain steps
  • (c)(i) I: Wittig reaction forming alkene from benzyl chloride → phosphonium ylide → cyclohexylidene product; II: Wolff-Kishner or hydrazone formation followed by reduction to methylene
  • (c)(ii) Correct selection of pair II (PhCHO + PhCH(CH₃)CHO) and pair III (PhCHO + CH₃CH₂CHO) as giving α,β-unsaturated carbonyls; justification requires one component with α-hydrogens and other without, avoiding self-condensation

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%2.5Correctly identifies all reaction types: [1,5]-sigmatropic shift for (a)(i), Diels-Alder cycloaddition for (a)(ii), carbocation rearrangement for (b)(i), radical anti-Markovnikov for (b)(ii), Wittig and Wolff-Kishner for (c)(i), and crossed aldol selectivity for (c)(ii)Identifies most reaction types but confuses pericyclic terminology or misapplies Markovnikov rules; may miss rearrangement in (b)(i) or misidentify one name reactionFundamental errors such as calling (a)(i) a [1,3]-shift, confusing Diels-Alder with electrocyclic reaction, or applying wrong regioselectivity rules throughout
Mechanism / equation25%2.5Complete arrow-pushing mechanisms for all parts: suprafacial [1,5]-H shift with orbital overlap, concerted Diels-Alder with endo preference, carbocation rearrangement with methyl shift, radical chain initiation/propagation/termination, ylide formation and Wittig mechanismShows mechanisms for major parts but omits key steps like radical termination, endo/exo distinction, or ylide formation; some arrow formalism errorsMissing or severely flawed mechanisms; no understanding of pericyclic orbital symmetry, no radical steps shown, or completely wrong mechanistic pathways
Numerical accuracy10%1Correct stoichiometry in all reactions, proper counting of atoms in scrambled products, accurate electron counting in mechanisms, correct oxidation states where relevantMinor counting errors in complex products or electron bookkeeping; generally correct but slips on detailed atom accountingMajor errors in product formulas, incorrect atom counts leading to impossible structures, or fundamental stoichiometric mistakes
Diagram / structure25%2.5Clear, correctly drawn structures: indene with numbered positions showing deuterium scrambling, endo-Diels-Alder adduct with stereochemistry, rearranged alkyl chloride, dibromide product, Wittig alkene and hydrazone/reduced products, with proper 3D representation where neededStructures mostly correct but stereochemistry ambiguous or missing in Diels-Alder adduct; acceptable but not crisp drawings; some missing productsIncorrect connectivity, missing double bonds, wrong ring fusion in bicyclic adduct, or no structures drawn despite question requirement
Application context15%1.5Correctly names all reactions (Wittig, Wolff-Kishner/DNP-hydrazone, Kharasch/anti-Markovnikov addition), explains why certain aldol pairs work (crossed aldol control), and relates pericyclic selectivity to Woodward-Hoffmann rulesNames most reactions correctly but may confuse Wolff-Kishner with Clemmensen or miss naming Kharasch effect; limited discussion of selectivity principlesFails to name reactions, no understanding of why specific aldehyde/ketone combinations are chosen for crossed aldol, no mention of orbital symmetry control

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