Chemistry 2022 Paper II 50 marks Explain

Q8

(a) (i) Among the following compounds I and II, which has more carbonyl stretching frequency in IR spectra? Explain: I II 5 marks (ii) Complete the following reaction. Write the structures of A, B and C, and explain how IR spectroscopy is helpful to distinguish among them: H₃C OH CH₃ H⁺ A + B + C 10 marks (b) (i) Calculate the λ_max values of the following compounds: (1) (2) 10 marks (ii) Explain why 1,3-butadiene exhibits a lower λ_max for π → π* transitions compared to that of 1,3,5-hexatriene. 5 marks (c) (i) What is McLafferty rearrangement? Discuss the mass spectral fragmentation of butyl butyrate with the following given data of ions: m/z 101, m/z 73, m/z 71 and m/z 56 Write the structures of fragment ions. 10 marks (ii) (1) A compound with MF C₂H₂BrCl exhibits two doublets (J = 16 Hz) in its PMR spectrum. Suggest a suitable structure along with other possible structures. (2) How can the structures of A and B be decided based on their UV spectral data? [ λ_max = 296 nm (ε_max = 10700) and λ_max = 281 nm (ε_max = 20800) ] 10 marks

हिंदी में प्रश्न पढ़ें

(a) (i) निम्नलिखित यौगिकों I तथा II में किसकी कार्बनिल तनन आवृत्ति IR स्पेक्ट्रा में अधिक है? व्याख्या कीजिए : I II 5 अंक (ii) निम्नलिखित अभिक्रिया को पूर्ण कीजिए। A, B और C की संरचना लिखिए तथा व्याख्या कीजिए किस प्रकार से IR स्पेक्ट्रमिकी इनमें अंतर बताने में सहायक है : H₃C OH CH₃ H⁺ A + B + C 10 अंक (b) (i) निम्नलिखित यौगिकों की λ_max मानों का परिकलन कीजिए : (1) (2) 10 अंक (ii) 1,3-ब्यूटाडाइन के π → π* संक्रमण का λ_max मान 1,3,5-हेक्साट्राइन की तुलना में कम क्यों है? व्याख्या कीजिए। 5 अंक (c) (i) मैकलफर्टी पुनर्विन्यास क्या है? ब्यूटिल ब्यूटिरेट के द्रव्यमान स्पेक्ट्रमी खंडन में निम्नलिखित आयन प्राप्त हुए हैं : m/z 101, m/z 73, m/z 71 और m/z 56 इनकी विवेचना कीजिए। खंड आयनों की संरचना लिखिए। 10 अंक (ii) (1) एक यौगिक, जिसका आण्विक सूत्र (MF) C₂H₂BrCl है, PMR स्पेक्ट्रम में दो डबलेट (J = 16 Hz) देता है। एक उपयुक्त संरचना का सुझाव दीजिए तथा संभावित दूसरी संरचनाएं भी दीजिए। (2) A तथा B की संरचना को UV स्पेक्ट्रमी आँकड़ों के आधार पर कैसे तय कर सकते हैं? [ λ_max = 296 nm (ε_max = 10700) और λ_max = 281 nm (ε_max = 20800) ] 10 अंक

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Directive word: Explain

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How this answer will be evaluated

Approach

Begin with a concise introduction on spectroscopic techniques as structural tools. For (a), allocate ~15 marks worth: explain carbonyl frequency trends using conjugation/hybridization effects, then complete the dehydration reaction identifying A (alkene), B (ether), C (alkene isomer), with IR distinguishing C=C vs C=O regions. For (b), spend ~15 marks: apply Woodward-Fieser rules for λ_max calculations, then explain conjugation length effect on HOMO-LUMO gap. For (c), allocate ~20 marks: define McLafferty rearrangement with radical mechanism, draw fragmentation pathway for butyl butyrate showing m/z 101 (acylium), 73, 71, 56; for (c)(ii) deduce trans-1-bromo-2-chloroethene from J=16 Hz coupling, and assign λ_max 296 nm to trans-stilbene (higher ε, planar) vs 281 nm to cis-stilbene using steric hindrance arguments. Conclude with integration of spectroscopic data for structural elucidation.

Key points expected

  • (a)(i) Correct identification of higher ν(C=O) in compound with less conjugation/stronger C=O bond; explanation via resonance and bond order effects
  • (a)(ii) Acid-catalyzed dehydration of 2-butanol giving 1-butene (A), 2-butene mixture (B=trans, C=cis or B=ether if intermolecular); IR distinguishes O-H (3300 cm⁻¹), C=C (1650 cm⁻¹), C-O (1100 cm⁻¹) regions
  • (b)(i) Accurate Woodward-Fieser rule application: base value + substituent increments + solvent corrections for both diene/triene systems
  • (b)(ii) Explanation of bathochromic shift with extended conjugation: decreased HOMO-LUMO energy gap in hexatriene vs butadiene
  • (c)(i) McLafferty rearrangement definition: γ-hydrogen transfer via six-membered cyclic TS with radical cation cleavage; specific fragment structures for butyl butyrate (m/z 101: CH₃CH₂CH₂CO⁺, m/z 73: CH₃CH₂CO⁺, m/z 71: C₄H₉⁺, m/z 56: C₄H₈⁺·)
  • (c)(ii)(1) trans-1-bromo-2-chloroethene from J=16 Hz (trans vicinal coupling); cis isomer excluded by large J value
  • (c)(ii)(2) UV assignment: higher λ_max with lower ε (296 nm, ε=10700) for trans-stilbene vs lower λ_max with higher ε (281 nm, ε=20800) for cis-stilbene due to steric hindrance reducing planarity and conjugation efficiency in cis form

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Demonstrates precise understanding of IR frequency-determining factors (conjugation, ring strain, H-bonding), Woodward-Fieser rules for dienes/enones, McLafferty rearrangement mechanism, and UV λ_max-conjugation relationships; correctly identifies trans-coupling constants and steric effects on chromophore planarityShows basic grasp of spectroscopic principles but confuses conjugation effects on IR frequencies, makes minor errors in Woodward-Fieser increments, or provides incomplete McLafferty mechanism without electron movement arrowsFundamental misconceptions: states wrong compound for higher ν(C=O), applies wrong base values for λ_max calculation, describes McLafferty as simple cleavage without rearrangement, or confuses cis/trans UV assignments
Mechanism / equation20%10Clear electron-pushing mechanism for McLafferty rearrangement showing six-membered cyclic transition state, γ-H transfer, and β-cleavage; acid-catalyzed dehydration mechanism with carbocation intermediate and regioselectivity explanationMechanism shown with some arrow errors or missing intermediates; McLafferty described verbally without proper electron flow; dehydration mechanism incomplete regarding E1/E2 competitionNo mechanism provided for McLafferty rearrangement; incorrect E2 mechanism for dehydration; arrows violating electron movement rules; missing key intermediates
Numerical accuracy15%7.5Precise λ_max calculations using correct Woodward-Fieser parameters: base value 214 nm (heteroannular) or 253 nm (homoannular), accurate substituent increments (5 nm for alkyl, 5 nm for exocyclic double bond, 30 nm for extending conjugation), correct solvent correctionsCorrect base value selection but arithmetic errors in summation; one increment missed or wrong value used; no solvent correction appliedWrong base values (confusing diene types), incorrect increment values, or no calculation attempted; J value mentioned without structural correlation
Diagram / structure25%12.5All structures clearly drawn: compounds I and II with proper stereochemistry, dehydration products A/B/C with correct connectivity, fragment ions with charges and radical dots explicitly shown, C₂H₂BrCl isomers with spatial arrangement indicating trans geometry; IR correlation table or spectrum regions sketchedStructures drawn but with ambiguous stereochemistry; missing charges on fragment ions; incomplete set of products; poor handwriting affecting clarityMissing critical structures (no fragment ion structures, no A/B/C identification); incorrect molecular formulas; no diagrams despite question requirement; structures contradict written explanation
Application context15%7.5Effectively integrates multiple spectroscopic techniques for structural proof: IR for functional group identification and reaction monitoring, UV for conjugation extent, MS for molecular weight and fragmentation pattern, PMR for stereochemistry; demonstrates how combined data solves real analytical problems in natural product or synthetic chemistryTreats each spectroscopic method in isolation without cross-validation; mentions applications generically without linking to specific problem; no discussion of complementary nature of techniquesNo application context provided; fails to explain how IR distinguishes A/B/C or how UV decides between stereoisomers; treats spectroscopy as purely theoretical exercise

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