(a) (i) Acetone shows a weak absorption at 280 nm and a strong absorption at 190 nm in the UV spectrum. Account for the observation. (5 marks)

(ii) Using Woodward-Fieser rules, calculate λ_max for the following compounds: A, B, C, D (12 marks)

(b)

Question

(a) (i) Acetone shows a weak absorption at 280 nm and a strong absorption at 190 nm in the UV spectrum. Account for the observation. (5 marks)

(ii) Using Woodward-Fieser rules, calculate λ_max for the following compounds: A, B, C, D (12 marks)

(b) (i) Rank the following dienes in order of increasing reactivity in a Diels-Alder reaction (1 = least reactive, 4 = most reactive). Briefly explain your answer: A, B, C, D (10 marks)

(ii) Write the structure of the product in the following reaction: OMe + CN 1) Heat 2) H⁺, H₂O ? (5 marks)

(c) (i) Write the structure of the product formed in the following sigmatropic rearrangement and categorize it with suitable explanation. Explain the thermal feasibility of this rearrangement by drawing orbital diagram: H H Bu 80 °C CCl₄ → ? (10 marks)

(ii) (1) Identify the mode of ring closure for each of the following electrocyclic reactions: (A), (B)
(2) Are the indicated hydrogens cis or trans? (10 marks)

UPSC Answer Check · Accepted Answer

Explain the spectroscopic and pericyclic phenomena across all sub-parts with precise chemical reasoning. Allocate approximately 35% of effort to part (a) covering UV transitions and Woodward-Fieser calculations (17 marks), 30% to part (b) on Diels-Alder reactivity and product prediction (15 marks), and 35% to part (c) on sigmatropic rearrangements and electrocyclic reactions (20 marks). Begin with clear identification of transition types and orbital interactions, proceed through systematic application of rules with drawn structures, and conclude with stereochemical outcomes and thermal feasibility justifications.
- For (a)(i): Explain n→π* (weak, 280 nm) and π→π* (strong, 190 nm) transitions in acetone, citing symmetry-allowed/forbidden nature and molar absorptivity differences
- For (a)(ii): Apply Woodward-Fieser rules for diene/ene-one λ_max calculation with correct base values, increment additions (homodienyl, exocyclic, substituent effects), and solvent corrections for compounds A-D
- For (b)(i): Rank dienes by s-cis conformation accessibility, electron-donating/withdrawing substituents, and steric factors; identify locked s-cis (most reactive) vs s-trans locked (least reactive)
- For (b)(ii): Predict Diels-Alder adduct with correct regioselectivity (ortho/para preference) and stereochemistry from electron-rich diene (OMe) and electron-deficient dienophile (CN)
- For (c)(i): Identify [3,3]-sigmatropic rearrangement (Cope/Claisen), draw product with stereochemical fidelity, and explain thermal feasibility via HOMO-LUMO orbital symmetry matching
- For (c)(ii): Determine conrotatory/disrotatory modes based on π-electron count (4n vs 4n+2) and thermal/photochemical conditions; assign cis/trans relationships of indicated hydrogens in products

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	20%	11	Precisely explains n→π* vs π→π* transition origins, Woodward-Fieser increment logic, Diels-Alder orbital overlap requirements, and Woodward-Hoffmann rules for electrocyclic/s sigmatropic reactions with correct electron counting	Identifies most transition types and reaction categories correctly but confuses allowed/forbidden nature or misapplies increment rules; minor errors in electron counting for pericyclic selections	Fundamental confusion between transition types, inability to distinguish pericyclic reaction classes, or gross misapplication of selection rules
Mechanism / equation	20%	11	Shows complete curved-arrow mechanisms for sigmatropic rearrangement with orbital overlap visualization; writes correct Diels-Alder concerted mechanism; explains rate-determining orbital interactions for diene reactivity ranking	Shows basic mechanistic understanding but misses orbital symmetry control or concerted nature; incomplete explanation of stereochemical outcome origins	Missing mechanisms entirely, or shows stepwise radical/ionic pathways for concerted pericyclic reactions; no orbital interaction discussion
Numerical accuracy	20%	10	All four Woodward-Fieser calculations correct with proper base values, exact increment tallies (homodienyl +39, exocyclic +5, alkyl +5, etc.), and final λ_max values within ±2 nm of literature	Correct base values but errors in increment counting or solvent corrections; final values within ±5-10 nm; partial credit for correct methodology	Incorrect base values, missing increments entirely, or values off by >15 nm; no demonstration of rule application
Diagram / structure	20%	10	Clear drawn structures for all compounds A-D with stereochemistry; accurate HOMO orbital diagrams for thermal [3,3]-sigmatropic showing constructive overlap; correct conrotatory/disrotatory orbital drawings with hydrogen positions labeled	Structures drawn but stereochemistry ambiguous or missing; orbital diagrams present but symmetry labels incorrect; some products missing	No structures drawn, or chemically impossible connectivity; missing orbital diagrams where explicitly required; illegible or irrelevant diagrams
Application context	20%	10	Connects acetone UV data to analytical applications (carbonyl detection); relates Diels-Alder to natural product synthesis (steroid construction, Indian pharmaceutical examples); explains sigmatropic rearrangements in terpene biosynthesis; cites solvent effects on λ_max for practical spectroscopy	Brief mention of synthetic utility without specific examples; generic statements about pericyclic reactions in organic synthesis	No application context provided; purely theoretical treatment without connection to laboratory practice or industrial relevance

Q7

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