(a) Calculate the value of λ_max in the following compounds using Woodward-Fieser rules : [structures A, B, C] (15 marks)

(b) Predict the structure of X, Y and Z in the following sequence of reactions : [reaction scheme with structures] (15 marks)

Question

(a) Calculate the value of λ_max in the following compounds using Woodward-Fieser rules : [structures A, B, C] (15 marks)

(b) Predict the structure of X, Y and Z in the following sequence of reactions : [reaction scheme with structures] (15 marks)

(c) (i) Given below are the NMR spectral characteristics of two isomeric compounds with molecular formula C₁₀H₁₂O₂ :
(1) ¹H NMR : δ 2·0 (3H, s), 2·93 (2H, t), 4·3 (2H, t), 7·3 (5H, s)
(2) ¹H NMR : δ 1·23 (3H, t), 3·72 (2H, s), 4·13 (2H, q), 7·3 (5H, s)
Both of these compounds exhibit a peculiar peak in IR spectra at 1730 cm⁻¹. Deduce the structures of these two compounds. (10 marks)

(ii) In the mass spectra of compounds I and II, prominent peaks at m/z 58 and m/z 92 are observed, respectively. Write the structures of the fragment ions and discuss their formation :
I : [structure], m/z 58
II : [structure], m/z 92 (10 marks)

UPSC Answer Check · Accepted Answer

Begin with the directive 'calculate' for part (a), applying Woodward-Fieser rules systematically for each enone/dienone structure. Allocate approximately 35% time to part (a) due to its 15 marks, 30% to part (b) for reaction sequence elucidation, 20% to part (c)(i) for NMR/IR spectral interpretation, and 15% to part (c)(ii) for mass fragmentation mechanisms. Structure the answer with clear sub-headings for each part, showing stepwise calculations first, then structural deductions with spectral reasoning, and concluding with fragmentation pathway diagrams.
- Part (a): Correct application of Woodward-Fieser rules—base value identification, increment addition for substituents (alkyl, exocyclic double bond, extended conjugation), and final λ_max calculation for each compound
- Part (b): Logical deduction of structures X, Y, and Z through analysis of reagents, reaction conditions, and stereochemical outcomes in the given sequence
- Part (c)(i): Structure elucidation of C₁₀H₁₂O₂ isomers—identification of phenylacetate ester vs. benzyl acetate from NMR splitting patterns and IR carbonyl stretch
- Part (c)(ii): McLafferty rearrangement mechanism for m/z 58 fragment from compound I and retro-Diels-Alder or α-cleavage pathway for m/z 92 from compound II
- Spectral correlation: Integration of IR (1730 cm⁻¹ ester), ¹H NMR (chemical shift, multiplicity, integration), and MS fragmentation data for unambiguous structure proof
- Numerical precision: Correct arithmetic in Woodward-Fieser calculations and accurate mass-to-charge ratio assignments in fragmentation analysis

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	20%	10	Demonstrates flawless understanding of Woodward-Fieser rules for dienones/enones; correctly identifies ester carbonyl from IR 1730 cm⁻¹; recognizes McLafferty rearrangement and retro-Diels-Alder fragmentation; applies (n+1) rule for NMR multiplicity analysis in both isomers	Shows basic understanding of spectroscopic principles but misapplies one increment in Woodward-Fieser or confuses ester vs. ketone carbonyl; identifies some but not all fragmentation pathways correctly	Fundamental errors in rule application (e.g., wrong base value), misidentifies functional groups from IR, or proposes impossible fragmentation mechanisms violating mass conservation
Mechanism / equation	20%	10	Clearly writes McLafferty rearrangement with six-membered cyclic transition state for compound I; depicts retro-Diels-Alder or α-cleavage with electron-pushing arrows for compound II; shows all curved arrows in reaction mechanism for part (b) intermediates	Shows fragmentation products correctly but arrow-pushing is incomplete or ambiguous; mechanism for part (b) lacks clarity in electron flow but reaches correct structures	Omits mechanisms entirely, shows incorrect bond cleavages, or violates electron-pushing conventions; no attempt at explaining how fragments form
Numerical accuracy	20%	10	All Woodward-Fieser calculations arithmetically correct with proper unit (nm); m/z values match calculated exact masses; integration ratios in NMR analysis numerically consistent; no calculation errors in any part	Minor arithmetic errors in one calculation (e.g., incorrect increment total) but method is sound; m/z values approximately correct but exact masses not verified	Multiple calculation errors, wrong final λ_max values, or impossible m/z assignments; NMR integration mismatches ignored or miscalculated
Diagram / structure	20%	10	All structures (A, B, C, X, Y, Z, two C₁₀H₁₂O₂ isomers, fragment ions) drawn clearly with proper stereochemistry; fragmentation pathways illustrated with structural diagrams showing bond cleavages; neat, labeled diagrams with correct bond angles	Structures present but stereochemistry ambiguous or missing; fragmentation shown as equations without structural diagrams; hand-drawn quality acceptable but lacks precision	Missing structures, incorrect connectivity (e.g., wrong substitution pattern on benzene ring), or illegible diagrams; no attempt to visualize fragmentation pathways
Application context	20%	10	Correlates spectral data logically across multiple techniques (UV-Vis, IR, N¹HMR, MS) for unambiguous identification; explains why phenylacetate shows singlet at δ 3.72 vs. benzyl acetate's distinct splitting; discusses diagnostic value of each spectral region	Treats each spectral technique in isolation without cross-validation; limited explanation of why particular structural features produce observed spectral features	No integration of spectroscopic data; presents answers as disconnected facts without demonstrating how structure follows from evidence; fails to explain significance of key peaks

Q7

Directive word: Calculate

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