(a) Write the structure of the product(s) and the intermediate formed in the following reaction:
Cl₂CH—COCl 1) Et₃N → ?
2) Cyclopentadiene, Heat
(10 marks)

(b) Deduce the structure of the starting material (A) and all the intermediates formed in eac

Question

(a) Write the structure of the product(s) and the intermediate formed in the following reaction:
Cl₂CH—COCl 1) Et₃N → ?
2) Cyclopentadiene, Heat
(10 marks)

(b) Deduce the structure of the starting material (A) and all the intermediates formed in each step that would lead to the formation of the following product through the defined reactions:
1) NaNH₂, EtI
A 2) Lindlar's cat., H₂ → OMe
3) NBS, ROOR, Δ
4) NaOMe, MeOH
(10 marks)

(c) A photochemical reaction takes place through T₁ state. S₀–S₁ and S₀–T₁ energy gaps correspond to 290 nm and 450 nm, respectively. To get an efficient photochemical reaction should we use light of 290 nm or 450 nm? Give your answer presenting the relevant Jablonski diagram.
(10 marks)

(d) (i) Which of the following molecules is/are active to rotational spectroscopy and why?
CH₄, H₂O, NH₃, BCl₃, XeF₄
(5 marks)

(ii) The spacing between lines in the microwave spectrum of CO decreases by substituting ¹²C by ¹³C. Why?
(5 marks)

(e) (i) In a 100 MHz NMR instrument, a particular set of protons absorbs at δ = 3.0 with J = 4.5 Hz. Find the chemical shift (in Hz) and the coupling constant J in a 500 MHz instrument for the same set of protons.
(5 marks)

(ii) The mass spectrum of n-butyl phenyl ketone (C₆H₅COCH₂CH₂CH₂CH₃) shows peaks at m/z 162, 120, 105 and 85. Predict the fragmentation pattern.
(5 marks)

UPSC Answer Check · Accepted Answer

Solve each sub-part systematically with clear structural drawings and calculations. Allocate approximately 15-18 minutes for parts (a) and (b) each (mechanistic organic chemistry), 12-15 minutes for part (c) (photochemistry with Jablonski diagram), 10-12 minutes for part (d) (rotational spectroscopy theory), and 8-10 minutes for part (e) (NMR calculations and mass fragmentation). Begin with structures/equations, show stepwise reasoning, and conclude with clear final answers for each sub-part.
- Part (a): Formation of dichloroketene via α-elimination with Et₃N, followed by [2+2] cycloaddition with cyclopentadiene to give bicyclic adduct; identify both ketene intermediate and final product structure
- Part (b): Retro-synthetic analysis revealing A as a terminal alkyne (propargyl alcohol derivative), with intermediates including alkynide anion, cis-alkene from Lindlar reduction, allylic bromide from NBS, and final SN2' product; correct stereochemistry at each step
- Part (c): Selection of 450 nm light for efficient T₁ photochemistry (direct S₀→T₁ excitation or sensitized pathway); Jablonski diagram showing S₀, S₁, T₁ states with correct energy gaps, ISC, and radiative/non-radiative pathways
- Part (d)(i): Rotational activity requires permanent dipole moment; H₂O and NH₃ active (asymmetric rotors), CH₄, BCl₃, XeF₄ inactive (zero dipole due to symmetry)
- Part (d)(ii): Explanation via reduced mass μ = m₁m₂/(m₁+m₂) increasing with ¹³C, causing decrease in rotational constant B = h/(8π²cI) and hence line spacing 2B
- Part (e)(i): Chemical shift scales with frequency (1500 Hz at 500 MHz), J remains constant at 4.5 Hz (field-independent)
- Part (e)(ii): Mass fragmentation via α-cleavage and McLafferty rearrangement: m/z 162 (M⁺), 120 (loss of C₃H₆), 105 (C₆H₅CO⁺), 85 (C₆H₁₃⁺ or rearrangement ion)

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	22%	13	Demonstrates flawless understanding across all sub-parts: correctly identifies ketene formation and pericyclic selection rules in (a), recognizes Lindlar's specificity for cis-alkenes in (b), explains ISC efficiency vs direct excitation in (c), applies rotational selection rules (μ≠0) and reduced mass effects in (d), and distinguishes frequency-dependent vs independent NMR parameters in (e)	Shows partially correct concepts with minor errors: recognizes ketene but confuses cycloaddition regioselectivity; understands Lindlar reduction but misassigns stereochemistry; knows T₁ is triplet but confuses excitation wavelength selection; identifies some dipolar molecules in (d) but misses symmetry arguments; gets chemical shift scaling but confuses J dependence	Fundamental conceptual errors: describes wrong reaction type for (a), fails to recognize alkyne chemistry in (b), cannot explain why 450 nm is preferred over 290 nm, states wrong selection criterion for rotational spectroscopy, or claims J scales with spectrometer frequency
Mechanism / equation	20%	12	Provides complete mechanistic arrows for α-elimination in (a), stepwise electron-pushing for each transformation in (b) including alkynide alkylation and allylic substitution, writes proper rate equations for photochemical processes, and shows clear fragmentation mechanisms with curved arrows for mass spectral pathways in (e)(ii)	Shows mechanisms with some arrow-pushing but incomplete detail: sketches ketene formation without showing Et₃N role, outlines reaction sequence in (b) without intermediate structures, describes photochemical process qualitatively without equations, presents fragmentation patterns without mechanistic arrows	Absent or incorrect mechanisms: no arrow-pushing shown, wrong mechanism type proposed (e.g., SN1 for obvious SN2 conditions), or completely misidentifies reaction classes (e.g., electrophilic addition instead of pericyclic)
Numerical accuracy	16%	10	Calculates chemical shift precisely as 1500 Hz (δ × 500 MHz) and confirms J = 4.5 Hz in (e)(i); correctly derives rotational constant ratio ~0.96 for ¹³CO vs ¹²CO in (d)(ii) using reduced mass calculation; performs energy/wavelength conversions accurately if needed for (c)	Shows correct formulas but arithmetic errors: calculates chemical shift scaling correctly but makes calculator error, sets up reduced mass ratio correctly but computes final B ratio incorrectly, or gives approximate values without showing work	Major numerical errors: claims chemical shift is 3.0 Hz at 500 MHz, states J becomes 22.5 Hz, or completely omits calculations where required; wrong wavelength-energy conversions affecting photochemical reasoning
Diagram / structure	22%	13	Draws clear structures for ketene intermediate and bicyclo[2.2.1] adduct in (a); shows all intermediates A through final product with correct stereochemistry in (b); presents complete Jablonski diagram with labeled S₀, S₁, T₁, vibrational levels, ISC, fluorescence, phosphorescence, and absorption arrows at 290 nm and 450 nm in (c); sketches molecular geometries for rotational analysis	Draws most structures correctly but with deficiencies: shows final product in (a) but omits ketene intermediate, presents some intermediates in (b) with vague connectivity, draws simplified Jablonski diagram missing key transitions or labels	Poor or absent structural representations: ambiguous line drawings, wrong ring sizes, missing double bonds, no Jablonski diagram, or diagrams that contradict written answers; illegible or chemically impossible structures
Application context	20%	12	Connects ketene cycloaddition to synthetic utility in natural product synthesis (e.g., gibberellin or taxol precursors); relates Lindlar reduction to industrial partial hydrogenation; discusses ISC efficiency in photodynamic therapy applications; links rotational spectroscopy to astrochemistry (detecting interstellar NH₃, H₂O) and NMR to pharmaceutical analysis (Indian drug quality control); references Indian contributions where relevant	Mentions practical relevance superficially: notes ketenes are reactive intermediates, states Lindlar gives cis-alkenes for synthesis, acknowledges rotational spectroscopy identifies molecules, or mentions NMR is used in industry without specific Indian or contemporary examples	No application context provided: treats all problems as purely academic exercises with no mention of synthetic utility, industrial relevance, or connection to analytical methods in research/industry; fails to recognize spectroscopic methods as tools for real-world problem solving

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