Chemistry 2025 Paper II 50 marks Justify

Q8

(a) (i) Estimate the expected splitting (coupling constant J in Hz) for the lettered protons in the ¹H NMR spectrum of the following compounds: (1) A, (2) B, (3) C (5 marks) (ii) Compare the chemical shifts of the labelled protons Hᵃ and Hᵇ in the ¹H NMR spectrum of the following compounds and justify your answer. (10 marks) (iii) Count the number of peaks observed in the ¹H NMR spectrum of the following compounds. Justify your answer: (1), (2) (5 marks) (b) (i) A halogenated ester shows M⁺ peak at m/z 166 (10%) and M+2 peak at m/z 168 (9·8%) in mass spectrum. ¹H NMR spectrum of this compound shows two triplets and a singlet at δ 2·9, 3·6 and 3·8 ppm, respectively in the intensity ratio 1:1:1·5. Deduce the structure of the compound. Justify your answer. (10 marks) (ii) Two isomeric alkenes with same molecular formula C₆H₁₂ show strong peaks at m/z 42 and 56 in the mass spectrum. Propose fragmentation pattern for both the peaks. (5 marks) (c) (i) (1) Phthalic acid diethyl ester shows a characteristic peak at m/z 149 in the mass spectrum. Account for the observance of this peak by fragmentation pattern. (2) The mass spectrum of ethylbenzene shows a characteristic peak at m/z 91 while n-propylbenzene shows strong peak at m/z 92. Explain with the help of fragmentation pattern. (10 marks) (ii) An unknown organic compound with molecular formula C₄H₅NO₂ displays a band at 2250 cm⁻¹ and a strong band at 1740 cm⁻¹ in the IR spectrum. The compound shows only two signals in 3:2 ratio in the ¹H NMR spectrum. Find out the structure of the compound. Justify your answer. (10 marks)

हिंदी में प्रश्न पढ़ें

(क) (i) निम्नलिखित यौगिकों के ¹H NMR स्पेक्ट्रम में अक्षरित प्रोटोनों के लिए अपेक्षित विपाटन (युग्मन स्थिरांक J, Hz में) का अनुमान लगाइए: (1) A, (2) B, (3) C (5 अंक) (ii) निम्नलिखित यौगिकों के ¹H NMR स्पेक्ट्रम में Hᵃ एवं Hᵇ अंकित प्रोटोनों की रासायनिक सृति की तुलना कीजिए तथा अपने उत्तर का औचित्य सिद्ध कीजिए। (10 अंक) (iii) निम्नलिखित यौगिकों के ¹H NMR स्पेक्ट्रम में दिखने वाले शिखरों की संख्या की गणना कीजिए। अपने उत्तर का औचित्य सिद्ध कीजिए: (1), (2) (5 अंक) (ख) (i) एक हैलोजीनेटेड एस्टर द्रव्यमान स्पेक्ट्रम में M⁺ शिखर m/z 166 (10%) पर और M+2 शिखर m/z 168 (9·8%) पर दर्शाता है। ¹H NMR स्पेक्ट्रम में यह यौगिक दो त्रिक व एक एकल क्रमशः δ 2·9, 3·6 और 3·8 ppm पर तीव्रता अनुपात 1:1:1·5 में दर्शाता है। यौगिक की संरचना का निगमन कीजिए। अपने उत्तर का औचित्य सिद्ध कीजिए। (10 अंक) (ii) दो समावयवी एल्कीन, जिनका समान आणविक सूत्र C₆H₁₂ है, द्रव्यमान स्पेक्ट्रम में प्रबल शिखर m/z 42 तथा 56 पर दर्शाते हैं। दोनों शिखरों के खण्ड प्रतिरूप को प्रस्तावित कीजिए। (5 अंक) (ग) (i) (1) थैलिक एसिड डाइएथिल एस्टर द्रव्यमान स्पेक्ट्रम में एक अभिलाक्षणिक शिखर m/z 149 पर दर्शाता है। खंडन प्रतिरूप दर्शाते हुए इस शिखर के प्रेक्षण का कारण बताइए। (2) एथिलबेंज़ीन द्रव्यमान स्पेक्ट्रम में एक अभिलाक्षणिक शिखर m/z 91 पर दर्शाता है, जबकि n-प्रोपिलबेंज़ीन m/z 92 पर प्रबल शिखर दर्शाता है। खंडन प्रतिरूप की सहायता से इसे समझाइए। (10 अंक) (ii) एक अज्ञात कार्बनिक यौगिक, जिसका आण्विक सूत्र C₄H₅NO₂ है, IR स्पेक्ट्रम में 2250 cm⁻¹ पर एक बैंड तथा 1740 cm⁻¹ पर एक प्रबल बैंड दर्शाता है। यह यौगिक ¹H NMR स्पेक्ट्रम में केवल दो सिग्नल 3:2 के अनुपात में दर्शाता है। यौगिक की संरचना का पता लगाइए। अपने उत्तर का औचित्य सिद्ध कीजिए। (10 अंक)

Evaluate your answer to this question → See all 2025 Chemistry questions

Directive word: Justify

This question asks you to justify. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

See our UPSC directive words guide for a full breakdown of how to respond to each command word.

How this answer will be evaluated

Approach

Begin with a concise introduction stating the principles of NMR spectroscopy (chemical shift, coupling, integration) and mass spectrometry (fragmentation, isotope patterns). For part (a), allocate ~20% time on coupling constants with typical J values (7-10 Hz for vicinal, 12-18 Hz for geminal), ~35% on chemical shift comparisons using anisotropic effects and electronegativity, and ~15% on peak counting with symmetry analysis. For part (b), spend ~18% on halogen identification via M/M+2 ratio (Br), ester deduction from NMR patterns, and alkene fragmentation mechanisms. For part (c), dedicate ~12% to McLafferty rearrangement in phthalate esters and tropylium ion formation in alkylbenzenes, concluding with ~10% on IR/NMR structure elucidation for the nitrile-ester compound. End with a brief synthesis statement on spectroscopic complementarity.

Key points expected

  • Part (a)(i): Estimate J values for vicinal (6-8 Hz), geminal (12-14 Hz), and long-range couplings (0-3 Hz) with correct multiplicity prediction using n+1 rule
  • Part (a)(ii): Compare Hᵃ and Hᵇ chemical shifts using anisotropic effects of C=O, C=C, aromatic ring currents, and electronegativity substituent effects
  • Part (a)(iii): Count distinct proton environments considering molecular symmetry elements (planes, centers of inversion) and diastereotopic protons
  • Part (b)(i): Identify bromine from M:M+2 ≈ 1:1 ratio, deduce ethyl bromoacetate structure matching integration ratio 2:2:3 for CH₂Br/CH₂/OCH₂CH₃
  • Part (b)(ii): Propose McLafferty rearrangement (m/z 56) and allylic cleavage (m/z 42) for C₆H₁₂ alkene isomers (2-hexene and 3-hexene or methylcyclopentane variants)
  • Part (c)(i): Explain m/z 149 from phthalic anhydride ion after ethyl loss, and m/z 91 (tropylium) vs m/z 92 (protonated toluene) via β-cleavage and hydride rearrangement
  • Part (c)(ii): Assign 2250 cm⁻¹ to C≡N and 1740 cm⁻¹ to ester C=O, deduce ethyl cyanoacetate (NC-CH₂-COOCH₂CH₃) from 3:2 integration ratio

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness22%12Correctly applies shielding/deshielding concepts for (a)(ii), identifies anisotropic zones of carbonyl and aromatic rings; recognizes McLafferty rearrangement and tropylium ion stability for (c)(i); correctly assigns nitrile and ester functionalities in (c)(ii)States general trends but confuses anisotropic effects or misidentifies fragmentation types; partial recognition of functional groups from IR dataFundamental errors like confusing chemical shift with coupling constant, or claiming M+2 indicates chlorine instead of bromine
Mechanism / equation20%11Draws clear fragmentation mechanisms with curved arrows for phthalate anhydride formation, tropylium ion generation, and McLafferty rearrangement; shows radical cation intermediates in mass spec fragmentsDescribes fragmentation outcomes without arrow-pushing mechanisms; mentions rearrangement without showing hydrogen transferNo mechanistic detail or incorrect bond cleavages proposed; confuses fragmentation with chemical reaction mechanisms
Numerical accuracy18%10Provides accurate J values: vicinal 6-8 Hz, geminal 12-14 Hz, allylic 1-3 Hz; correct mass calculations confirming C₄H₅NO₂; precise m/z values for all fragments; correct integration ratio analysisApproximate J values without ranges; minor errors in mass calculations or fragment assignmentsWildly incorrect J values (e.g., 50 Hz for vicinal), wrong molecular formula calculations, or ignores M+2 intensity data
Diagram / structure22%12Draws all structures clearly: correct ester with bromine position, accurate alkene isomers, phthalic anhydride fragment structure, tropylium ion resonance forms, and final cyanoacetate structure with correct connectivityStructures drawn but with minor errors in connectivity or missing stereochemical considerations where relevantMissing structures, incorrect functional group placement, or illegible drawings; no attempt to show fragmentation structures
Application context18%10Demonstrates how spectroscopic data complement each other: uses IR to confirm functional groups, NMR for connectivity, MS for molecular formula and substructures; cites relevance to pharmaceutical analysis (e.g., phthalates as plasticizers, drug metabolite identification)Treats each spectroscopic method in isolation without integration; generic mention of analytical applicationsNo contextual linkage between techniques; fails to justify structural assignments with cross-method evidence

Practice this exact question

Write your answer, then get a detailed evaluation from our AI trained on UPSC's answer-writing standards. Free first evaluation — no signup needed to start.

Evaluate my answer →

More from Chemistry 2025 Paper II