Chemistry 2023 Paper II 50 marks Solve

Q8

(a)(i) For the following compound : (i) Identify the site of initial ionization under EI conditions. (ii) Draw the structure of ion having m/z = 58 value. (iii) Calculate the m/z value of metastable ion formed during the formation of above ion. 10 (a)(ii) Draw the structural formula of a ketone with MF C₇H₁₂O that absorbs in the UV with λₘₐₓ = 249 nm. 5 (b)(i) (A) Which of the above molecule(s) will show a microwave rotational spectrum ? Justify your answer. H₂, HCl, BrF, CH₄ and H₂O (B) The first rotational absorption of ¹²C¹⁶O was observed at 3·84235 cm⁻¹ while that of ¹³C¹⁶O was observed at 3·67337 cm⁻¹. Find their rotational constants. 10 (b)(ii) Estimate the expected splitting (J in Hz) for the lettered protons in the above compounds : 5 (c)(i) In the ¹H NMR spectrum of acetyl acetone the height of the integration curve at methylene (–CH₂) signal was found to be 10 mm and that of methine (=CH) signal was 22 mm. Calculate the percentage of keto and enol form in the sample. 10 (c)(ii) How could the above pair of isomeric compounds be differentiated by mass spectrometry ? 5 (c)(iii) How many fundamental vibrational frequencies would be expected to be observed in the IR spectrum of CO₂ ? Sketch them. Identify the IR-active vibrations with justification. 5

हिंदी में प्रश्न पढ़ें

(क)(i) निम्नलिखित यौगिक में : (i) ई आई (EI) अवस्था के तहत प्रारंभिक आयनन स्थल की पहचान करें । (ii) m/z = 58 मान वाले आयन की संरचना बनाइए । (iii) उपरोक्त आयन के निर्माण के दौरान बनने वाले (metastable) मितस्थायी आयन के m/z मान की गणना करें । 10 (क)(ii) आणविक सूत्र C₇H₁₂O वाले एक कीटोन का संरचनात्मक सूत्र बनाइए जो पराबैंगनी प्रकाश में λₘₐₓ = 249 nm पर अवशोषित करता है । 5 (ख)(i) (A) निम्नलिखित में से कौन सा अणु सूक्ष्मतरंग घूर्णनात्मक स्पेक्ट्रम दिखाएगा ? अपने उत्तर का औचित्य दीजिए । H₂, HCl, BrF, CH₄ and H₂O (B) ¹²C¹⁶O का प्रथम घूर्णनात्मक अवशोषण 3·84235 cm⁻¹ पर देखा गया जबकि ¹³C¹⁶O का 3·67337 cm⁻¹ पर दिखा । उनका घूर्णनात्मक स्थिरांक ज्ञात कीजिए । 10 (ख)(ii) निम्नलिखित यौगिकों में अक्षरित प्रोटोनों के लिए अपेक्षित विपाटन (J in Hz) का अनुमान लगाइए । 5 (ग)(i) एसिटाइल एसीटोन के ¹H NMR स्पेक्ट्रम की समाकलन वक्र रेखा में मेथिलीन (–CH₂) सिग्नल की ऊँचाई 10 mm और मेथाइन (=CH) सिग्नल की ऊँचाई 22 mm पाई गई है । प्रतिदर्श में कीटो और एनोल रूप के प्रतिशत की गणना करें । 10 (ग)(ii) मास स्पेक्ट्रोमेट्री द्वारा समावयवी यौगिकों की निम्नलिखित जोड़ी को कैसे विभेदित किया जा सकता है ? 5 (ग)(iii) CO₂ के अवरक्त स्पेक्ट्रम में कितनी मौलिक कंपन आवृत्तियों की अपेक्षा की जा सकती है ? उनका आरेख (स्केच) बनाइए । ओचित्य के साथ अवरक्त सक्रिय कंपनों की पहचान करें । 5

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Approach

Solve this multi-part spectroscopy problem by allocating time proportionally to marks: spend ~40% on part (a) [15 marks], ~30% on part (b) [15 marks], and ~30% on part (c) [20 marks]. Begin with clear identification of the compound in (a)(i), then systematically work through each sub-part showing all calculations and structures. For rotational and NMR problems, state relevant formulas before substituting values. Conclude each section with boxed final answers.

Key points expected

  • (a)(i) Correct identification of ionization site under EI (lone pair on oxygen), accurate structure of m/z=58 fragment (McLafferty rearrangement product), and correct metastable ion calculation using m* = (m₂)²/m₁
  • (a)(ii) Correct structural formula of C₇H₁₂O ketone with extended conjugation matching λₘₐₓ=249 nm (Woodward-Fieser rules application)
  • (b)(i)(A) Correct selection of microwave-active molecules (HCl, BrF, H₂O) with justification based on permanent dipole moment requirement for pure rotational spectra
  • (b)(i)(B) Accurate calculation of rotational constants B for both isotopologues using ṽ = 2B(J+1) with J=0→1 transition
  • (b)(ii) Correct estimation of spin-spin coupling constants (J values) for lettered protons using typical vicinal, geminal, and long-range coupling constants
  • (c)(i) Correct calculation of keto-enol equilibrium percentages using integration ratio and the 2:1 proton count relationship
  • (c)(ii) Appropriate differentiation strategy using mass spectrometry (fragmentation patterns, McLafferty rearrangement, or molecular ion stability differences)
  • (c)(iii) Correct determination of 3N-5 = 4 fundamental vibrations for linear CO₂, with proper sketching and IR activity assignment (asymmetric stretch active, symmetric stretch and bends inactive in IR)

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%10Demonstrates flawless understanding across all sub-parts: correctly identifies EI ionization sites, applies Woodward-Fieser rules for UV prediction, explains rotational selection rules (μ≠0), understands NMR integration-keto-enol relationship, and correctly applies 3N-5 rule for CO₂ vibrationsShows basic understanding of most concepts but has minor errors in selection rules or misapplies Woodward-Fieser corrections; may confuse IR/Raman activity or misidentify microwave-active moleculesFundamental misconceptions: believes homonuclear diatomics show rotational spectra, confuses IR and Raman activity, or completely misapplies integration ratios for keto-enol calculation
Mechanism / equation20%10States and correctly applies all key equations: McLafferty rearrangement mechanism, rotational energy level formula ṽ = 2B(J+1), metastable ion equation m* = (m₂)²/m₁, and clearly explains the physical basis for each spectroscopic phenomenonUses correct equations but with incomplete explanations of underlying mechanisms; may state formulas without showing derivation or physical significanceMissing or incorrect equations; confuses formulas (e.g., uses 3N-6 for linear molecule) or fails to explain the mechanism of fragmentation or energy level transitions
Numerical accuracy20%10All calculations precise: rotational constants B(¹²C¹⁶O) = 1.92118 cm⁻¹ and B(¹³C¹⁶O) = 1.83669 cm⁻¹; metastable ion m/z correct; keto-enol percentages accurately derived from 10:22 integration ratio accounting for 2:1 proton equivalenceCorrect approach with minor arithmetic errors or incorrect significant figures; may miscalculate B values by factor of 2 or make rounding errors in final percentagesMajor calculation errors: incorrect formula substitution, wrong unit conversions, or completely wrong final answers despite correct approach
Diagram / structure20%10Clear, accurate structures: m/z=58 ion with proper arrow pushing for McLafferty rearrangement; correct ketone structure with extended conjugation; labeled CO₂ vibration modes with +/− displacement vectors; proper representation of rotational energy levelsStructures drawn but with minor errors in arrow pushing, missing stereochemistry, or unclear vibration mode diagrams; labels may be incompleteMissing essential diagrams, incorrect structures (wrong connectivity), or failure to show electron movement in fragmentation mechanisms
Application context20%10Effectively connects spectroscopic techniques to real analytical problems: explains how EI-MS identifies fragmentation pathways, how UV-Vis determines conjugation extent, how microwave spectroscopy measures bond lengths (relevant to ISRO's atmospheric studies), how NMR quantifies tautomeric equilibria, and how IR identifies molecular symmetryMentions applications but in generic terms without specific connections to the compounds or techniques in the questionNo application context provided; treats problem as purely mathematical exercise without understanding analytical purpose of each technique

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