Q5 50M Compulsory explain Organic spectroscopy and reaction mechanisms
(a) How many signals would you expect in the ¹H NMR spectrum of above compounds ? Mark these protons. 10 marks
(b) Compare the C=C stretching vibrations in the above compounds and give a suitable explanation for your answer. 10 marks
(c) (i) Identify A and B. The polymerisation of the rearranged product of B, unaided by any catalyst, gives rise to a 'synthetic rubber'. Name this rubber along with its structural formula.
2 CH≡CH →[Cu₂Cl₂/NH₄Cl] A →[HCl] B
(ii) When rubber balls and other objects made of rubber are exposed to the air for long periods of time, they turn brittle and crack. This does not happen to objects made of polyethylene. Explain. 10 marks
(d) Reduction of camphor with LiAlH₄ leads to 90% of the isomer in which the OH group is cis to the bridge. Give a suitable explanation of this observation. 10 marks
(e) Identify the products in the above reactions with plausible mechanism.
(i) CH₃—C(=O)—CH₃ →[hν/Vapour phase]
(ii) CH₃—C(=O)—CH₃ →[hν/Room temperature] 10 marks
हिंदी में पढ़ें
(a) आप निम्न यौगिकों के ¹H NMR स्पेक्ट्रम में कितने शीर्ष (सिग्नल) का अनुमान लगाते हैं ? इन प्रोटोनों को चिह्नित कीजिए । 10
(b) निम्न यौगिकों में C=C तनन आवृत्ति की तुलना कीजिए तथा अपने उत्तर का औचित्य सिद्ध करने के लिए उपयुक्त स्पष्टीकरण दीजिए । 10
(c) (i) निम्नलिखित अभिक्रिया अनुक्रम में A तथा B यौगिकों को पहचानिए । किसी भी उत्प्रेरक की सहायता के बिना B के पुनर्व्यवस्थित उत्पाद के बहुलकन से एक सिंथेटिक रबड़ बनता है । इस रबड़ का संरचनात्मक सूत्र सहित नाम लिखिए ।
2 CH≡CH →[Cu₂Cl₂/NH₄Cl] A →[HCl] B
(ii) जब रबड़ की गेंदों और रबड़ से बनी अन्य वस्तुओं को ज्यादा समय के लिए हवा के संपर्क में रखा जाता है, तो वे भंगुर हो जाती हैं और टूट जाती हैं । पॉलीथीन से बनी वस्तुओं के साथ ऐसा नहीं होता है । व्याख्या कीजिए । 10
(d) कपूर (camphor) को LiAlH₄ द्वारा अपचयन करने पर 90% मात्रा में एक ऐसा समावयव मिलता है जिसमें हाइड्रोक्सल (OH) सेतु के समीप है। इस अवलोकन के लिए एक उपयुक्त स्पष्टीकरण दें। 10
(e) निम्न अभिक्रियाओं में उत्पादों की पहचान कीजिए व क्रियाविधि लिखिए।
(i) CH₃—C(=O)—CH₃ →[hν/Vapour phase]
(ii) CH₃—C(=O)—CH₃ →[hν/Room temperature] 10
Answer approach & key points
Explain the spectroscopic, mechanistic, and synthetic aspects across all seven sub-parts with balanced coverage: allocate ~15% each to (a) NMR signals and (b) IR vibrations; ~25% to (c) acetylene chemistry and rubber degradation; ~20% to (d) camphor stereochemistry; and ~25% to (e) photochemical mechanisms. Begin with clear structural diagrams, proceed with systematic analysis using chemical principles, and conclude with real-world applications where relevant.
- For (a): Correct number of ¹H NMR signals with proton labeling on given compounds, explaining chemical equivalence and splitting patterns
- For (b): Comparison of C=C stretching frequencies with explanation based on conjugation, ring strain, and substituent effects on bond strength
- For (c)(i): Identification of A as vinylacetylene (CH₂=CH-C≡CH) and B as chloroprene (2-chloro-1,3-butadiene), with neoprene rubber structure and polymerization
- For (c)(ii): Explanation of rubber oxidation via allylic hydrogen abstraction and peroxide formation versus polyethylene stability due to saturated backbone
- For (d): Stereochemical outcome of LiAlH₄ reduction of camphor favoring endo alcohol (isoborneol) via steric approach control and Cieplak model
- For (e)(i): Vapor phase photolysis of acetone giving biacetyl and methane via Norrish Type II cleavage
- For (e)(ii): Room temperature photolysis giving pinacol via radical coupling in liquid phase with different cage effects
Q6 50M solve Spectroscopy, polymer chemistry and organic structure determination
(a)(i) The frequencies of vibration of the following molecules in their v = 0 states are HCl : 2885 cm⁻¹; D₂ : 2990 cm⁻¹; DCl : 1990 cm⁻¹ and HD : 3627 cm⁻¹. Calculate the energy change of the following reaction : HCl + D₂ → DCl + HD. Determine whether energy is liberated or absorbed.
[Given : h = 6·626 × 10⁻³⁴ Js, c = 2·998 × 10⁸ ms⁻¹, Nₐ = 6·022 × 10²³ mol⁻¹] 10 marks
(a)(ii) The IR spectra of butyric acid and ethyl butyrate show sharp strong singlet absorption at 1725 cm⁻¹ and 1740 cm⁻¹, respectively. By contrast, the IR spectrum of butyric anhydride shows a broad, sharp doublet at 1750 cm⁻¹ and 1825 cm⁻¹. Why are these so different ? 5 marks
(b)(i) Write the structure of product(s) in the above reactions : 10 marks
(b)(ii) What is meant by 'Tacticity' of a polymer ? Distinguish among isotactic, syndiotactic and atactic polymers. 5 marks
(c)(i) An organic compound having molecular formula C₁₆H₂₅NO gave following IR and ¹H NMR data : IR(cm⁻¹) = 1690; ¹H NMR(CDCl₃, 400 MHz) : δ 1·11(t, J = 7Hz, 6H), 1·29(d, J = 7Hz 6H), 2·40 (q, J = 7Hz, 4H), 2·55(t, J = 7Hz, 2H), 2·65(t, J = 7 Hz, 2H), 3·12(septet, 1H), 7·21(d, J = 8Hz, 2H), 7·81(d, J = 8Hz, 2H). Determine the structure of the compound. 10 marks
(c)(ii) Assign and arrange the lettered protons in the increasing order of their chemical shift value in ¹H NMR spectrum. 10 marks
हिंदी में पढ़ें
(a)(i) ब्यूटिरिक एसिड व एथिल ब्यूटिरेट अवरक्त (IR) स्पेक्ट्रम में क्रमशः: 1725 cm⁻¹ व 1740 cm⁻¹ पर एक प्रबल अवशोषण दिखाते हैं। इसके विपरीत, ब्यूटिरिक एनहाइड्राइड अवरक्त (IR) स्पेक्ट्रम में 1750 cm⁻¹ व 1825 cm⁻¹ पर द्विक अवशोषण दिखाता है। ये इतने अलग क्यों हैं ? 5
(b)(i) निम्नलिखित अभिक्रियाओं में उत्पाद(ओं) की संरचना लिखिए : 10
(b)(ii) बहुलक की टैक्टिसिटी (व्यवस्था) से क्या अभिप्राय है ? समव्यवस्थ (आइसोटैक्टिक), एकान्तर व्यवस्थ (सिन्डियोटैक्टिक) तथा अव्यवस्थ (एटैक्टिक) बहुलकों के बीच अंतर बताएं। 5
(c)(i) एक कार्बनिक यौगिक जिसका आण्विक सूत्र C₁₆H₂₅NO है वह निम्नलिखित IR व ¹H NMR आँकड़ा देता है : IR(cm⁻¹) = 1690; ¹H NMR(CDCl₃, 400 MHz) : δ 1·11(t, J = 7Hz, 6H), 1·29(d, J = 7Hz 6H), 2·40 (q, J = 7Hz, 4H), 2·55(t, J = 7Hz, 2H), 2·65(t, J = 7 Hz, 2H), 3·12(septet, 1H), 7·21(d, J = 8Hz, 2H), 7·81(d, J = 8Hz, 2H). यौगिक की संरचना निर्धारित कीजिए। 10
(c)(ii) अक्षरों से चिह्नित प्रोटोनों को उनके ¹H NMR स्पेक्ट्रम में रासायनिक सृति मानों के अनुसार आरोही क्रम में लिखें। 10
Answer approach & key points
Solve this multi-part spectroscopy and polymer chemistry problem by allocating approximately 35% time to part (a) covering vibrational energy calculations and IR interpretation, 25% to part (b) on reaction products and tacticity definitions, and 40% to part (c) involving complete structure elucidation from spectral data. Begin each sub-part with clear identification of the chemical principle involved, show all calculations with proper units, draw unambiguous structures with stereochemistry where relevant, and conclude with explicit answers to each directive.
- For (a)(i): Apply zero-point energy formula E = ½hcν̃ for each molecule, calculate ΔE = [E(DCl) + E(HD)] - [E(HCl) + E(D₂)], convert to kJ/mol using Avogadro's number, and correctly identify energy liberation (exothermic)
- For (a)(ii): Explain Fermi resonance in anhydrides (coupling of C=O stretch with overtone of C-O stretch), symmetric/asymmetric stretching modes, and contrast with isolated C=O in esters/acids
- For (b)(i): Draw correct product structures for unspecified reactions (typically Grignard, reduction, or substitution sequences common in UPSC papers) with proper stereochemistry
- For (b)(ii): Define tacticity as stereochemical arrangement of substituents; distinguish isotactic (same side), syndiotactic (alternating), and atactic (random) with 3D representations
- For (c)(i): Deduce structure as N,N-diisopropyl-4-ethylbutyrylbenzamide or similar amide from IR (1690 cm⁻¹, amide C=O), molecular formula, and complete NMR analysis including coupling patterns and integration
- For (c)(ii): Assign all lettered protons and arrange in order: methyl/methylene (δ 0.9-2.8) < methine (δ 3.1) < aromatic (δ 7.2-7.8), citing shielding/deshielding effects
Q7 50M draw Nucleic acids, photochemistry and organic spectroscopy
(a) (i) Draw the structure of 2'-deoxycytidine-3'-monophosphate.
(ii) Why nucleotides and nucleic acids are acidic ? Mention the factor(s) that stabilize the DNA duplex.
(iii) Show the hydrogen bonding in between cytosine and guanine base pair. 15
(b)(i) Identify all the possible products in the above reaction showing the sequential pathways of their formation : 10
(b)(ii) 1,3-Butadiene solution on irradiation with UV light follows path I. However, irradiating 1,3-butadiene in the presence of benzophenone at 366 nm follows path II. Give a suitable explanation. 5
(c)(i) Write the structure of the compounds (A), (B), (C), (D) and (E). 10
(c)(ii) From the reaction sequence below, answer the following questions :
(I) Write structure of (A) and (B)
(II) Write the oxidation state of Osmium in the intermediate (A) and its geometry. 5
(c)(iii) Write the structure of (A) and (B) in the above reactions. 5
हिंदी में पढ़ें
(क) (i) 2'-डिऑक्सीसाइटिडिन-3'-मोनोफॉस्फेट की संरचना लिखिए।
(ii) न्यूक्लिओटाइड तथा न्यूक्लिक अम्ल अम्लीय क्यों होते हैं ? DNA डबल हेलिक्स संरचना को स्थिर करने वाले कारकों का उल्लेख कीजिए।
(iii) साइटोसीन व गुआनिन शार युग्म के मध्य हाइड्रोजन आबंधन को दिखाइए। 15
(ख)(i) निम्नलिखित अभिक्रिया में सभी संभावित उत्पादों की पहचान करें जो उनके गठन का अनुक्रमिक मार्ग दिखाते हैं। 10
(ख)(ii) 1,3-ब्यूटाडाइन विलयन पराबैंगनी किरण पर पथ I का अनुसरण करता है। हालांकि बेंजोफेनॉन की उपस्थिति में 366 nm पर 1,3-ब्यूटाडाइन का पराबैंगनी किरण पथ II का अनुसरण करता है। उपयुक्त स्पष्टीकरण दीजिए। 5
(ग)(i) यौगिकों (A), (B), (C), (D) तथा (E) की संरचना लिखिए। 10
(ग)(ii) नीचे दिए गए अभिक्रिया क्रम से, निम्नलिखित प्रश्नों का उत्तर दें :
(I) (A) तथा (B) की संरचना लिखिए ।
(II) मध्यवर्ती (A) में ऑस्मियम की ऑक्सीकरण अवस्था तथा ज्यामिति लिखिए । 5
(ग)(iii) निम्नलिखित अभिक्रियाओं में (A) तथा (B) की संरचना लिखिए । 5
Answer approach & key points
Begin with (a) parts (i)-(iii) on nucleic acids (15 marks), spending ~30% time on accurate structural drawings and explanations of acidity/stabilization. For (b)(i)-(ii) photochemistry (15 marks), allocate ~30% time to identify products with mechanistic pathways and explain photosensitization by benzophenone. Devote ~40% time to (c) spectroscopy/organometallic problems (20 marks), solving all five structures in (c)(i) and the osmium complex in (c)(ii)-(iii) with correct oxidation states and geometries. Use clear sequential numbering for all structures.
- For (a)(i): Correct Haworth projection of 2'-deoxycytidine-3'-monophosphate showing β-configuration at C-1', 2'-deoxy (no OH), phosphate at 3'-position, and cytosine base
- For (a)(ii): Explanation that phosphate groups (pKa ~1-2) make nucleotides acidic; DNA duplex stabilization via Watson-Crick H-bonding, base stacking (van der Waals), and hydrophobic effects in aqueous medium
- For (a)(iii): Accurate depiction of three hydrogen bonds between cytosine (N-3, O-2, N-4) and guanine (N-1, N-2, O-6) with correct donor-acceptor geometry
- For (b)(i)-(ii): Identification of electrocyclic ring closure products (4π conrotatory thermal vs photochemical); explanation of benzophenone as triplet sensitizer enabling intersystem crossing to T1 state of butadiene for disrotatory closure
- For (c)(i): Spectroscopic deduction of five compounds using IR, NMR, and MS data with correct functional group identification and structural elucidation
- For (c)(ii)-(iii): Osmium tetroxide dihydroxylation mechanism showing cyclic osmate ester intermediate with Os(VI) oxidation state and trigonal bipyramidal/octahedral geometry; correct diol products
Q8 50M solve Spectroscopy: mass, UV, rotational, NMR and IR
(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 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
Answer approach & key points
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.
- (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)