Q5 50M Compulsory explain Organic Chemistry - Spectroscopy and Reaction Mechanisms
(a) 2,4-Pentadione exhibits five signal in ¹H NMR spectrum. Account for the observation. Also write the approximate chemical shift. (10 marks)
(b) (i) Arrange the above monomers in order of decreasing ability to undergo anionic polymerization. (5 marks)
(ii) Draw the structures of synthetic rubber and natural rubber and discuss their configurations. (5 marks)
(c) Predict the product(s) and suggest mechanism in each of the following chemical transformations:
I. [Structure: Acetophenone] SeO₂ → ?
II. [Structure: Benzoic acid] Na, NH₃/C₂H₅OH → ? (10 marks)
(d) Arrange the compounds in the above groups for ease of Norrish type-II H-abstraction in decreasing order giving the plausible explanations.
(i) Ph — CO — CH₂CH₂CH₃ (I), Ph — CO — CH₂CH₂CH₂CH₃ (II), Ph — CO — CH₂CH₂ — CH(CH₃)₂ (III)
(ii) Ph — CO — CH₂CH₂CH₃ (I)
(e) 2-Pentanone and 3-Pentanone are structural isomers. Only one of the isomers exhibits McLafferty rearrangement. Identify and show the McLafferty fragmentation for this isomer. Also propose base peak for the other isomer, which does not exhibit the McLafferty rearrangement. (10 marks)
हिंदी में पढ़ें
(a) ¹H NMR स्पेक्ट्रम में 2,4-पेंटाडाइओन पाँच सिग्नल दर्शाता है। इस प्रेक्षण का लेखा दो व अनुमानित रासायनिक विस्थापन (केमिकल शिफ्ट) भी लिखे। (10 अंक)
(b) (i) निम्नलिखित एकलकों में ऋणायनी बहुलकन की घटती हुई क्षमता का क्रम बताएं। (5 अंक)
(ii) कृत्रिम रबड़ और प्राकृतिक रबड़ की संरचना लिखें और उनके विन्यासों की विवेचना करें। (5 अंक)
(c) निम्नलिखित रासायनिक रूपांतरणों में उत्पाद/उत्पादों का अनुमान लगाएं और प्रत्येक की क्रियाविधि का सुझाव दें।
I. [संरचना: एसीटोफीनोन] SeO₂ → ?
II. [संरचना: बेंजोइक अम्ल] Na, NH₃/C₂H₅OH → ? (10 अंक)
(d) निम्नलिखित समूहों में यौगिकों को उनके नॉरिस टाइप-II हाइड्रोजन प्रत्याकरण की सरलता के घटते हुए क्रमानुसार लिखिए व उपयुक्त व्याख्या दीजिए।
(i) Ph — CO — CH₂CH₂CH₃ (I), Ph — CO — CH₂CH₂CH₂CH₃ (II), Ph — CO — CH₂CH₂ — CH(CH₃)₂ (III)
(ii) Ph — CO — CH₂CH₂CH₃ (I)
(e) 2-पेंटेनोन और 3-पेंटेनोन संरचनात्मक समावयवी हैं। इनमें से केवल एक समावयवी मैक्लैफर्टी पुनर्विन्यास दर्शाता है। इस समावयवी की मैक्लैफर्टी खंडन को चिह्नित करें और दर्शाएं। दूसरे समावयवी, जो मैक्लैफर्टी खंडन नहीं दर्शाता है उसके आधार शिखर की प्रस्तावना करें। (10 अंक)
Answer approach & key points
Explain the spectroscopic observations and reaction mechanisms with clarity, allocating approximately 20% time to part (a) on ¹H NMR of 2,4-pentadione, 20% to part (b) on polymerization and rubber structures, 20% to part (c) on SeO₂ oxidation and Birch reduction mechanisms, 20% to part (d) on Norrish type-II photochemistry with stereoelectronic reasoning, and 20% to part (e) on McLafferty rearrangement in mass spectrometry. Begin with clear structural diagrams, proceed through mechanistic arrows and electron flow, and conclude with comparative analyses where requested.
- Part (a): Explanation of keto-enol tautomerism in 2,4-pentadione creating two distinct species; identification of five non-equivalent proton environments (two methyls in keto form, one methyl in enol form, enolic OH, and CH) with approximate δ values (enol OH ~15 ppm, enol CH ~5.5 ppm, keto CH₂ ~3.5 ppm, methyls ~2.0-2.2 ppm)
- Part (b)(i): Arrangement of monomers (isoprene, butadiene, styrene derivatives) by anionic polymerization ability based on electron-withdrawing/donating effects and carbanion stability; nitrile-substituted > carbonyl > simple alkene
- Part (b)(ii): Structures of cis-1,4-polyisoprene (natural rubber, all-cis) vs trans-1,4-polyisoprene (gutta-percha) and synthetic polybutadiene/styrene-butadiene rubber; discussion of stereoregularity and conformational properties
- Part (c): SeO₂ oxidation of acetophenone via ene mechanism to phenylglyoxal; Birch reduction of benzoic acid to 1,4-cyclohexadiene-1-carboxylic acid with electron/proton transfer steps
- Part (d): Norrish type-II ordering based on γ-hydrogen accessibility and transition state stability: III (branched, more stable 6-membered TS) > II (straight chain) > I; stereoelectronic requirement for coplanar γ-C-H with carbonyl n→π* orbital
- Part (e): 2-Pentanone shows McLafferty rearrangement (γ-hydrogen transfer to carbonyl oxygen, cleavage to m/z 58 and 42); 3-pentanone cannot form 6-membered cyclic TS, gives base peak at m/z 57 (C₃H₅O⁺ or C₄H₉⁺) via α-cleavage
- Integration of spectroscopic data with mechanistic reasoning across all parts, demonstrating mastery of organic structure determination methods
Q6 50M explain Organic Chemistry - Spectroscopy, Proteins and Photochemistry
(a)(i) The molecule obtained on treatment of acetone with dilute sodium hydroxide exhibits the following spectral data. Propose the structure of this molecule.
IR : 1620 cm⁻¹ and 1695 cm⁻¹
¹H NMR : δ 1·9(s, 3H), 2·1(s, 6H), 6·15(s, 1H) (15 marks)
(ii) Identify the compound in each of the following pairs, that can be expected to exhibit carbonyl stretching signal at higher frequency:
I. [two structures]
II. CH₃—C(=O)—O—C₂H₅ and C₆H₅—C(=O)—O—C₂H₅ (5 marks)
(b) Show salt bridge, hydrogen bond, van der Waals' interaction and disulfide bridge for stabilization of protein by choosing appropriate amino acid residues in the protein chain. (15 marks)
(c) Complete the following reactions by giving the suitable mechanisms:
I. [diagram] hv ? (5 marks)
II. [diagram] hv ? (10 marks)
हिंदी में पढ़ें
(a)(i) ऐसीटोन और तनु सोडियम हाइड्रोक्साइड की अभिक्रिया में बनने वाला अणु निम्नलिखित स्पेक्ट्रमी आँकड़ा दर्शाता है। इस अणु की संरचना लिखें।
IR : 1620 cm⁻¹ and 1695 cm⁻¹
¹H NMR : δ 1·9(s, 3H), 2·1(s, 6H), 6·15(s, 1H) (15 अंक)
(ii) निम्नलिखित प्रत्येक युगलों में उस यौगिक की पहचान करें जो उच्चतर आवृत्ति का कार्बोनिल तनन सिग्नल दिखाता है :
I. [दो संरचनाएँ]
II. CH₃—C—O—C₂H₅ और C₆H₅—C—O—C₂H₅
‖ ‖
O O (5 अंक)
(b) उपयुक्त एमीनो अम्ल का चयन करते हुए लवण सेतु, हाइड्रोजन आबंध, वान्डरवाल्स अन्योन्यक्रिया और डाइसल्फाइड सेतु द्वारा प्रोटीन के स्थायीकरण को दर्शाइए। (15 अंक)
(c) निम्नलिखित अभिक्रियाओं को उनकी क्रियाविधि के साथ पूर्ण करें :
I. [आरेख] hv ? (5 अंक)
II. [आरेख] hv ? (10 अंक)
Answer approach & key points
Explain the structural elucidation in (a)(i), comparative IR analysis in (a)(ii), protein stabilization interactions in (b), and photochemical mechanisms in (c). Allocate approximately 35% time to part (a) combined (20 marks), 30% to part (b) (15 marks), and 35% to part (c) (15 marks). Begin with clear structure proposals supported by spectral data, proceed through systematic comparison of carbonyl frequencies, detailed illustration of protein interactions with specific amino acid examples, and conclude with arrow-pushing mechanisms for photochemical transformations showing excited state chemistry.
- For (a)(i): Identify the aldol condensation product of acetone as 4-methyl-3-penten-2-one (mesityl oxide), explaining IR peaks at 1620 cm⁻¹ (C=C conjugated) and 1695 cm⁻¹ (conjugated ketone), and NMR signals including the vinylic proton at δ 6.15
- For (a)(ii): Compare carbonyl stretching frequencies based on conjugation, inductive effects, and ring strain; identify that esters with electron-withdrawing groups or less conjugation show higher frequency, and that phenyl conjugation lowers frequency in aromatic esters
- For (b): Illustrate salt bridge (Asp/Glu with Lys/Arg), hydrogen bond (Ser/Thr/Tyr with backbone carbonyl), van der Waals interaction (Ala/Val/Leu/Ile side chains), and disulfide bridge (Cys-Cys) with specific amino acid residues and their positions in protein structure
- For (c) I and II: Show photochemical mechanisms involving Norrish Type I or Type II cleavage, [2+2] cycloaddition, or cis-trans isomerization with proper excited state notation (S₁, T₁) and arrow-pushing for radical or pericyclic pathways
- Demonstrate understanding of how hydrogen bonding and conjugation affect IR frequencies, and how photochemical reactions differ from thermal reactions due to spin states and orbital symmetry
Q7 50M deduce Organic chemistry reaction mechanisms and spectroscopy
7.(a) Complete the above reaction sequence by writing the structures of A, B and C.
I. C₆H₅—CH = CH₂ n-BuLi A 1,3-butadiene B H₂O C (excess)
II. [diagram] NH₂OH A 55% H₂SO₄ B Δ C
(b) Discuss the solvent compatibility for LiAlH₄ and NaBH₄ reagents and the factors responsible for differential reactivity. Also suggest preferred reagent between the two for the above transformations.
(c) A molecule with molecular formula C₁₀H₁₄O exhibits a broad band at 3464 cm⁻¹ in IR spectrum. Its mass spectrum exhibits base peak at m/z 135 and the ¹H NMR spectrum exhibits the following signals: δ 1·3 (d, 6H); 2·4 (s, 3H), 3·4 (m, 1H), 4·6 (s, D₂O exchangeable), 6·6 (s, 1H), 6·8 (d, 1H) and 7·1 (d, 1H). Deduce the structure.
हिंदी में पढ़ें
७.(क) निम्नलिखित अभिक्रियाओं के अनुक्रमों में A, B और C की संरचना लिखकर पूर्ण करें :
I. C₆H₅—CH = CH₂ n-BuLi A 1,3-ब्यूटाडाइीन B H₂O C (excess) 1,3-butadiene
II. [diagram] NH₂OH A 55% H₂SO₄ B Δ C
(ख) LiAlH₄ और NaBH₄ अभिकर्मकों की विलायक के प्रति अनुकूलता और उनकी अवकल अभिक्रियाशीलता के उत्तरदायी कारणों की विवेचना कीजिए । निम्नलिखित रूपांतरणों के लिए इनमें से कौन सा अभिकर्मक ज्यादा अच्छा है वह भी बताएं ।
(ग) एक अणु जिसका आण्विक सूत्र C₁₀H₁₄O है वह IR स्पेक्ट्रम में एक विस्तृत बैंड 3464 सेमी⁻¹ पर दर्शाता है । इसके मास स्पेक्ट्रम में m/z 135 पर आधार शिखर और ¹H NMR स्पेक्ट्रम में निम्नलिखित सिग्नल प्रदर्शित करता है : δ 1·3 (d, 6H); 2·4 (s, 3H), 3·4 (m, 1H), 4·6 (s, D₂O विनिमय (exchangeable)), 6·6 (s, 1H), 6·8 (d, 1H) और 7·1 (d, 1H). इस अणु की संरचना करें ।
Answer approach & key points
Begin with a brief introduction acknowledging the multi-part nature of this organic chemistry problem. For part (a), deduce structures A, B, and C for both sequences showing clear mechanistic reasoning—allocate ~35% effort here. For part (b), discuss solvent compatibility and differential reactivity of LiAlH₄ vs NaBH₄ with specific solvent examples (THF, ether, alcohols, water), explaining hydride nucleophilicity and metal ion effects—allocate ~30% effort. For part (c), systematically deduce the structure using DBE calculation, IR interpretation (O-H stretch), MS fragmentation (loss of CH₃ to give m/z 135), and detailed NMR analysis including coupling patterns and D₂O exchange—allocate ~35% effort. Conclude with a summary table of structures and reagent preferences.
- Part (a) Sequence I: Correct identification of A as benzyllithium (or lithiated styrene derivative), B as the Diels-Alder adduct from 1,3-butadiene, and C as the hydrolyzed alcohol product; Sequence II: Correct identification of A as oxime, B as Beckmann rearrangement product (amide), and C as hydrolyzed carboxylic acid/amine products
- Part (b): Explanation that LiAlH₄ requires aprotic solvents (THF, dry ether) due to violent reaction with protic solvents, while NaBH₄ tolerates protic solvents (alcohols, even water); factors include Al³⁺ vs Na⁺ Lewis acidity, hydride hardness/softness, and reducing power differences
- Part (b): Preferred reagent selection—NaBH₄ for selective reduction of aldehydes/ketones in protic media; LiAlH₄ for comprehensive reduction of esters, carboxylic acids, amides, nitriles; specific mention of chemoselectivity in complex molecule synthesis
- Part (c): Correct DBE calculation (4 degrees of unsaturation), IR assignment of 3464 cm⁻¹ to hydrogen-bonded O-H (phenolic/alcoholic), MS fragmentation pattern showing loss of 15 Da (methyl) giving base peak at m/z 135
- Part (c): Complete NMR interpretation—isopropyl group (δ 1.3 d, 6H; δ 3.4 m, 1H), methyl ketone or methyl aromatic (δ 2.4 s, 3H), exchangeable proton (δ 4.6 s, phenolic OH), ABX or meta-disubstituted aromatic pattern (δ 6.6, 6.8, 7.1); final structure identified as thymol (2-isopropyl-5-methylphenol) or carvacrol isomer
- Part (c): Stereochemical and positional reasoning—integration matches, coupling constants consistent with meta-coupling (~2 Hz) and ortho-coupling (~8 Hz), confirming substitution pattern on aromatic ring
Q8 50M explain Organic chemistry transformations and spectroscopy
8.(a) (i) Write the products X/Y in the above chemical transformations.
I. MCPBA → X
II. OsO₄ → X NaIO₄ → Y
III. OH | H₃C—CH—COOH BH₃/THF X
IV. CrO₃-Pyridine CH₂Cl₂ X
(ii) Predict the product in the reaction of 2-methyl-1-butene with diborane. Account for the regioselectivity observed in the reaction.
(b) (i) A molecule with molecular formula C₉H₁₈O exhibits only one signal at δ 1·2 ppm in ¹H NMR spectrum. This also exhibits a strong absorption at 1710 cm⁻¹ in IR spectrum. Propose structure for this molecule.
(ii) The UV spectrum of acetone exhibits two signals of different intensities, one at λₘₐₓ 280 nm and the other at λₘₐₓ 190 nm. Assign corresponding electronic transitions to the observed signals.
(c) (i) Predict the number of signals, nature of the signals (s/d/t/m) and approximate chemical shifts in ¹H NMR spectrum of methyl propenoate.
(ii) Acetone exhibits only one carbonyl stretching frequency in IR spectrum whereas chloroacetone exhibits two at 1725 and 1745 cm⁻¹. Explain why.
हिंदी में पढ़ें
८.(क) (i) निम्नलिखित रासायनिक रूपांतरणों में उत्पादित यौगिकों X/Y की संरचना लिखिए :
I. MCPBA → X
II. OsO₄ → X NaIO₄ → Y
III. OH | H₃C—CH—COOH BH₃/THF X
IV. CrO₃-पिरिडीन CH₂Cl₂ X
(ii) 2-मेथिल-1-ब्यूटीन और डाइबोरेन की अभिक्रिया के उत्पाद का अनुमान लगाएं। इस अभिक्रिया में जो रीजियोसेलेक्टिविटी देखी गई उसका लेखा दें।
(ख) (i) एक अणु जिसका आणविक सूत्र C₉H₁₈O है वह ¹H NMR स्पेक्ट्रम में केवल एक सिग्नल δ 1·2 ppm पर देता है। यह IR स्पेक्ट्रम में भी एक प्रबल अवशोषण 1710 से.मी.⁻¹ पर देता है। इस अणु की संरचना अनुमानित करें।
(ii) ऐसीटोन का UV स्पेक्ट्रम विभिन्न तीव्रताओं के दो सिग्नल — एक λₘₐₓ 280 nm पर और दूसरा λₘₐₓ 190 nm पर दर्शाता है। इन सिग्नलों के संगत इलेक्ट्रॉनिक संक्रमण को अंकित करें।
(ग) (i) मेथिल प्रोपीनोएट के ¹H NMR स्पेक्ट्रम में सिग्नलों की संख्या, सिग्नलों का स्वरूप (s/d/t/m) और सन्निकट रासायनिक विस्थापन का अनुमान लगाएं।
(ii) IR स्पेक्ट्रम में ऐसीटोन केवल एक कार्बोनिल तनन आवृत्ति जबकि क्लोरोऐसीटोन दो 1725 से.मी.⁻¹ और 1745 से.मी.⁻¹ पर दर्शाती है। ऐसा क्यों, समझाइए।
Answer approach & key points
Begin by identifying products X/Y for each transformation in 8(a)(i), then explain hydroboration regioselectivity with anti-Markovnikov rationale in (a)(ii). For (b), deduce the symmetrical ketone structure from NMR/IR data and assign n→π* and π→π* transitions for acetone UV. In (c), analyze methyl propenoate NMR splitting patterns and explain Fermi resonance in chloroacetone IR. Allocate approximately 35% effort to part (a) given its dual sub-parts with mechanisms, 35% to part (b) for integrated spectroscopic reasoning, and 30% to part (c) for detailed spectral interpretation.
- For 8(a)(i): Identify X as epoxide from MCPBA oxidation; X as syn-diol from OsO₄, Y as carbonyl cleavage product from NaIO₄; X as anti-Markovnikov alcohol from BH₃/THF reduction of lactic acid derivative; X as aldehyde from CrO₃-pyridine (PCC) oxidation
- For 8(a)(ii): Predict 2-methyl-1-butanol as product; explain anti-Markovnikov regioselectivity via boron attaching to less substituted carbon due to steric and electronic factors in the four-centered transition state
- For 8(b)(i): Propose di-tert-butyl ketone or 2,2,4,4-tetramethyl-3-pentanone structure based on molecular formula C₉H₁₈O, single ¹H NMR signal indicating symmetry, and 1710 cm⁻¹ confirming saturated ketone
- For 8(b)(ii): Assign λₘₐₓ 280 nm to weak n→π* (R-band) transition and λₘₐₓ 190 nm to strong π→π* (K-band) transition in acetone carbonyl chromophore
- For 8(c)(i): Predict three signals for methyl propenoate: OCH₃ singlet (~3.7 ppm), =CH₂ doublet of doublets (~6.3 ppm), =CH- doublet of doublets (~5.8 ppm) with appropriate coupling constants
- For 8(c)(ii): Explain single carbonyl stretch in acetone (1740 cm⁻¹ region) versus two bands in chloroacetone due to Fermi resonance between C=O stretch and overtone of C-Cl stretch, enhanced by α-chlorine inductive effect