Paper II — Q5
(a) ¹H NMR spectrum me 2,4-pentadione paanch signal darsata hai. Is prashn ka lekh do v anumanit rasayanik vishapan (chemical…
¹H NMR spectrum me 2,4-pentadione paanch signal darsata hai. Is prashn ka lekh do v anumanit rasayanik vishapan (chemical shift) bhi likhve. 2,4-Pentadione exhibits five signal in ¹H NMR spectrum. Account for the observation. Also write the approximate chemical shift. 10 marks
5.
Nimanlikhit ekalon me jharnayani bhulkaran ki chhati hui karyata ka kram batave. [figure] Arrange the above monomers in order of decreasing ability to undergo anionic polymerization. 5 marks
Krtrim rabz aur prakritik rabz ki sanshna likhve aur unke vinyason ki vivechana kare. Draw the structures of synthetic rubber and natural rubber and discuss their configurations. 5 marks
5.
Nimanlikhit rasayanik kshapitronon me utpad/utpadon ka anuman lagave, aur pratyek ki kriyavidhi ka sujav de. [figure] Predict the product(s) and suggest mechanism in each of the above chemical transformations. 10 marks
5.
Nimanlikhit samuhon me yogikon ko unke noris type-II haizojan puthkaran ki saralta ke chhate hue kramanusar likhve v upyuk vyaakhya dejiye. Ph-CO-CH2CH2CH3 Ph-CO-CH2CH2CH2CH3 Ph-CO-CH2CH2-CH(CH3)2 I II III [figure]
Ph-CO-CH2CH2CH3 [figure] [figure] I II III Arrange the compounds in the above groups for ease of Norrish type-II H-abstraction in decreasing order giving the plausible explanations. 5 marks
5.
2-pentanone aur 3-pentanone sanshantak samavayi hain. Inme se keval ek samavayi McLafferty punnivyas darsata hai. Is samavayi ki McLafferty khadan ko chichhit kare aur darsave. Dusera samavayi, jo McLafferty khadan nahi darsata hai uske aahar shikhar ki prastavana kare. 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 प्रश्न पढ़ें
¹H NMR स्पेक्ट्रम में 2,4-पेन्टाडाइओन पाँच सिग्नल दर्शाता है। इस प्रश्न का लेख दो व अनुमानित रासायनिक विस्थापन (केमिकल शिफ्ट) भी लिखें। 2,4-Pentadione exhibits five signal in ¹H NMR spectrum. Account for the observation. Also write the approximate chemical shift. (10 अंक)
5.
निम्नलिखित एकलों में जड़ानायनी बहुलकन की घटी हुई कार्यता का क्रम बताएं। [figure] Arrange the above monomers in order of decreasing ability to undergo anionic polymerization. (5 अंक)
कृत्रिम रबड़ और प्राकृतिक रबड़ की संरचना लिखें और उनके वinyasों की विवेचना करें। Draw the structures of synthetic rubber and natural rubber and discuss their configurations. (5 अंक)
5.
निम्नलिखित रासायनिक रूपान्तरणों में उत्पाद/उत्पादों का अनुमान लगाएं, और प्रत्येक की क्रियाविधि का सुझाव दें। [figure] Predict the product(s) and suggest mechanism in each of the above chemical transformations. (10 अंक)
5.
निम्नलिखित समूहों में यौगिकों को उनके नोरिस टाइप-II हाइड्रोजन पृथक्करण की सरलता के घटते हुए क्रमानुसार लिखिए व उपयुक्त व्याख्या दीजिए। Ph-CO-CH2CH2CH3 Ph-CO-CH2CH2CH2CH3 Ph-CO-CH2CH2-CH(CH3)2 I II III [figure]
Ph-CO-CH2CH2CH3 [figure] [figure] I II III Arrange the compounds in the above groups for ease of Norrish type-II H-abstraction in decreasing order giving the plausible explanations. (5 अंक)
5.
2-पेन्टेनोन और 3-पेन्टेनोन संरचनात्मक समावयवी हैं। इनमें से केवल एक समावयवी मैकलेफर्टी पुनर्व्यास दर्शाता है। इस समावयवी की मैकलेफर्टी खंडन को चिह्नित करें और दर्शाएं। दूसरे समावयवी, जो मैकलेफर्टी खंडन नहीं दर्शाता है उसके आधार शिखर की प्रस्तावना करें। 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 अंक)
The figure this question refers to, in words
The question paper is a scan and the diagram did not survive as text. This is the figure as read from the original page — every component, value and label — so the question can be worked from the text below.
(b) Three chemical structures of substituted styrene monomers are shown side-by-side, labeled I, II, and III. Each structure consists of a benzene ring with a vinyl group (CH=CH2) attached at the top position (position 1) and a substituent at the bottom position (position 4). Structure I has a methyl group (CH3) at the bottom. Structure II has a nitro group (NO2) at the bottom. Structure III has a methoxy group (OCH3) at the bottom.
(c) Two chemical reaction schemes are listed as I and II. Reaction I shows the structure of acetophenone (a benzene ring attached to a carbonyl group, which is attached to a methyl group) reacting with SeO2 to form an unknown product indicated by a question mark. Reaction II shows the structure of benzoic acid (a benzene ring attached to a COOH group) reacting with Na, NH3 in C2H5OH to form an unknown product indicated by a question mark.
(d) Part (i) lists three chemical structures labeled I, II, and III. Structure I is Ph-CO-CH2CH2CH3. Structure II is Ph-CO-CH2CH2CH2CH3. Structure III is Ph-CO-CH2CH2-CH(CH3)2. Part (ii) lists three structures labeled I, II, and III. Structure I is the same as in part (i): Ph-CO-CH2CH2CH3. Structure II is a bicyclic ketone: a benzene ring fused to a six-membered ring containing a ketone group (C=O) and a propyl group (-CH2CH2CH3) attached to the carbon adjacent to the carbonyl. Structure III is a bicyclic ketone: a norbornane skeleton with a ketone group (COPh) attached to the bridgehead carbon.
Model answer
Drafted with AI by UPSC Answer Check against this question's marking rubric, to the expected length. UPSC does not publish answers for Mains — this is one way to score well, not an official key. How we build rubrics and answers.
(a) ¹H NMR of 2,4-pentanedione Pentane-2,4-dione exists as an equilibrium of keto and enol tautomers, and interconversion is slow on the NMR time scale, so each form shows its own signals, five in all. The enol (about 80% neat; stabilised by conjugation and a strong intramolecular O–H···O hydrogen bond) shows three: CH₃ (6H, δ ≈ 2.0, s), vinyl =CH (1H, δ ≈ 5.5, s) and the hydrogen-bonded enolic OH (1H, δ ≈ 15–16, broad s). The keto form (about 20%) shows two: CH₃ (6H, δ ≈ 2.2, s) and the CH₂ between the carbonyls (2H, δ ≈ 3.6, s). Symmetry makes the two methyls equivalent in each tautomer. The ratio of integrals gives the enol content.
(b)(i) Order of anionic polymerisation of p-substituted styrenes Anionic polymerisation proceeds through a benzylic carbanion at the growing chain end, so electron-withdrawing groups that delocalise the negative charge accelerate it. Order: II (p-NO₂) > I (p-CH₃) > III (p-OCH₃). NO₂ stabilises the carbanion by strong –M and –I effects (charge delocalised onto the nitro oxygens). CH₃ is weakly donating (+I, hyperconjugation) and slightly destabilises. OCH₃ donates strongly by +M, intensifying the negative charge, so the carbanion is least stable and the monomer is least reactive.
(b)(ii) Natural and synthetic rubber Natural rubber (Hevea) is cis-1,4-polyisoprene: –[CH₂–C(CH₃)=CH–CH₂]ₙ– with all chains having the CH₂ groups on the same side of every C=C. The coiled, amorphous chains give elasticity. Its trans isomer (gutta-percha, balata) is hard and crystalline. Synthetic rubbers: synthetic cis-1,4-polyisoprene made with Ziegler–Natta catalyst (about 98% cis), cis-polybutadiene, SBR (styrene–butadiene copolymer, random), and neoprene (mainly trans-1,4-polychloroprene). Stereoregularity set by the catalyst controls configuration and properties.
(c) Predicting products with mechanisms I. Acetophenone with SeO₂ gives phenylglyoxal, PhCO–CHO (Riley oxidation of the active α-CH₃). The enol form attacks electrophilic selenium, a [2,3]-sigmatropic shift/loss of water follows, and the intermediate decomposes to the 1,2-dicarbonyl and Se(0) (red precipitate). II. Benzoic acid with Na/liquid NH₃ and C₂H₅OH undergoes Birch reduction. The solvated electron adds to the ring to give a radical anion, ethanol protonates it (para to the electron-withdrawing COOH), a second electron adds and the resulting carbanion is protonated again. Electron-withdrawing groups end up on the sp³ carbon, giving cyclohexa-2,5-diene-1-carboxylic acid (1,4-dihydrobenzoic acid).
(d)(i) Norrish type II H-abstraction ease Norrish type II needs the excited carbonyl oxygen to abstract a γ-hydrogen through a six-membered transition state. Ease follows the weakness of the C–H bond, III > II > I. Compound III has a tertiary γ-H (–CH(CH₃)₂), II has secondary γ-H (CH₂), and I has only primary γ-H (CH₃). The tertiary 1,4-biradical formed is the most stable.
(d)(ii) Norrish type II in cyclic and acyclic ketones Order: II > I > III. In II, γ-hydrogens are secondary (propyl CH₂), and the ring holds the C=O in a conformation from which the propyl chain can reach it through a six-membered transition state. In I, the flexible chain easily reaches, but γ-H is primary. In III (benzoylnorbornane), the rigid bicycle keeps γ-hydrogens too far and misaligned from the carbonyl oxygen, and the biradical would be strained, so abstraction is the slowest.
(e) McLafferty rearrangement 2-Pentanone (CH₃COCH₂CH₂CH₃) shows it, as it has a γ-hydrogen (on the terminal CH₃). The molecular ion (m/z 86) transfers the γ-H to the carbonyl oxygen through a six-membered transition state, then the α–β C–C bond breaks, expelling ethene (28) and leaving the enol radical cation CH₂=C(OH)CH₃⁺• at m/z 58. 3-Pentanone (CH₃CH₂COCH₂CH₃) has no γ-hydrogen, only α and β, so cannot undergo it. α-Cleavage gives the acylium ion CH₃CH₂CO⁺ at m/z 57, the base peak.
How this answer will be evaluated
What earns full marks: All parts answered with correct mechanisms, structures, and justifications.
Evaluation rubric
Each sub-part is marked on its own, against the marks and word limit printed on the paper.
- (a) Explain the five 1H NMR signals of 2,4-pentadione and their chemical shifts. 10 marks
account for— state the phenomenon → the causes in order of weight → conclusion
Must cover
- Identify the five distinct proton environments
- Assign approximate shifts: keto CH3 about 2.2 (6H), CH2 about 3.6 (2H); enol CH3 about 2.0 (6H), =CH about 5.5 (1H), enolic OH about 15-16 (1H)
- Explain keto-enol tautomerism in slow exchange, so both forms give separate signals
- Distinguish between enol and keto form protons
Loses marks
- Listing signals without explaining the cause
- Ignoring the enol form entirely
- Providing incorrect chemical shift ranges
Earns more
- Mention the symmetry of the molecule
- Note the coupling patterns if visible
- Reference the specific enol content
- Discuss the effect of conjugation on shifts
Extra mark
- Draw the enol and keto structures explicitly
- Cite the specific percentage of enol form
- (b)(i) Arrange the monomers in decreasing order of anionic polymerization ability. 5 marks
justify— claim → 3-4 reasons → evidence → conclusion
Must cover
- Provide the correct order: II > I > III
- Explain the electron-withdrawing effect of NO2
- Explain the electron-donating effect of OCH3
- Relate substituent effects to carbanion stability
Loses marks
- Incorrect order without justification
- Confusing anionic with cationic polymerization
- Ignoring the substituent effects
Earns more
- Mention the resonance structures
- Discuss the inductive effects
- Compare the stability of the transition states
Extra mark
- Draw the resonance structures for the carbanions
- Mention the specific pKa values if known
- (b)(ii) Draw structures of synthetic and natural rubber and discuss their configurations. 5 marks
describe— define → structure or process in order → labelled diagram → significance
Must cover
- Draw the structure of natural rubber (cis-1,4-polyisoprene)
- Draw the structure of synthetic rubber (e.g., SBR or cis-1,4-polyisoprene)
- Identify the cis configuration in natural rubber
- Discuss the stereochemical differences
Loses marks
- Drawing incorrect structures
- Failing to mention the cis/trans configuration
- Confusing the monomers
Earns more
- Mention the specific monomers used
- Discuss the physical properties related to configuration
- Note the industrial synthesis methods
Extra mark
- Draw the trans isomer for comparison
- Mention the specific commercial names
- (c) Predict products and suggest mechanisms for the given transformations. 10 marks
suggest— the problem in one line → implementable measures → who acts → conclusion
Must cover
- Predict the SeO2 product: phenylglyoxal (Riley oxidation of the active alpha-CH3, not allylic)
- Predict the Birch product: cyclohexa-2,5-diene-1-carboxylic acid (1,4-dihydrobenzoic acid)
- Show the mechanism for the SeO2 reaction
- Show the mechanism for the Birch reduction
Loses marks
- Predicting incorrect products
- Failing to show the mechanisms
- Ignoring the regioselectivity
Earns more
- Mention the specific intermediates
- Discuss the regioselectivity of the reactions
- Note the stereochemical outcomes
Extra mark
- Draw the transition states
- Mention the specific conditions required
- (d)(i) Arrange the compounds in decreasing order of Norrish type-II H-abstraction ease. 5 marks
justify— claim → 3-4 reasons → evidence → conclusion
Must cover
- Correct order: III > II > I (tertiary > secondary > primary gamma-H)
- Explain the role of the gamma-hydrogen
- Discuss the steric effects on H-abstraction
- Relate the structure to the reaction mechanism
Loses marks
- Incorrect order without justification
- Ignoring the steric effects
- Confusing Norrish type-I and type-II
Earns more
- Mention the specific gamma-hydrogens involved
- Discuss the stability of the resulting radical
- Note the conformational preferences
Extra mark
- Draw the transition states for H-abstraction
- Mention the specific bond dissociation energies
- (d)(ii) Arrange the compounds in decreasing order of Norrish type-II H-abstraction ease. 5 marks
justify— claim → 3-4 reasons → evidence → conclusion
Must cover
- Correct order: II > I > III (accessible secondary gamma-H; rigid norbornyl ketone cannot reach the six-membered transition state)
- Explain the role of the gamma-hydrogen
- Discuss the steric effects on H-abstraction
- Relate the structure to the reaction mechanism
Loses marks
- Incorrect order without justification
- Ignoring the steric effects
- Confusing Norrish type-I and type-II
Earns more
- Mention the specific gamma-hydrogens involved
- Discuss the stability of the resulting radical
- Note the conformational preferences
Extra mark
- Draw the transition states for H-abstraction
- Mention the specific bond dissociation energies
- (e) Identify the isomer showing McLafferty rearrangement and propose the base peak for the other. 10 marks
justify— claim → 3-4 reasons → evidence → conclusion
Must cover
- Identify 2-pentanone as the isomer showing McLafferty rearrangement
- Show the McLafferty fragmentation mechanism for 2-pentanone
- Identify 3-pentanone as the isomer not showing McLafferty rearrangement
- Propose base peak for 3-pentanone: m/z 57 (CH3CH2CO+)
Loses marks
- Identifying the wrong isomer for McLafferty rearrangement
- Failing to show the mechanism
- Proposing an incorrect base peak
Earns more
- Mention the specific m/z values
- Discuss the stability of the fragments
- Note the specific bond cleavages
Extra mark
- Draw the fragmentation pathways
- Mention the specific ion structures
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