All 8 questions from UPSC Civil Services Mains Chemistry 2021 Paper II (400 marks total). Every stem reproduced in full, with directive-word analysis, marks, word limits, and answer-approach pointers.
8Questions
400Total marks
2021Year
Paper IIPaper
Topics covered
Organic reaction mechanisms and stereochemistry (2)Stereochemistry and reaction mechanisms (1)Organic chemistry reactions and mechanisms (1)NMR, polymerization, reactions, Norrish, McLafferty (1)Organic Chemistry - Spectroscopy, Proteins and Photochemistry (1)Organic chemistry reaction mechanisms and spectroscopy (1)Organic chemistry transformations and spectroscopy (1)
A
Q1
50MCompulsoryexplainOrganic reaction mechanisms and stereochemistry
(a) In the reaction R—COO⁻ + Br—CN → R—CN + Br⁻ + CO₂↑ What is the origin of —CN group in the product ? Explain by using isotopic labelling technique. 10 marks
(b) Discuss the product(s) formation when above quaternary ammonium salt is treated with sodium amide at low temperature and at high temperature. 10 marks
(c) Write the products A and B in the above reaction. Also give the mechanism of their formation. Which one of these is the major product and why ? [Diagram: Cyclohexane ring with C(CH₃)₂OH substituent] BF₃ : OEt₂ → A + B 10 marks
(d) Discuss the reactivity of following compounds towards nucleophile in the presence of BF₃ : OEt₂ : (i) p-trifluoromethyl benzaldehyde (ii) p-tolualdehyde 10 marks
(e) Complete the above transformations. (i) (ii) 10 marks
हिंदी में पढ़ें
(a) इस अभिक्रिया में R—COO⁻ + Br—CN → R—CN + Br⁻ + CO₂↑ सायनाइड (—CN) समूह उत्पाद में कहाँ से उत्पन्न होता है ? समस्थानिक लेबलिंग प्रविधि के प्रयोग द्वारा स्पष्ट करें । 10 अंक
(b) निम्नलिखित चतुष्क अमोनियम लवण को जब सोडियम एमाइड के साथ निम्न तापमान और उच्च तापमान पर अभिक्रियत करते हैं तो उत्पादों के बनने का विवरण दें । (CH₃)₃N⁺—CH₂—C₆H₅ X⁻ 10 अंक
(c) निम्नलिखित अभिक्रिया के उत्पाद A और B लिखें । उनके बनने की क्रियाविधि भी दें । इनमें से कौन सा मुख्य उत्पाद है और क्यों ? [आरेख: साइक्लोहेक्सेन वलय C(CH₃)₂OH प्रतिस्थापक के साथ] BF₃ : OEt₂ → A + B 10 अंक
(d) निम्नलिखित यौगिकों की नाभिकस्नेही के प्रति BF₃ : OEt₂ की उपस्थिति में अभिक्रियता की विवेचना कीजिये : (i) पैरा(p)-ट्राइफ्लुओरोमेथिल बेंज़ैल्डिहाइड (ii) पैरा(p)-टॉलुएल्डिहाइड 10 अंक
(e) निम्नलिखित रूपान्तरणों को पूर्ण करें : (i) (ii) 10 अंक
Answer approach & key points
(a) explain: definition/context > points in order > small example > short close | (b) discuss: intro > 3-4 dimensions > example > balanced close | (c) explain: definition/context > points in order > small example > short close | (d) discuss: intro > 3-4 dimensions > example > balanced close | (e) describe: define > structure or process in order > labelled diagram > significance Full marks: Complete mechanisms, correct products, clear reasoning, all parts addressed
Propose isotopic labelling experiment (RC14OO- with BrCN, or Br14CN) and compare activity of nitrile and CO2
Show mechanism of nucleophilic attack on Br-CN
Identify origin of -CN group in product
Explain fate of CO2 in the reaction
Identify ylide formation at the benzylic carbon with NaNH2
(a) Involving the stereochemical concept, compare the elimination behaviour of compounds A and B in the presence of base. Also give the product(s) of the reactions. 20 marks
(b) (i) Comment upon the structure and stability of cyclopropylmethyl carbocation. (ii) Which one of the above compound is more acidic and why ? (iii) Write the product(s) in the above reaction. 15 marks
(c) (i) Indicating the stereochemistry of the product, complete the above reaction with mechanism. 10 marks (ii) Write down the products in the above reactions. 5 marks
हिंदी में पढ़ें
(a) त्रिविम रासायनिक संकल्पना को सम्मिलित करते हुए क्षार की उपस्थिति में यौगिक A और B के उन्मूलन व्यवहार की तुलना करें । अभिक्रिया के उत्पादों को भी लिखें । 20 अंक
(b) (i) साइक्लोप्रोपाइलमेथिल कार्बोकैटायन की संरचना और स्थायित्व पर टिप्पणी करें । (ii) निम्नलिखित यौगिकों में कौन सा ज़्यादा अम्लीय है और क्यों ? (iii) निम्नलिखित अभिक्रिया में उत्पाद/उत्पादों को लिखें : 15 अंक
(c) (i) उत्पाद की त्रिविम रसायन को दर्शाते हुए निम्नलिखित अभिक्रिया को क्रियाविधि देते हुए पूर्ण करें : 10 अंक (ii) निम्नलिखित अभिक्रियाओं के उत्पादों को लिखें : 5 अंक
Answer approach & key points
(a) compare: paired headings or table > key differences > significance > conclusion | (b) comment: context > arguments both sides > judgment > close | (c(i)) explain: definition/context > points in order > small example > short close | (c(ii)) enumerate: list the items in order > one line each > no commentary Full marks: Complete mechanisms with correct stereochemistry, all products drawn accurately, clear reasoning throughout.
Identify A as trans and B as cis isomer
Apply anti-periplanar requirement for E2 elimination
Show B (cis) eliminates easily from the stable chair (axial Cl, axial C2-H) to give mainly 1-methylcyclohexene (Zaitsev), minor 3-methylcyclohexene
Show A (trans) needs diaxial ring flip and, with C2-H equatorial, gives only 3-methylcyclohexene (slower)
Explain cyclopropylmethyl cation stability by sigma-pi (Walsh, bent-bond) conjugation in the bisected conformation
Identify cyclohexane-1,3-dione as more acidic: delocalised planar enolate; Bredt's rule prevents enolisation of the bridged dione
(iii) Buchner ring expansion: carbene from the diazoester adds to benzene to give norcaradiene in equilibrium with cycloheptatriene
50MexplainOrganic reaction mechanisms and stereochemistry
(a) Complete the following reactions with mechanisms:
(i) Conc. H₂SO₄ (10 marks)
(ii) EtO⁻ (10 marks)
(b) (i) Write the structure of the reaction product between 3-chlorocyclopropene and SbCl₅. What is the unique feature of this product in ¹H NMR spectrum?
(ii) Which one of the above compounds is more acidic and why? (Structures A and B shown)
(iii) Predict the aromaticity of tropolones and sydnones.
(c) Complete the following reactions along with mechanisms:
(i) meso-2,3-dibromobutane →(I⁻) A →(OEt⁻) B
(ii) CH₃—C—CHI—CH₃ →(AgNO₃, EtOH, Δ) ?
|
CH₃
हिंदी में पढ़ें
(a) निम्नलिखित अभिक्रियाओं को उनकी क्रियाविधि दर्शाते हुए पूर्ण करें :
(i) Conc. H₂SO₄ (10)
(ii) EtO⁻ (10)
(b) (i) 3-क्लोरोसाइक्लोप्रोपीन और SbCl₅ की परस्पर अभिक्रिया के उत्पाद की संरचना लिखें। इस उत्पाद की ¹H NMR स्पेक्ट्रम में अनूठी विशेषता क्या है?
(ii) निम्नलिखित यौगिकों में कौन सा ज्यादा अम्लीय है और क्यों? (A और B की संरचनाएं दिखाई गई हैं)
(iii) ट्रोपोलोन्स और सिड्नोन्स की ऐरोमैटिकता का अनुमान लगाएं।
(c) निम्नलिखित अभिक्रियाओं को उनकी क्रियाविधि के साथ पूर्ण करें :
(i) मेसो-2,3-डाइब्रोमोब्यूटेन →(I⁻) A →(OEt⁻) B
(ii) CH₃—C—CHI—CH₃ →(AgNO₃, EtOH, Δ) ?
|
CH₃
Answer approach & key points
(a(i)) describe: define > structure or process in order > labelled diagram > significance | (a(ii)) describe: define > structure or process in order > labelled diagram > significance | (b) explain: definition/context > points in order > small example > short close | (c(i)) describe: define > structure or process in order > labelled diagram > significance | (c(ii)) describe: define > structure or process in order > labelled diagram > significance Full marks: Complete mechanisms with correct stereochemistry and reasoning
Protonation of hydroxyl group
Loss of water from the diphenylcarbinol carbon to give the more stable diphenylmethyl cation
Stabilization by phenyl groups
1,2-alkyl (ring) migration to the cation with ring expansion, then loss of H+ to give 2,2-diphenylcyclohexanone
Identify tertiary alpha-chloroketone with alpha-prime hydrogens (CH3CO)
Enolate formation and intramolecular displacement of Cl to the spiro-cyclopropanone
Ethoxide attack on C=O and ring opening to the more stable (primary) carbanion
Final product ethyl 1,4-dimethylcyclohexane-1-carboxylate; cis/trans mixture
Structure of cyclopropenyl cation product
Single peak in 1H NMR due to symmetry
Comparison of acidity based on conjugate base stability
Aromaticity prediction for tropolones and sydnones
A: (E)-but-2-ene by anti-elimination (debromination) with iodide
B: (E)-2-bromobut-2-ene by anti E2 with ethoxide
Anti-periplanar geometry shown for the meso isomer (Newman projection)
Stereochemical outcome: meso gives E alkene (A) and E bromoalkene (B)
Ag+ abstracts iodide giving secondary carbocation
1,2-methyl shift gives tertiary carbocation, which ethanol attacks
Final ether 2-ethoxy-2,3-dimethylbutane (E1 alkenes minor)
50MexplainOrganic chemistry reactions and mechanisms
(a) (i) Heating of 3-deuterioindene causes scrambling of the deuterium. Explain with mechanism. (5 marks)
(ii) Write the structure of product(s) formed during the thermal reaction of maleic anhydride with cyclopentadiene. Explain with mechanism. (20 marks)
4. (b) Write the product(s) of the above reactions.
(i) (CH3)3C-CH=CH2 --HCl--> ? (5 marks)
(ii) CH2=CH-CH2-Br --HBr--> ? (10 marks)
CH2=CH-CH2-Br --HBr, benzoyl peroxide--> ?
CH2=CH-CH2-Br --HBr, benzoyl peroxide--> ?
4. (c) (i) Predict the product in the above chemical conversions and also identify the name reaction involved. (10 marks)
I. PhCH2Cl --(1) (C6H5)3P, (2) RLi, (3) [cyclohexanone structure]--> ?
II. PhNHNH2 + [cyclohexanone structure] --Glacial acetic acid--> ?
(ii) Which of the following pair(s) gives α,β-unsaturated carbonyl compound in presence of base? Justify the answer. (5 marks)
I. HCHO and PhCHO
II. PhCHO and Ph-CH-CHO (with CH3 below CH)
III. PhCHO and CH3CH2CHO
IV. HCHO and PhCOPh
SECTION 'B'
हिंदी में पढ़ें
(a) (i) 3-ड्यूटेरोइंडीन को गर्म करने पर ड्यूटेरियम का व्यमिश्रण होता है। क्रियाविधि देते हुए समझाइए। (5 अंक)
(ii) मैलेइक एनहाइड्राइड की साइक्लोपेन्टाडाइन के साथ उष्मीय अभिक्रिया में बने उत्पादों की संरचना लिखें। क्रियाविधि के साथ समझाइए। (20 अंक)
4. (b) निम्नलिखित अभिक्रियाओं के उत्पाद/उत्पादों को लिखें:
(i) (CH3)3C-CH=CH2 --HCl--> ? (5 अंक)
(ii) CH2=CH-CH2-Br --HBr--> ? (10 अंक)
CH2=CH-CH2-Br --HBr, बेंज़ॉयल परॉक्साइड--> ?
CH2=CH-CH2-Br --HBr, बेंज़ॉयल परॉक्साइड--> ?
4. (c) (i) निम्नलिखित रासायनिक रूपान्तरणों का उत्पाद लिखें और संलिप्त अभिक्रिया को पहचानते हुए नाम लिखें: (10 अंक)
I. PhCH2Cl --(1) (C6H5)3P, (2) RLi, (3) [साइक्लोहेक्सेनोन संरचना]--> ?
II. PhNHNH2 + [साइक्लोहेक्सेनोन संरचना] --ग्लेशियल ऐसिटिक अम्ल--> ?
(ii) निम्नलिखित में से कौन सा युग्म धार की उपस्थिति में, α,β-असंतुप्त कार्बोनिल यौगिक देता है? उत्तर को सिद्ध करें: (5 अंक)
I. HCHO और PhCHO
II. PhCHO और Ph-CH-CHO (CH3 नीचे CH के नीचे)
III. PhCHO और CH3CH2CHO
IV. HCHO और PhCOPh
खण्ड 'B'
Answer approach & key points
(a) explain: definition/context > points in order > small example > short close | (b(i)) explain: definition/context > points in order > small example > short close | (b(ii)) explain: definition/context > points in order > small example > short close | (c(i)) derive: given > assumptions > stepwise derivation > result > check | (c(ii)) justify: claim > 3-4 reasons > evidence > conclusion Full marks: Accurate mechanisms with correct stereochemistry and clear justification for all parts.
Concerted [1,5]-sigmatropic shift mechanism for deuterium scrambling
Diels-Alder reaction mechanism with correct arrow pushing
Endo product structure for the Diels-Alder adduct
Explanation of thermal conditions favoring the reaction
Markovnikov protonation gives the secondary carbocation (CH3)3C-CH(+)-CH3
1,2-methyl shift gives the more stable tertiary carbocation (CH3)2C(+)-CH(CH3)2
(a) 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. (b) (i) 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)
(ii) 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. (c) 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. (d) Nimanlikhit samuhon me yogikon ko unke noris type-II haizojan puthkaran ki saralta ke chhate hue kramanusar likhve v upyuk vyaakhya dejiye.
(i) Ph-CO-CH2CH2CH3 Ph-CO-CH2CH2CH2CH3 Ph-CO-CH2CH2-CH(CH3)2
I II III
[figure]
(ii) 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. (e) 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 पढ़ें
(a) 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. (b) (i) 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)
(ii) 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. (c) 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. (d) निम्नलिखित समूहों में यौगिकों को उनके नोरिस टाइप-II हाइड्रोजन पृथक्करण की सरलता के घटते हुए क्रमानुसार लिखिए व उपयुक्त व्याख्या दीजिए।
(i) Ph-CO-CH2CH2CH3 Ph-CO-CH2CH2CH2CH3 Ph-CO-CH2CH2-CH(CH3)2
I II III
[figure]
(ii) 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. (e) 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)
Answer approach & key points
(a) account for: state the phenomenon > the causes in order of weight > conclusion | (b(i)) justify: claim > 3-4 reasons > evidence > conclusion | (b(ii)) describe: define > structure or process in order > labelled diagram > significance | (c) suggest: the problem in one line > implementable measures > who acts > conclusion | (d(i)) justify: claim > 3-4 reasons > evidence > conclusion | (d(ii)) justify: claim > 3-4 reasons > evidence > conclusion | (e) justify: claim > 3-4 reasons > evidence > conclusion Full marks: All parts answered with correct mechanisms, structures, and justifications.
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
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
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
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
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
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
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+)
50MexplainOrganic 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)
50MdeduceOrganic 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
Framework: Concept > Structure or mechanism > Reasoning > Result. (a) trace: start point > the stages in sequence > end point > what changed | (b) discuss: intro > 3-4 dimensions > example > balanced close | (c) analyse: intro > causes > effects > stakeholders/linkages > way forward Full marks: Correct structures and reagents with clear reasoning; accurate spectral interpretation.
Sequence I is living anionic polymerisation: n-BuLi adds to styrene and, with excess styrene, A is polystyryl-lithium, Bu–(CH2–CHPh)n–CH2–CH(Ph)⁻ Li⁺ (a living carbanion chain end)
B: the living chain adds 1,3-butadiene to give the block copolymer polystyrene-b-polybutadienyl-lithium (1,4- and 1,2-units)
C: water protonates the chain end, terminating the living polymer: the styrene–butadiene diblock copolymer
Sequence II: A cyclohexanone oxime; B ε-caprolactam by Beckmann rearrangement in 55% H2SO4; C nylon-6 by ring-opening polymerisation of caprolactam on heating
LiAlH4: reacts violently with protic solvents, needs dry ether or THF, reduces esters, acids, amides, nitriles and, via the aluminate, propargylic alcohols; NaBH4: usable in water or alcohols, reduces only aldehydes and ketones (and acyl halides) under normal conditions
Reason for the difference: Al–H is more hydridic (Al less electronegative, larger, weaker Al–H bond) than B–H, so LiAlH4 is the stronger and less selective donor
First transformation (aryl ketone reduced with the primary amide untouched): NaBH4 in methanol or ethanol, because LiAlH4 would also reduce the amide to the amine
Second transformation (propargylic alcohol to the (E)-allylic alcohol): LiAlH4 (or Red-Al) — the alkoxide directs intramolecular trans hydroalumination, giving the trans alkene after work-up; NaBH4 does not reduce alkynes
Identify the structure as thymol (2-isopropyl-5-methylphenol), a 1,2,4-trisubstituted phenol
IR 3464 cm-1 (broad) is a hydrogen-bonded phenolic O-H
MS base peak m/z 135 is M-15 (150-15), loss of CH3 from the isopropyl group
NMR 1.3 (d, 6H) with 3.4 (m, 1H) is an isopropyl group on the ring
50MexplainOrganic 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
Framework: Concept > Structure or mechanism > Reasoning > Result. (a(i)) explain: definition/context > points in order > small example > short close | (a(ii)) account for: state the phenomenon > the causes in order of weight > conclusion | (b(i)) suggest: the problem in one line > implementable measures > who acts > conclusion | (b(ii)) explain: definition/context > points in order > small example > short close | (c(i)) derive: given > assumptions > stepwise derivation > result > check | (c(ii)) explain: definition/context > points in order > small example > short close Full marks: All products/structures correct with mechanisms and full reasoning
I: MCPBA epoxidises the ring C3=C4 double bond of 1-phenyl-1,2-dihydronaphthalene: X is the 3,4-epoxide (oxirane fused to the dihydronaphthalene), formed on the face anti to the phenyl group
II: OsO4 gives syn dihydroxylation of 1,4-dihydronaphthalene: X is the cis-2,3-diol of tetralin; NaIO4 cleaves the diol: Y is o-phenylenediacetaldehyde (2,2′-(1,2-phenylene)diacetaldehyde)
III: BH3/THF reduces the carboxylic acid selectively: 3-hydroxybutanoic acid gives butane-1,3-diol; the secondary OH survives
IV: CrO3·pyridine (Collins reagent) in CH2Cl2 performs allylic oxidation of cyclohexene: X is cyclohex-2-en-1-one
Product is 2-methyl-1-butanol
Anti-Markovnikov addition explained
Boron attaches to less substituted carbon
Steric and electronic factors discussed
Structure is 2,2,4,4-tetramethyl-3-pentanone
Formula C9H18O verified
Single NMR signal explained by symmetry
1710 cm-1 assigned to ketone C=O
280 nm assigned to n→π* transition
190 nm assigned to π→π* transition
Relative intensities explained
Orbital energy levels referenced
Four signals: OCH3 plus three non-equivalent vinylic H
Splitting correct: OCH3 singlet; each vinylic H a doublet of doublets
Chemical shifts approximate and reasonable
Integration ratio 3:1:1:1
Chloroacetone exists as two rotational isomers (conformers) seen separately in IR
C-Cl eclipsing C=O (cisoid) gives the higher band (about 1745) by dipolar/field effect; the gauche form gives the lower (about 1725)
Both conformers coexist, so two bands appear
Acetone has no such rotamers, so it shows one band