Q5 50M Compulsory predict Biomolecules, photochemistry, spectroscopy and organic reactions
(a) Write the structures of the bases present in DNA and RNA. Compare the stability of DNA and RNA. (10 marks)
(b) Predict the structure of P, Q and R in the following sequence of reactions : (10 marks)
(c) Write down the product(s) in the following reactions :
(i) C₆H₅COCOOC₂H₅ + CH₃CHOHC₂H₅ $\xrightarrow{h\nu}$
(ii) CH₃COCOOC₂H₅ + CH₃OH $\xrightarrow{h\nu}$ (10 marks)
(d) In UV spectra of the following pairs, which compound will have higher λ_max?
(i) and
A B
(ii) and
A B
(iii) and
A B
(iv) and
A B
(v) and
A B (10 marks)
(e) In ¹H NMR spectrum of the following compounds, how many signals will be observed?
I II III
In each case, label and arrange the hydrogens in the order of increasing chemical shift. (10 marks)
हिंदी में पढ़ें
(a) डी० एन० ए० तथा आर० एन० ए० में उपस्थित बेसों की संरचना लिखिए। डी० एन० ए० तथा आर० एन० ए० की स्थिरता की तुलना कीजिए। (10 अंक)
(b) निम्नलिखित अभिक्रिया क्रम में P, Q तथा R की संरचना का अनुमान लगाइए : (10 अंक)
(c) निम्नलिखित अभिक्रियाओं में उत्पाद/उत्पादों को लिखिए :
(i) C₆H₅COCOOC₂H₅ + CH₃CHOHC₂H₅ $\xrightarrow{h\nu}$
(ii) CH₃COCOOC₂H₅ + CH₃OH $\xrightarrow{h\nu}$ (10 अंक)
(d) निम्नलिखित युग्मों के UV स्पेक्ट्रा में किस यौगिक का λ_max अधिक होगा?
(i) और
A B
(ii) और
A B
(iii) और
A B
(iv) और
A B
(v) और
A B (10 अंक)
(e) निम्नलिखित यौगिकों के ¹H NMR स्पेक्ट्रम में कितने सिग्नल दिखाई देंगे?
I II III
प्रत्येक केस में हाइड्रोजनों को लेबल कीजिए और उन्हें रासायनिक सूति (शिफ्ट) के बढ़ते क्रम में व्यवस्थित कीजिए। (10 अंक)
Answer approach & key points
The directive 'predict' in part (b) requires logical deduction of reaction intermediates and products, while other parts demand 'write', 'compare', and analytical reasoning. Allocate approximately 20% time to each sub-part (a-e) as all carry equal 10 marks. Begin with clear structural drawings for DNA/RNA bases in (a), then systematically work through the photochemical mechanisms in (b) and (c) showing radical intermediates, apply Woodward-Fieser rules for UV comparisons in (d), and conclude with careful symmetry analysis for NMR signal counting in (e). Ensure all structures are neatly drawn with proper stereochemistry indicated where relevant.
- Part (a): Structures of five nitrogenous bases (adenine, guanine, cytosine, thymine, uracil) with correct hydrogen bonding patterns; comparison of DNA vs RNA stability citing 2'-OH in ribose, base pairing (A-T vs A-U), and double helix structure
- Part (b): Identification of P, Q, R as photochemical reaction intermediates/products—likely involving Norrish Type I/II cleavage or Paternò-Büchi reaction products with correct stereochemical assignments
- Part (c)(i): Photochemical reduction product of phenylglyoxylate ester with isopropanol—pinacol-type coupling or radical addition product with proper structural representation
- Part (c)(ii): Photochemical reaction of pyruvate ester with methanol—decarbonylation or ester exchange via radical mechanism showing the α-hydroxy ester or fragmentation products
- Part (d): Application of Woodward-Fieser rules for λ_max prediction—identifying extended conjugation, auxochrome effects, and steric factors in each A vs B pair (likely enones, dienes, or aromatic systems)
- Part (e): ¹H NMR signal counting using symmetry elements—chemical shift ordering based on electronegativity, anisotropic effects, and hybridization for compounds I, II, III with δ values in ppm
Q6 50M explain Polymer chemistry, photochemistry and reaction mechanisms
(a) (i) Explain the various steps involved in benzoyl peroxide-initiated polymerization of ethylene to give polyethylene. (10 marks)
(ii) Predict the physical properties of atactic, syndiotactic and isotactic polystyrenes based on their structure. (5 marks)
(b) Write down the mechanism for the formation of compounds B and C from compound A on photoirradiation : (15 marks)
(c) Identify A, B, C and D in the following reaction sequence :
A $\xrightarrow[\text{MeOH}]{\text{NaBH}_4}$ [cyclohexanol structure] $\xrightarrow{\text{H}^+}$ B $\xrightarrow{\text{C}}$ [3-bromocyclohexene structure] $\xrightarrow{\text{mCPBA}}$ D
Write the mechanism of the first step of the above reaction sequence. (20 marks)
हिंदी में पढ़ें
(a) (i) बेंज़ॉयल परॉक्साइड द्वारा प्रारंभ किए गए एथिलीन के बहुलकन, जिसमें पॉलीएथिलीन बनता है, में सम्मिलित विभिन्न चरणों की व्याख्या कीजिए। (10 अंक)
(ii) अव्यवस्थ (एटैक्टिक), एकांतर व्यवस्थ (सिन्डियोटैक्टिक) तथा समव्यवस्थ (आइसोटैक्टिक) पॉलिस्टाइरीनों के भौतिक गुणधर्मों का उनकी संरचना के आधार पर अनुमान लगाइए। (5 अंक)
(b) यौगिक A के प्रकाशिक किरणन पर यौगिकों B तथा C के बनने की क्रियाविधि लिखिए : (15 अंक)
(c) निम्नलिखित अभिक्रिया क्रम में A, B, C तथा D की पहचान कीजिए :
उपर्युक्त अभिक्रिया क्रम में प्रथम चरण की क्रियाविधि लिखिए। (20 अंक)
Answer approach & key points
Begin with a brief introduction defining radical polymerization and stereoregularity. For part (a)(i), explain the three-step mechanism (initiation, propagation, termination) with clear radical structures; for (a)(ii), compare tacticity effects on crystallinity and Tg using diagrams. Part (b) requires detailed photochemical mechanisms with curved arrows showing excited state chemistry. Part (c) demands identification of all four compounds and a complete mechanism for NaBH4 reduction. Allocate approximately 25% time to (a)(i), 10% to (a)(ii), 30% to (b), and 35% to (c) based on marks distribution. Conclude with industrial relevance of polyethylene and polystyrene in Indian manufacturing context.
- For (a)(i): Homolytic cleavage of benzoyl peroxide to benzoyloxy radicals, then phenyl radicals; initiation by addition to ethylene; propagation with radical chain growth; termination by coupling or disproportionation
- For (a)(ii): Isotactic PS has regular packing, high crystallinity, higher Tm and mechanical strength; syndiotactic has alternating stereochemistry with moderate crystallinity; atactic is amorphous, transparent, lower Tg, used in disposable cups
- For (b): Photoirradiation of compound A (typically a carbonyl or alkene) generates excited singlet/triplet states; Norrish Type I or II cleavage, or [2+2] cycloaddition leads to products B and C with complete arrow-pushing mechanisms
- For (c): A is cyclohexanone; B is cyclohexene; C is HBr or NBS/hν; D is 3-bromocyclohexene epoxide; mechanism shows NaBH4 delivering hydride to carbonyl, alkoxide protonation, then acid-catalyzed dehydration
- For (c) mechanism: Concerted or stepwise hydride transfer, axial/equatorial stereochemistry consideration, formation of tetrahedral intermediate
- Industrial context: LDPE/HDPE production in India (Reliance, GAIL); polystyrene applications in packaging and insulation
Q7 50M calculate Spectroscopy and structure elucidation
(a) Calculate the value of λ_max in the following compounds using Woodward-Fieser rules : [structures A, B, C] (15 marks)
(b) Predict the structure of X, Y and Z in the following sequence of reactions : [reaction scheme with structures] (15 marks)
(c) (i) Given below are the NMR spectral characteristics of two isomeric compounds with molecular formula C₁₀H₁₂O₂ :
(1) ¹H NMR : δ 2·0 (3H, s), 2·93 (2H, t), 4·3 (2H, t), 7·3 (5H, s)
(2) ¹H NMR : δ 1·23 (3H, t), 3·72 (2H, s), 4·13 (2H, q), 7·3 (5H, s)
Both of these compounds exhibit a peculiar peak in IR spectra at 1730 cm⁻¹. Deduce the structures of these two compounds. (10 marks)
(ii) In the mass spectra of compounds I and II, prominent peaks at m/z 58 and m/z 92 are observed, respectively. Write the structures of the fragment ions and discuss their formation :
I : [structure], m/z 58
II : [structure], m/z 92 (10 marks)
हिंदी में पढ़ें
(a) वुडवर्ड-फीजर नियमों का उपयोग कर निम्नलिखित यौगिकों के λ_max के मान की गणना कीजिए : [संरचनाएँ A, B, C] (15 अंक)
(b) निम्नलिखित अभिक्रिया क्रम में X, Y तथा Z की संरचना का अनुमान लगाइए : [अभिक्रिया योजना संरचनाओं सहित] (15 अंक)
(c) (i) दो समावयवी यौगिकों के, जिनका आण्विक सूत्र C₁₀H₁₂O₂ है, NMR स्पेक्ट्रमी अभिलक्षण नीचे दिए गए हैं :
(1) ¹H NMR : δ 2·0 (3H, s), 2·93 (2H, t), 4·3 (2H, t), 7·3 (5H, s)
(2) ¹H NMR : δ 1·23 (3H, t), 3·72 (2H, s), 4·13 (2H, q), 7·3 (5H, s)
ये दोनों यौगिक IR स्पेक्ट्रा में 1730 cm⁻¹ पर एक विशेष शिखर दर्शाते हैं। इन दोनों यौगिकों की संरचना लिखिए। (10 अंक)
(ii) यौगिकों I तथा II के द्रव्यमान स्पेक्ट्रा में प्रमुख शिखर क्रमशः : m/z 58 तथा m/z 92 पर दर्शाते हैं। इन खंड आयनों की संरचनाएँ लिखिए तथा इनके बनने पर विवेचन कीजिए :
I : [संरचना], m/z 58
II : [संरचना], m/z 92 (10 अंक)
Answer approach & key points
Begin with the directive 'calculate' for part (a), applying Woodward-Fieser rules systematically for each enone/dienone structure. Allocate approximately 35% time to part (a) due to its 15 marks, 30% to part (b) for reaction sequence elucidation, 20% to part (c)(i) for NMR/IR spectral interpretation, and 15% to part (c)(ii) for mass fragmentation mechanisms. Structure the answer with clear sub-headings for each part, showing stepwise calculations first, then structural deductions with spectral reasoning, and concluding with fragmentation pathway diagrams.
- Part (a): Correct application of Woodward-Fieser rules—base value identification, increment addition for substituents (alkyl, exocyclic double bond, extended conjugation), and final λ_max calculation for each compound
- Part (b): Logical deduction of structures X, Y, and Z through analysis of reagents, reaction conditions, and stereochemical outcomes in the given sequence
- Part (c)(i): Structure elucidation of C₁₀H₁₂O₂ isomers—identification of phenylacetate ester vs. benzyl acetate from NMR splitting patterns and IR carbonyl stretch
- Part (c)(ii): McLafferty rearrangement mechanism for m/z 58 fragment from compound I and retro-Diels-Alder or α-cleavage pathway for m/z 92 from compound II
- Spectral correlation: Integration of IR (1730 cm⁻¹ ester), ¹H NMR (chemical shift, multiplicity, integration), and MS fragmentation data for unambiguous structure proof
- Numerical precision: Correct arithmetic in Woodward-Fieser calculations and accurate mass-to-charge ratio assignments in fragmentation analysis
Q8 50M explain Spectroscopy, photochemistry and physical chemistry
(a) Ethanolic solution of compound I on irradiation leads to the formation of compounds II, III and IV. The resulting reaction mixture exhibits bands at 1787, 1740, 1715 and 1685 cm⁻¹ for νC=O. Assign these C=O stretching frequencies to the corresponding compounds giving reasons : (15 marks)
(b) (i) 2,2-Dimethyl cyclopropanone undergoes ring opening when attacked by methoxide ion and the product obtained possesses the following spectral data :
IR (ν, cm⁻¹) : 1740, 1160
¹H NMR (δ) : 3·6 (3H, s), 1·2 (9H, s)
Mass (m/z) : 116, 85, 59, 31
Deduce the structure of the product with reasons. Write down the structure of another possible product. (10 marks)
(ii) Arrange the following compounds in the order of increasing coupling constant values (J_{Ha-Hb}) :
I
II
III (5 marks)
(c) (i) 2-Chloro-2,3-dimethyl butane on dehydrohalogenation can lead to the formation of two products. Explain how the two can be distinguished using ¹H NMR and IR spectral data. (10 marks)
(ii) The mass spectral data of diethyl ether is as under :
m/z 74, m/z 59, m/z 45, m/z 31, m/z 29
Explain the fragmentation pattern. (5 marks)
(iii) Using the following data, calculate the bond length of HCl :
I = 2·70×10⁻⁴⁷ kg m²
1 a.m.u. = 1·661×10⁻²⁷ kg (5 marks)
हिंदी में पढ़ें
(a) यौगिक I के एथेनॉलिक विलयन का किरणन करने पर यौगिक II, III तथा IV बनते हैं। परिणामी अभिक्रिया मिश्रण 1787, 1740, 1715 तथा 1685 cm⁻¹ पर कार्बोनिल (νC=O) बैंड दर्शाता है। इन C=O तरंग आवृत्तियों को संबंधित यौगिकों में निर्दिष्ट कीजिए तथा उसका कारण दीजिए : (15 अंक)
(b) (i) 2,2-डाइमेथिल साइक्लोप्रोपेनोन की मेथॉक्साइड आयन के साथ अभिक्रिया में वलय विवर्तन हो जाता है एवं बनने वाले उत्पाद का स्पेक्ट्रमी आँकड़ा निम्नलिखित है :
IR (ν, cm⁻¹) : 1740, 1160
¹H NMR (δ) : 3·6 (3H, s), 1·2 (9H, s)
Mass (m/z) : 116, 85, 59, 31
कारणों के साथ उत्पाद की संरचना निकालिए। दूसरे संभावित उत्पाद की संरचना लिखिए। (10 अंक)
(ii) निम्नलिखित यौगिकों को उनके युग्मन स्थिरांक मानों (J_{Ha-Hb}) के आरोही क्रमानुसार व्यवस्थित कीजिए :
I
II
III (5 अंक)
(c) (i) 2-क्लोरो-2,3-डाइमेथिल ब्यूटेन के विहाइड्रोहैलोजनन से दो उत्पाद बन सकते हैं। व्याख्या कीजिए कि ¹H NMR तथा IR स्पेक्ट्रमी आँकड़ों द्वारा दोनों उत्पादों का विभेदन कैसे कर सकते हैं। (10 अंक)
(ii) डाइएथिल ईथर का द्रव्यमान स्पेक्ट्रमी आँकड़ा इस प्रकार दिया गया है :
m/z 74, m/z 59, m/z 45, m/z 31, m/z 29
खंड प्रतिरूप की व्याख्या कीजिए। (5 अंक)
(iii) निम्नलिखित आँकड़ों के द्वारा HCl की आबंध लंबाई की गणना कीजिए :
I = 2·70×10⁻⁴⁷ kg m²
1 a.m.u. = 1·661×10⁻²⁷ kg (5 अंक)
Answer approach & key points
This question demands explanation of spectroscopic assignments, structural elucidation, and calculations across five sub-parts. Allocate approximately 30% time/words to part (a) [15 marks] covering Norrish Type I/II photochemistry and carbonyl stretching frequency assignments; 20% to (b)(i) [10 marks] for ring-opening mechanism and spectral interpretation; 10% to (b)(ii) [5 marks] for dihedral angle-coupling constant relationship; 20% to (c)(i) [10 marks] for elimination product distinction; and 20% combined to (c)(ii)-(iii) [10 marks] for fragmentation pattern and bond length calculation. Structure as: brief introduction on spectroscopic principles, systematic treatment of each sub-part with structures and reasoning, and concluding summary.
- Part (a): Assignment of 1787 cm⁻¹ to strained ketone (II), 1740 cm⁻¹ to ester (III), 1715 cm⁻¹ to ketone (IV), and 1685 cm⁻¹ to α,β-unsaturated ketone with explanation of ring strain, conjugation, and hydrogen bonding effects on νC=O
- Part (b)(i): Structure elucidation as methyl 3,3-dimethylbutanoate from IR (ester C=O and C-O), NMR (OCH₃ singlet, t-butyl singlet), and MS (m/z 116 molecular ion, McLafferty rearrangement to m/z 85); alternative product as methyl 2,2-dimethylpropanoate from nucleophilic attack at less hindered carbon
- Part (b)(ii): Correct order of J(Ha-Hb) based on Karplus equation: I (anti-periplanar, ~16-18 Hz) > III (gauche, ~2-4 Hz) > II (near 90°, ~0-1 Hz) or equivalent based on given structures
- Part (c)(i): Distinction between 2,3-dimethyl-2-butene (more substituted, IR weak/absent =C-H stretch, NMR vinylic H absent) and 2,3-dimethyl-1-butene (less substituted, IR =C-H stretch ~3100 cm⁻¹, NMR two vinylic H signals, coupling pattern)
- Part (c)(ii): Fragmentation pattern of diethyl ether: m/z 74 (M⁺•), m/z 59 (α-cleavage losing CH₃•), m/z 45 (α-cleavage losing C₂H₅•), m/z 31 (CH₂=OH⁺ base peak), m/z 29 (C₂H₅⁺)
- Part (c)(iii): Calculation of reduced mass μ = (1×35.5)/(1+35.5) × 1.661×10⁻²⁷ kg, then bond length r = √(I/μ) = 1.27 Å or equivalent correct calculation with proper unit conversion