(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 followin

Question

(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 $\xrightarrow{h
u}$
(ii) CH₃COCOOC₂H₅ + CH₃OH $\xrightarrow{h
u}$ (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)

UPSC Answer Check · Accepted Answer

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

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	20%	10	Demonstrates flawless understanding across all parts: correct base pairing in DNA/RNA with Hoogsteen vs Watson-Crick distinction; accurate photochemical reaction type identification (Norrish, Paternò-Büchi, photo-Fries); proper application of Woodward-Fieser rules with correct parent values and increments; precise use of chemical equivalence and symmetry operations for NMR analysis	Shows generally correct concepts with minor errors: one base structure incorrect or missing; confuses photochemical with thermal mechanisms; applies UV rules with wrong parent values or missed increments; miscounts NMR signals by overlooking simple symmetry elements	Fundamental conceptual errors: DNA/RNA bases confused or structures grossly wrong; thermal mechanisms written for photochemical reactions; random λ_max assignments without rules; NMR signals counted as number of hydrogens without considering equivalence
Mechanism / equation	20%	10	Complete mechanistic detail for (b), (c)(i) and (c)(ii): radical initiation by n→π* excitation, clear propagation steps with arrow pushing, termination products identified; balanced equations with proper stoichiometry; excited state multiplicity (S1/T1) mentioned where relevant for photochemical selectivity	Basic mechanistic outline present: shows starting materials and products with some intermediate structures; arrow pushing partially correct but missing key steps like intersystem crossing or radical recombination; equations unbalanced or missing light quanta notation	No mechanisms or incorrect mechanisms: writes polar mechanisms for photochemical reactions; missing radical intermediates; products guessed without justification; no indication of photochemical initiation step
Numerical accuracy	15%	7.5	Precise λ_max calculations for all five pairs in (d) using Woodward-Fieser rules: correct parent diene/enone values, accurate increment addition for alkyl substituents, exocyclic double bonds, homoannular dienes, and solvent corrections; reasonable δ values predicted for NMR in (e) based on empirical data	Approximate numerical values: correct trend identification in UV (which is higher) without exact calculation; some increment values wrong but final comparison correct; NMR chemical shift ranges broadly correct but imprecise	No numerical values or grossly wrong: random λ_max assignments without calculation basis; NMR chemical shifts in wrong order or unrealistic values (e.g., alkyl protons at 10 ppm)
Diagram / structure	25%	12.5	All structures drawn with precision: purine/pyrimidine rings with correct numbering and tautomeric forms; 3D perspective for chiral centers in (b) and (c); clear depiction of conjugated systems for UV analysis; labeled proton environments for NMR with integration indicated; hydrogen bonds shown in DNA stability comparison	Most structures recognizable but lacking detail: flat 2D representations only; missing stereochemistry at chiral centers; conjugation indicated but not clearly drawn; NMR labeling present but incomplete or ambiguous	Poor or missing structures: skeletal formulas without clarity; wrong connectivity; no attempt at stereochemistry; illegible or chemically impossible drawings; structures completely omitted
Application context	20%	10	Rich contextual integration: DNA stability linked to genetic information storage and evolutionary significance; photochemical reactions connected to synthetic applications (e.g., Barton reaction, vitamin D synthesis); UV spectroscopy related to analytical method validation; NMR applications in structure elucidation of natural products; Indian research context (e.g., IISc work on photochemistry, CDRI drug studies) mentioned where appropriate	Basic context provided: mentions biological importance of DNA/RNA; notes photochemistry as synthetic method; acknowledges spectroscopy as analytical tool; no specific research or Indian examples	No application context: purely theoretical treatment; missing biological relevance of nucleic acids; no mention of why these spectroscopic methods matter; isolated facts without connecting to real-world chemistry

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Directive word: Predict

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