(a) Write the structures of nucleosides and nucleotides, and discuss the primary structures of DNA and RNA. (10 marks)
(b) Write down the products obtained after photolysis of 2-methylcyclohexanone in solution phase. Explain the formation of products

Question

(a) Write the structures of nucleosides and nucleotides, and discuss the primary structures of DNA and RNA. (10 marks)
(b) Write down the products obtained after photolysis of 2-methylcyclohexanone in solution phase. Explain the formation of products. (10 marks)
(c) (i) By using appropriate reactants, reagents and conditions and using acetylene as starting material, how will you synthesize the following compound?
(ii) Identify the products W, X, Y and Z in the following reactions: (10 marks)
(d) (i) How will you distinguish between NH stretching absorption of a primary amine and a secondary amine by using IR spectroscopy?
(ii) How will you distinguish among primary, secondary and tertiary alcohols on the basis of PMR spectroscopy? (8 marks)
(e) Given below are the IR and NMR spectral characteristics of three compounds:
(i) IR : 1750 cm⁻¹; NMR : δ 2·0 (s, 3H), 5·1 (s, 2H) and 7·3 (s, 5H)
(ii) IR : 1740 cm⁻¹; NMR : δ 3·5 (s, 3H), 3·6 (s, 2H) and 7·4 (s, 5H)
(iii) IR : 3200–2800 (various bands) and 1700 cm⁻¹; NMR : δ 2·75 (t, 2H), 2·95 (t, 2H), 7·4 (s, 5H) and 12·0 (s, 1H)
Match each of these spectral data with one of the following structures: (1), (2), (3), (4), (5) (10 marks)

UPSC Answer Check · Accepted Answer

This multi-part question requires balanced coverage across 8 sub-parts totaling 48 marks. Allocate approximately 20% time to part (a) on nucleic acids, 20% to part (b) on photochemistry, 20% to part (c) on synthesis and reaction identification, 17% to part (d) on spectroscopic distinctions, and 23% to part (e) on spectral interpretation. Begin each sub-part with clear structural diagrams, follow with mechanistic or explanatory text, and conclude with specific applications or distinctions requested.
- Part (a): Correct structures of nucleoside (base + sugar) and nucleotide (base + sugar + phosphate); DNA vs RNA primary structure differences in sugar (deoxyribose vs ribose) and bases (T vs U)
- Part (b): Norrish Type I cleavage products from 2-methylcyclohexanone; biradical intermediate formation and subsequent fragmentation pathways in solution phase
- Part (c)(i): Multi-step synthesis from acetylene using appropriate reagents (NaNH₂, alkyl halides, hydration, oxidation, etc.); (c)(ii): Identification of products W, X, Y, Z with correct structures
- Part (d)(i): IR distinction of 1° vs 2° amines by NH stretching (doublet vs singlet, ~3400-3300 cm⁻¹ region); (d)(ii): PMR distinction using OH proton exchange, coupling patterns, and chemical shift differences
- Part (e): Correct matching of spectral data to structures—(i) ester with benzyl group, (ii) methyl ester with CH₂, (iii) carboxylic acid with broad OH and characteristic α-protons

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	22%	11	Precise definitions for (a): nucleoside vs nucleotide linkage chemistry; for (d): correct IR frequency ranges and PMR exchange phenomena; for (e): accurate interpretation of all spectral features including multiplicity and integration	Basic definitions correct but missing subtle distinctions like 2'-OH in RNA or conflating NH stretching frequencies; partial spectral interpretation	Fundamental errors such as confusing nucleoside with nucleotide, wrong IR regions for functional groups, or incorrect NMR splitting pattern analysis
Mechanism / equation	20%	10	For (b): detailed Norrish Type I mechanism showing α-cleavage, biradical formation, and subsequent decarbonylation or recombination pathways with electron-pushing arrows; for (c): complete synthetic scheme with all reagents and conditions	Correct products identified for (b) but incomplete mechanistic explanation; synthetic steps present but missing key reagents or conditions in (c)	Incorrect mechanism type (confusing Type I with Type II) or missing biradical intermediate; synthetic route chemically implausible or missing critical steps
Numerical accuracy	16%	8	Exact chemical shift values quoted for (d) and (e): 1° amine NH ~3400-3300 cm⁻¹ (doublet), 2° amine ~3350-3310 cm⁻¹ (singlet); precise δ values matched to structural environments in spectral problems	Approximate values given with minor deviations; correct trends but imprecise numerical recall for IR frequencies or chemical shifts	Grossly incorrect numerical values (e.g., IR carbonyl at 1500 cm⁻¹) or failure to use numerical data in spectral assignments
Diagram / structure	22%	10	Clear, correctly drawn structures for all nucleosides/nucleotides in (a); accurate 2-methylcyclohexanone photolysis products with stereochemistry where relevant; complete synthetic schemes in (c); proper structural assignments for W, X, Y, Z; correct matched structures for (e)	Structures generally correct but missing stereochemical details, incomplete synthetic schemes, or minor errors in product identification	Missing critical structures, incorrect connectivity in nucleotide drawings, wrong photolysis products, or failure to draw structures for spectral identification
Application context	20%	9	For (d): practical spectroscopic applications in pharmaceutical analysis (e.g., distinguishing drug metabolites); for (e): logical systematic approach to spectral problem-solving with clear reasoning for each assignment; connects photochemistry to synthetic methodology	Basic applications mentioned without specific context; spectral assignments made without clear stepwise reasoning	No application context provided; spectral assignments arbitrary without justification; fails to explain why particular distinctions matter analytically

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