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

Question

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.

UPSC Answer Check · Accepted Answer

Begin with a brief introduction acknowledging the multi-part nature of this organic chemistry problem. For part (a), deduce structures A, B, and C for both sequences showing clear mechanistic reasoning—allocate ~35% effort here. For part (b), discuss solvent compatibility and differential reactivity of LiAlH₄ vs NaBH₄ with specific solvent examples (THF, ether, alcohols, water), explaining hydride nucleophilicity and metal ion effects—allocate ~30% effort. For part (c), systematically deduce the structure using DBE calculation, IR interpretation (O-H stretch), MS fragmentation (loss of CH₃ to give m/z 135), and detailed NMR analysis including coupling patterns and D₂O exchange—allocate ~35% effort. Conclude with a summary table of structures and reagent preferences.
- Part (a) Sequence I: Correct identification of A as benzyllithium (or lithiated styrene derivative), B as the Diels-Alder adduct from 1,3-butadiene, and C as the hydrolyzed alcohol product; Sequence II: Correct identification of A as oxime, B as Beckmann rearrangement product (amide), and C as hydrolyzed carboxylic acid/amine products
- Part (b): Explanation that LiAlH₄ requires aprotic solvents (THF, dry ether) due to violent reaction with protic solvents, while NaBH₄ tolerates protic solvents (alcohols, even water); factors include Al³⁺ vs Na⁺ Lewis acidity, hydride hardness/softness, and reducing power differences
- Part (b): Preferred reagent selection—NaBH₄ for selective reduction of aldehydes/ketones in protic media; LiAlH₄ for comprehensive reduction of esters, carboxylic acids, amides, nitriles; specific mention of chemoselectivity in complex molecule synthesis
- Part (c): Correct DBE calculation (4 degrees of unsaturation), IR assignment of 3464 cm⁻¹ to hydrogen-bonded O-H (phenolic/alcoholic), MS fragmentation pattern showing loss of 15 Da (methyl) giving base peak at m/z 135
- Part (c): Complete NMR interpretation—isopropyl group (δ 1.3 d, 6H; δ 3.4 m, 1H), methyl ketone or methyl aromatic (δ 2.4 s, 3H), exchangeable proton (δ 4.6 s, phenolic OH), ABX or meta-disubstituted aromatic pattern (δ 6.6, 6.8, 7.1); final structure identified as thymol (2-isopropyl-5-methylphenol) or carvacrol isomer
- Part (c): Stereochemical and positional reasoning—integration matches, coupling constants consistent with meta-coupling (~2 Hz) and ortho-coupling (~8 Hz), confirming substitution pattern on aromatic ring

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	20%	10	Demonstrates flawless understanding of organolithium chemistry, Diels-Alder regioselectivity, Beckmann rearrangement mechanism, hydride reducing agent chemistry, and spectroscopic principles; correctly applies IUPAC nomenclature and stereochemical concepts throughout	Shows basic understanding of most concepts but makes minor errors in mechanism directionality, solvent compatibility, or spectroscopic interpretation; some confusion between LiAlH₄ and NaBH₄ scope or misassigns one NMR signal	Fundamental misconceptions evident—confuses oxime geometry in Beckmann rearrangement, assigns wrong hydride reactivity order, or completely misinterprets NMR multiplicity; fails to recognize phenolic vs alcoholic OH in IR
Mechanism / equation	20%	10	Provides complete curved-arrow mechanisms for lithiation, Diels-Alder cycloaddition with stereochemistry, Beckmann rearrangement with migratory aptitude explanation, and clear rationale for reagent selection; all equations balanced with proper conditions	Writes most mechanisms correctly but misses key details—omits stereochemistry in Diels-Alder, shows Beckmann without explaining anti-periplanar requirement, or gives unbalanced equations; mechanisms present but incomplete	Mechanisms missing or fundamentally wrong—shows carbocation intermediates where not applicable, incorrect electron flow in Beckmann rearrangement, or no mechanistic rationale for reagent preference; equations unbalanced or absent
Numerical accuracy	15%	7.5	Accurate DBE calculation (C₁₀H₁₄O: DBE = 4), correct mass fragmentation analysis (150 → 135 confirms methyl loss), precise chemical shift predictions within ±0.2 ppm, and correct integration ratios; all numerical data interpreted correctly	Correct DBE but minor errors in mass spec interpretation or chemical shift assignment; integration ratios understood but coupling constant analysis approximate; one numerical error in molecular formula verification	Incorrect DBE calculation, misinterprets m/z 135 as molecular ion, chemical shifts completely misassigned, or ignores integration data; numerical analysis essentially absent or consistently wrong
Diagram / structure	25%	12.5	All nine structures (A, B, C for both sequences plus final deduced structure) drawn with clear stereochemistry, proper bond angles, and correct connectivity; includes transition state for Diels-Alder if relevant; structures labeled unambiguously with IUPAC names	Most structures correct but stereochemistry omitted or ambiguous in Diels-Alder adduct; connectivity correct but bond angles poor; final structure correct but missing substituent positions or drawn unclearly	Multiple incorrect structures, missing stereochemistry where critical, or structures not drawn (only named); final structure completely wrong or not attempted; poor presentation making evaluation difficult
Application context	20%	10	Cites specific applications—LiAlH₄ in Indian pharmaceutical industry (e.g., synthesis of artemisinin derivatives), NaBH₄ in green chemistry protocols; connects thymol structure to natural sources (Thymus vulgaris, Indian ajwain) and biological activity; discusses industrial solvent safety	Mentions general applications of reducing agents in organic synthesis without specific Indian/industrial examples; notes natural product origin of final compound but lacks detail; some context present but underdeveloped	No application context provided; fails to connect reagents to real-world use or natural product significance; answer purely academic without mention of pharmaceutical, agricultural, or industrial relevance

Q7

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