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. A

Question

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.

UPSC Answer Check · Accepted Answer

Begin by identifying products X/Y for each transformation in 8(a)(i), then explain hydroboration regioselectivity with anti-Markovnikov rationale in (a)(ii). For (b), deduce the symmetrical ketone structure from NMR/IR data and assign n→π* and π→π* transitions for acetone UV. In (c), analyze methyl propenoate NMR splitting patterns and explain Fermi resonance in chloroacetone IR. Allocate approximately 35% effort to part (a) given its dual sub-parts with mechanisms, 35% to part (b) for integrated spectroscopic reasoning, and 30% to part (c) for detailed spectral interpretation.
- For 8(a)(i): Identify X as epoxide from MCPBA oxidation; X as syn-diol from OsO₄, Y as carbonyl cleavage product from NaIO₄; X as anti-Markovnikov alcohol from BH₃/THF reduction of lactic acid derivative; X as aldehyde from CrO₃-pyridine (PCC) oxidation
- For 8(a)(ii): Predict 2-methyl-1-butanol as product; explain anti-Markovnikov regioselectivity via boron attaching to less substituted carbon due to steric and electronic factors in the four-centered transition state
- For 8(b)(i): Propose di-tert-butyl ketone or 2,2,4,4-tetramethyl-3-pentanone structure based on molecular formula C₉H₁₈O, single ¹H NMR signal indicating symmetry, and 1710 cm⁻¹ confirming saturated ketone
- For 8(b)(ii): Assign λₘₐₓ 280 nm to weak n→π* (R-band) transition and λₘₐₓ 190 nm to strong π→π* (K-band) transition in acetone carbonyl chromophore
- For 8(c)(i): Predict three signals for methyl propenoate: OCH₃ singlet (~3.7 ppm), =CH₂ doublet of doublets (~6.3 ppm), =CH- doublet of doublets (~5.8 ppm) with appropriate coupling constants
- For 8(c)(ii): Explain single carbonyl stretch in acetone (1740 cm⁻¹ region) versus two bands in chloroacetone due to Fermi resonance between C=O stretch and overtone of C-Cl stretch, enhanced by α-chlorine inductive effect

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	22%	11	Correctly identifies all reagent-specific outcomes: epoxidation, syn-dihydroxylation/cleavage, selective reduction, controlled oxidation; accurately applies anti-Markovnikov rule, symmetry analysis for NMR, and electronic transition principles; no conceptual errors in stereochemistry or regiochemistry	Identifies most products correctly but confuses reagent selectivity (e.g., PCC vs Jones oxidation) or misapplies regioselectivity explanation; partial understanding of spectroscopic principles with minor errors in transition assignment or symmetry analysis	Major conceptual errors: confuses MCPBA with OsO₄ outcomes, misidentifies hydroboration regioselectivity, proposes incorrect structures ignoring NMR symmetry, or fundamentally misunderstands n→π* vs π→π* transitions
Mechanism / equation	20%	10	Writes balanced equations with proper arrow-pushing for concerted epoxidation, cyclic osmate ester formation, hydroboration four-centered transition state; explains electron flow in carbonyl cleavage and oxidation mechanisms with clarity	Shows correct products but mechanisms lack detail or contain minor arrow-pushing errors; omits transition state geometry for hydroboration or cyclic intermediate for OsO₄; partial mechanistic explanation for regioselectivity	No mechanisms provided or fundamentally incorrect electron flow; confuses ionic with concerted mechanisms; fails to explain how borane achieves anti-Markovnikov selectivity or how NaIO₄ cleaves diols
Numerical accuracy	16%	8	Precise chemical shift predictions: OCH₃ at ~3.7 ppm, vinylic protons at 5.8-6.5 ppm with correct splitting (dd); accurate IR frequencies (1710 cm⁻¹ for ketone, 1725/1745 cm⁻¹ for chloroacetone); correct λₘₐₓ values with extinction coefficient trends	Approximate chemical shifts within ±0.5 ppm, correct IR region identification but imprecise values; knows 280 nm is weak and 190 nm is strong but cannot relate to extinction coefficients; minor numerical errors in coupling constant predictions	Wildly incorrect numerical values (e.g., aldehyde proton at 1.2 ppm, carbonyl stretch at 2000 cm⁻¹); confuses UV and IR numerical ranges; no quantitative basis for spectral interpretation
Diagram / structure	20%	10	Clear structural drawings: epoxide stereochemistry, cyclic osmate ester, four-centered hydroboration TS, 2,2,4,4-tetramethyl-3-pentanone with symmetry plane; proper splitting diagrams for methyl propenoate showing coupling tree; labeled IR correlation diagram for Fermi resonance	Structures drawn but stereochemistry omitted or ambiguous; products shown without intermediates; NMR splitting shown as simple description rather than diagram; adequate but not exemplary structural communication	No structures drawn or unrecognizable drawings; incorrect connectivity (e.g., 10 carbons for C₉ formula); impossible geometries; failure to illustrate key intermediates that explain selectivity
Application context	22%	11	Connects reagents to synthetic utility: PCC for aldehyde synthesis stopping, hydroboration-oxidation for anti-Markovnikov alcohols in natural product synthesis; relates spectroscopic methods to structure elucidation in pharmaceutical analysis (e.g., Indian drug industry QC); explains why Fermi resonance matters for functional group identification	Mentions synthetic applications in general terms without specific examples; acknowledges spectroscopy for structure determination but lacks industry or research context; superficial connection between theory and practice	No application context provided; treats reactions as isolated facts without synthetic relevance; fails to explain why chemists need to distinguish n→π* from π→π* or why IR doublets matter for structure proof

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