(a)(i) The molecule obtained on treatment of acetone with dilute sodium hydroxide exhibits the following spectral data. Propose the structure of this molecule.
IR : 1620 cm⁻¹ and 1695 cm⁻¹
¹H NMR : δ 1·9(s, 3H), 2·1(s, 6H), 6·15(s, 1H) (15 marks)

(i

Question

(a)(i) The molecule obtained on treatment of acetone with dilute sodium hydroxide exhibits the following spectral data. Propose the structure of this molecule.
IR : 1620 cm⁻¹ and 1695 cm⁻¹
¹H NMR : δ 1·9(s, 3H), 2·1(s, 6H), 6·15(s, 1H) (15 marks)

(ii) Identify the compound in each of the following pairs, that can be expected to exhibit carbonyl stretching signal at higher frequency:
I. [two structures]
II. CH₃—C(=O)—O—C₂H₅ and C₆H₅—C(=O)—O—C₂H₅ (5 marks)

(b) Show salt bridge, hydrogen bond, van der Waals' interaction and disulfide bridge for stabilization of protein by choosing appropriate amino acid residues in the protein chain. (15 marks)

(c) Complete the following reactions by giving the suitable mechanisms:
I. [diagram] hv ? (5 marks)
II. [diagram] hv ? (10 marks)

UPSC Answer Check · Accepted Answer

Explain the structural elucidation in (a)(i), comparative IR analysis in (a)(ii), protein stabilization interactions in (b), and photochemical mechanisms in (c). Allocate approximately 35% time to part (a) combined (20 marks), 30% to part (b) (15 marks), and 35% to part (c) (15 marks). Begin with clear structure proposals supported by spectral data, proceed through systematic comparison of carbonyl frequencies, detailed illustration of protein interactions with specific amino acid examples, and conclude with arrow-pushing mechanisms for photochemical transformations showing excited state chemistry.
- For (a)(i): Identify the aldol condensation product of acetone as 4-methyl-3-penten-2-one (mesityl oxide), explaining IR peaks at 1620 cm⁻¹ (C=C conjugated) and 1695 cm⁻¹ (conjugated ketone), and NMR signals including the vinylic proton at δ 6.15
- For (a)(ii): Compare carbonyl stretching frequencies based on conjugation, inductive effects, and ring strain; identify that esters with electron-withdrawing groups or less conjugation show higher frequency, and that phenyl conjugation lowers frequency in aromatic esters
- For (b): Illustrate salt bridge (Asp/Glu with Lys/Arg), hydrogen bond (Ser/Thr/Tyr with backbone carbonyl), van der Waals interaction (Ala/Val/Leu/Ile side chains), and disulfide bridge (Cys-Cys) with specific amino acid residues and their positions in protein structure
- For (c) I and II: Show photochemical mechanisms involving Norrish Type I or Type II cleavage, [2+2] cycloaddition, or cis-trans isomerization with proper excited state notation (S₁, T₁) and arrow-pushing for radical or pericyclic pathways
- Demonstrate understanding of how hydrogen bonding and conjugation affect IR frequencies, and how photochemical reactions differ from thermal reactions due to spin states and orbital symmetry

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	25%	12.5	Correctly identifies mesityl oxide structure from spectral data; accurately explains that conjugation lowers carbonyl frequency; precisely describes all four protein stabilization forces with correct amino acid chemistry; demonstrates clear understanding of photochemical excited states and their reactivity differences from ground state	Identifies basic aldol product but misassigns some spectral features; understands general trend for carbonyl frequencies but lacks precision in explanation; describes most protein interactions correctly but confuses residue types or bond strengths; shows partial understanding of photochemistry with some mechanistic errors	Incorrect structure proposal or major spectral misinterpretation; fundamentally misunderstands factors affecting IR frequencies; confuses salt bridges with hydrogen bonds or disulfides; fails to distinguish photochemical from thermal mechanisms or omits excited state considerations
Mechanism / equation	25%	12.5	Writes complete aldol condensation mechanism with enolate formation and dehydration; provides clear stepwise photochemical mechanisms with proper arrow-pushing for bond cleavage/formation, radical intermediates or pericyclic transitions, and identifies reaction type (Norrish I/II, Paternò-Büchi, etc.)	Shows basic aldol mechanism but misses dehydration step or reversibility; presents photochemical mechanisms with correct products but incomplete arrow-pushing or missing intermediate structures; identifies some mechanistic steps but lacks clarity on spin states or orbital interactions	Omits mechanisms entirely or shows fundamentally incorrect electron flow; confuses acid/base catalysis in aldol; presents thermal rather than photochemical pathways; fails to show any arrow-pushing or intermediate structures
Numerical accuracy	15%	7.5	Precisely interprets all NMR chemical shifts and integration values; correctly calculates expected frequency differences based on conjugation and substituent effects; accurately predicts IR values within ±10 cm⁻¹ for comparison purposes	Correctly identifies most NMR signals but misassigns one chemical shift or integration; understands qualitative trends for IR frequencies but lacks quantitative reasoning; makes minor arithmetic errors in spectral interpretation	Major errors in NMR interpretation (e.g., confusing sextet for singlet); completely ignores numerical data in spectral analysis; makes no attempt to quantify or compare frequency values; invents incorrect chemical shift values
Diagram / structure	20%	10	Draws clear structural formulas for mesityl oxide with proper geometry; illustrates both ester structures for comparison; presents detailed protein segment showing spatial arrangement of all four interaction types with correct amino acid side chains; depicts photochemical mechanisms with clear orbital diagrams or energy level representations	Draws acceptable structures but with poor stereochemistry or missing double bonds; shows protein interactions as separate diagrams rather than integrated structure; presents photochemical steps as linear equations without orbital visualization; hand-drawn quality acceptable but lacks precision	Structures with major errors (wrong connectivity, missing functional groups); protein diagram shows only generic labels without specific residues; no diagrams for photochemical steps; illegible or chemically impossible drawings
Application context	15%	7.5	Connects spectral analysis to pharmaceutical quality control (e.g., Indian generic drug industry); relates protein stabilization to enzyme engineering or therapeutic protein stability (relevant to Indian biotechnology sector); discusses photochemistry applications in photodynamic therapy or solar energy research in Indian context	Mentions general applications without specific Indian relevance; briefly notes importance of spectroscopy in organic synthesis or protein structure in biochemistry; makes passing reference to photochemical applications without elaboration	No application context provided; fails to relate any part of the question to real-world chemistry; treats question as purely academic exercise with no mention of industrial, medicinal, or environmental relevance

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