(a) How many signals would you expect in the ¹H NMR spectrum of above compounds ? Mark these protons. 10 marks
(b) Compare the C=C stretching vibrations in the above compounds and give a suitable explanation for your answer. 10 marks
(c) (i) Identify

Question

(a) How many signals would you expect in the ¹H NMR spectrum of above compounds ? Mark these protons. 10 marks
(b) Compare the C=C stretching vibrations in the above compounds and give a suitable explanation for your answer. 10 marks
(c) (i) Identify A and B. The polymerisation of the rearranged product of B, unaided by any catalyst, gives rise to a 'synthetic rubber'. Name this rubber along with its structural formula.
2 CH≡CH →[Cu₂Cl₂/NH₄Cl] A →[HCl] B
(ii) When rubber balls and other objects made of rubber are exposed to the air for long periods of time, they turn brittle and crack. This does not happen to objects made of polyethylene. Explain. 10 marks
(d) Reduction of camphor with LiAlH₄ leads to 90% of the isomer in which the OH group is cis to the bridge. Give a suitable explanation of this observation. 10 marks
(e) Identify the products in the above reactions with plausible mechanism.
(i) CH₃—C(=O)—CH₃ →[hν/Vapour phase]
(ii) CH₃—C(=O)—CH₃ →[hν/Room temperature] 10 marks

UPSC Answer Check · Accepted Answer

Explain the spectroscopic, mechanistic, and synthetic aspects across all seven sub-parts with balanced coverage: allocate ~15% each to (a) NMR signals and (b) IR vibrations; ~25% to (c) acetylene chemistry and rubber degradation; ~20% to (d) camphor stereochemistry; and ~25% to (e) photochemical mechanisms. Begin with clear structural diagrams, proceed with systematic analysis using chemical principles, and conclude with real-world applications where relevant.
- For (a): Correct number of ¹H NMR signals with proton labeling on given compounds, explaining chemical equivalence and splitting patterns
- For (b): Comparison of C=C stretching frequencies with explanation based on conjugation, ring strain, and substituent effects on bond strength
- For (c)(i): Identification of A as vinylacetylene (CH₂=CH-C≡CH) and B as chloroprene (2-chloro-1,3-butadiene), with neoprene rubber structure and polymerization
- For (c)(ii): Explanation of rubber oxidation via allylic hydrogen abstraction and peroxide formation versus polyethylene stability due to saturated backbone
- For (d): Stereochemical outcome of LiAlH₄ reduction of camphor favoring endo alcohol (isoborneol) via steric approach control and Cieplak model
- For (e)(i): Vapor phase photolysis of acetone giving biacetyl and methane via Norrish Type II cleavage
- For (e)(ii): Room temperature photolysis giving pinacol via radical coupling in liquid phase with different cage effects

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	20%	10	Demonstrates flawless grasp of NMR equivalence, IR frequency-structure relationships, alkyne coupling chemistry, rubber autoxidation, camphor stereochemistry, and photochemical reaction types; correctly identifies all compounds and explains selectivity	Shows reasonable understanding of most concepts but makes minor errors in compound identification, misassigns stereochemical outcomes, or confuses Norrish Type I/II pathways	Fundamental misconceptions about chemical equivalence, incorrect identification of A/B, confusion between oxidation mechanisms of rubber vs polyethylene, or major stereochemical errors
Mechanism / equation	20%	10	Provides complete, arrow-pushing mechanisms for Cu-catalyzed coupling, chloroprene formation, LiAlH₄ reduction stereochemistry, and both photochemical pathways with proper radical/intermediate structures	Shows mechanisms with some correct steps but missing key intermediates, incomplete electron flow, or incorrect stereochemical rationale for camphor reduction	Absent or severely flawed mechanisms; no understanding of how Cu₂Cl₂ facilitates dimerization, no radical mechanisms for photolysis, or completely wrong reduction pathway
Numerical accuracy	15%	7.5	Precisely states expected NMR signal counts, correct IR frequency ranges (conjugated vs isolated C=C), accurate 90:10 product ratio for camphor reduction, and proper stoichiometry in all equations	Generally correct numerical values but minor errors in signal counting or approximate IR frequencies without specific values	Major numerical errors, incorrect signal counts, confused ratios, or missing quantitative aspects entirely
Diagram / structure	25%	12.5	Clear, well-labeled structures for all compounds including camphor with bridge notation, chloroprene and neoprene repeat unit, proper depiction of endo/exo stereochemistry, and transition state representations	Adequate structures but poor stereochemical representation, missing neoprene structure, or unclear labeling of proton environments in NMR	Missing or incorrect structures, inability to draw bicyclic camphor framework, no polymer structure, or confusing/illegible diagrams
Application context	20%	10	Effectively connects neoprene to Indian synthetic rubber industry, explains real-world rubber degradation relevance to tire manufacturing, and relates photochemistry to industrial ketone chemistry; cites Cieplak model or Felkin-Anh for stereochemistry	Mentions applications superficially without depth, generic rubber discussion without Indian context, or missing connection between theory and practical material science	No application context provided, fails to explain why stereoselectivity matters, or completely ignores industrial relevance of neoprene and rubber oxidation

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