Chemistry 2025 Paper II 50 marks Elucidate

Q3

(a) (i) Write the structure of the product formed in the following reactions: (1) (2) (10 marks) (ii) Describe the synthesis of ketone (A) when only isobutanol is available as the starting material: (5 marks) (b) (i) Write the steps involved in the following conversion: (10 marks) (ii) Elucidate the structure of C and D in the following conversion: (5 marks) (c) (i) Provide the structure of missing reagents and intermediates in the following synthesis: [diagram] (10 marks) (ii) Write the structure of the product formed and the steps involved in the following reaction: [diagram] (10 marks)

हिंदी में प्रश्न पढ़ें

(a) (i) निम्नलिखित अभिक्रियाओं में बनने वाले उत्पाद की संरचना लिखिए : (1) (2) (10) (ii) कीटोन (A) के संश्लेषण का वर्णन कीजिए जब केवल आइसोब्यूटेनॉल प्रारंभिक पदार्थ के रूप में उपलब्ध हो : (5) (b) (i) निम्नलिखित रूपांतरण में सम्मिलित चरणों को लिखिए : (10) (ii) निम्नलिखित रूपांतरण में C तथा D की संरचना को स्पष्ट कीजिए : (5) (c) (i) निम्नलिखित संश्लेषण में लुप्त अभिकर्मकों तथा मध्यवर्तियों की संरचना उपलब्ध कराइए : [diagram] (10) (ii) निम्नलिखित अभिक्रिया में बने उत्पाद की संरचना तथा सम्मिलित चरणों को लिखिए : [diagram] (10)

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Directive word: Elucidate

This question asks you to elucidate. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

See our UPSC directive words guide for a full breakdown of how to respond to each command word.

How this answer will be evaluated

Approach

The directive 'elucidate' demands clear, detailed explanation with structural clarity. Allocate time proportionally: ~35% (18 minutes) for (a)(i) product structures and (a)(ii) isobutanol-to-ketone synthesis; ~20% (10 minutes) for (b)(i) conversion steps; ~10% (5 minutes) for (b)(ii) structures C and D; ~20% (10 minutes) for (c)(i) reagents/intermediates; and ~15% (7 minutes) for (c)(ii) product and mechanism. Begin with clear product structures, proceed through logical synthetic sequences with reagents and conditions, and conclude with mechanistic arrows where required.

Key points expected

  • For (a)(i): Correct product structures from given reactions with proper stereochemistry and regioselectivity considerations
  • For (a)(ii): Complete synthesis of ketone (A) from isobutanol via oxidation to isobutyraldehyde, then Grignard/appropriate carbon-carbon bond formation or oxidation sequence
  • For (b)(i): Stepwise conversion with all reagents, conditions, and intermediate structures clearly indicated
  • For (b)(ii): Accurate structural elucidation of C and D based on spectroscopic or chemical evidence implied in the conversion
  • For (c)(i): Missing reagents (e.g., PCC, Jones reagent, LiAlH4, Grignard reagents, Wittig reagents) and all intermediate structures in multi-step synthesis
  • For (c)(ii): Final product structure with curved-arrow mechanism showing electron flow, transition states where relevant, and stereochemical outcome

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%10Correctly identifies reaction types (SN1/SN2, E1/E2, addition, elimination, rearrangement) for all sub-parts; applies Baldwin's rules, Cram's rule, or Felkin-Anh model where stereochemistry is involved; recognizes protecting group strategies in (c)(i)Identifies most reaction types correctly but misclassifies one mechanism (e.g., confuses SN2 with SN1); basic understanding of stereochemical outcomes but misses nuancesFundamental errors in reaction classification; incorrect identification of nucleophiles/electrophiles; fails to recognize basic transformations like oxidation vs reduction
Mechanism / equation25%12.5Complete curved-arrow mechanisms for (c)(ii) with proper electron flow; balanced equations with all byproducts (e.g., Mg(OH)Br from Grignard workup); arrow-pushing in pericyclic reactions or carbocation rearrangements in (a)(i) if applicableShows major mechanistic steps but omits some arrow-pushing details; correct products but incomplete byproduct identification; misses rearrangement stepsNo mechanistic detail; only products shown without how formed; incorrect arrow direction; violates octet rule in intermediates
Numerical accuracy10%5Correct stoichiometric coefficients in all equations; accurate oxidation state assignments for carbon in redox transformations; proper yield calculations if impliedMinor errors in balancing hydrogen atoms; correct organic skeleton but incorrect counts of inorganic reagentsUnbalanced equations throughout; incorrect formula representations; arithmetic errors in any quantitative aspect
Diagram / structure30%15Clear, unambiguous structural formulas with wedge-dash stereochemistry where relevant; correct connectivity in all intermediates; proper representation of cyclic systems and aromatic compounds; IUPAC-acceptable shorthandCorrect connectivity but poor stereochemical representation; readable but cramped structures; minor errors in bond angles or ring representationsAmbiguous structures (e.g., missing double bonds, incorrect valency); illegible handwriting in diagrams; missing structures for key intermediates
Application context15%7.5Cites specific reagent choices based on chemoselectivity (e.g., PCC vs Jones for aldehyde synthesis without over-oxidation); mentions industrial relevance (e.g., pharmaceutical intermediates, fragrance synthesis); notes green chemistry alternatives where applicableBasic awareness of why certain reagents chosen; mentions reaction conditions (temperature, solvent) without full justificationNo context for reagent selection; ignores solvent effects; fails to distinguish between similar reagents (e.g., NaBH4 vs LiAlH4)

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