Chemistry 2025 Paper II 50 marks Explain

Q2

(a) (i) In the presence of sodium ethoxide, the following transformation occurs. Explain : (ii) Propose a suitable mechanism for the following transformation : (b) (i) The following reaction does not produce the product shown : (1) Predict the major product from the conditions shown above, and write a detailed mechanism for its formation. (2) Write that reaction conditions which would lead to successful synthesis of the product shown above (i.e., 3,3-dimethyl-2-butanol). (ii) Write the structure of the major product(s) formed in the following reaction. Justify your answer : (c) Write the structure of the major product(s) formed in the following reactions : (i) (ii) (iii) (iv)

हिंदी में प्रश्न पढ़ें

(a) (i) सोडियम एथॉक्साइड की उपस्थिति में निम्नलिखित रूपांतरण होता है। व्याख्या कीजिए। (ii) निम्नलिखित रूपांतरण के लिए उपयुक्त क्रियाविधि प्रस्तावित कीजिए : (b) (i) निम्नलिखित अभिक्रिया में दर्शाया हुआ उत्पाद नहीं बनता है : (1) उपर दर्शाई गई अवस्था में बनने वाले मुख्य उत्पाद का पूर्वानुमान लगाइए व इसके बनने की क्रियाविधि की विस्तृत जानकारी दीजिए। (2) उस अभिक्रिया अवस्था को लिखिए, जिससे उपयुक्त उत्पाद (अर्थात् 3,3-डाइमेथिल-2-ब्यूटेनॉल) का सफल संश्लेषण किया जा सके। (ii) निम्नलिखित अभिक्रिया में बनने वाले मुख्य उत्पाद/उत्पादों की संरचना लिखिए। अपने उत्तर का औचित्य सिद्ध कीजिए : (c) निम्नलिखित अभिक्रियाओं में बनने वाले मुख्य उत्पाद/उत्पादों की संरचना लिखिए : (i) (ii) (iii) (iv)

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Directive word: Explain

This question asks you to explain. 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

Explain the mechanistic rationale for each transformation across all sub-parts, allocating approximately 25% time to (a)(i)-(ii) on base-catalyzed rearrangements, 35% to (b)(i)-(ii) on Grignard reactions and stereochemical outcomes, and 40% to (c)(i)-(iv) on pericyclic and photochemical reactions. Begin with clear structure drawings, follow with curved-arrow mechanisms, and conclude with stereochemical justifications where applicable.

Key points expected

  • For (a)(i): Explanation of Favorskii rearrangement or base-catalyzed ring contraction/expansion with sodium ethoxide driving elimination-addition pathway
  • For (a)(ii): Detailed E1cB or SN2 mechanism with proper curved arrows showing nucleophilic attack and leaving group departure
  • For (b)(i)(1): Prediction of tertiary alkoxide or elimination product instead of desired alcohol due to steric hindrance with t-BuMgBr; mechanism showing competing pathways
  • For (b)(i)(2): Alternative conditions using less hindered Grignard reagent or different carbonyl compound (acetone + isopropylmagnesium bromide) for successful synthesis
  • For (b)(ii): Identification of major product based on Cram's rule or Felkin-Anh model with stereochemical justification for nucleophilic addition to chiral carbonyl
  • For (c)(i)-(iv): Structures of products from Diels-Alder, photochemical [2+2] cycloaddition, sigmatropic rearrangement, or electrocyclic reactions with correct stereochemistry
  • For all parts: Proper representation of stereochemistry (R/S, E/Z, syn/anti) in product structures where applicable

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%10Correctly identifies (a)(i) as Favorskii or related rearrangement; recognizes (b)(i) failure due to steric hindrance with neopentyl-like systems; applies proper pericyclic selection rules for (c); no conceptual errors in reaction typesIdentifies most reaction types correctly but confuses mechanism categories (e.g., SN1 vs SN2) or misapplies pericyclic rules; minor errors in recognizing steric constraintsFundamental misconceptions such as calling all base-catalyzed reactions E2, ignoring carbocation stability, or violating Woodward-Hoffmann rules without justification
Mechanism / equation25%12.5Curved-arrow mechanisms for (a)(ii) and (b)(i) show complete electron flow with proper intermediates (enolate, tetrahedral intermediate); explains competing pathways; balanced equations with conditionsMechanisms mostly correct but missing key intermediates or incorrect arrow directions in 1-2 steps; equations present but conditions omitted or incorrectMissing mechanisms for required parts; arrows showing incorrect electron movement; no recognition of rate-determining steps or intermediate stability
Numerical accuracy10%5Correct stoichiometric coefficients in all equations; proper counting of carbon atoms in rearrangement products; accurate yield predictions where relevantMinor errors in balancing or carbon count in complex rearrangements; stoichiometry correct for simple reactions but not for multi-step sequencesUnbalanced equations; incorrect carbon skeleton in products showing loss/gain of atoms; no attention to stoichiometric requirements
Diagram / structure25%12.5All seven sub-part products drawn with correct stereochemistry (wedge-dash, Fischer projections where appropriate); clear distinction between starting materials and products; proper ring conformations in cyclic systemsStructures correct but stereochemistry ambiguous or missing in 2-3 sub-parts; acceptable line-angle formulas but poor spatial representationIncorrect connectivity in products; missing stereochemistry where critical; confusing or illegible structures; wrong functional groups in final products
Application context20%10For (b)(i)(2), provides industrially viable alternative conditions; discusses stereoselectivity relevance in pharmaceutical synthesis (e.g., single enantiomer drugs); connects pericyclic reactions to natural product synthesisMentions practical alternatives without detail; limited discussion of stereochemical consequences in real synthesis; generic references to organic synthesisNo practical alternatives offered for failed reaction; ignores stereochemical implications; no connection to applied organic chemistry or industrial relevance

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