Q2
(a) Involving the stereochemical concept, compare the elimination behaviour of compounds A and B in the presence of base. Also give the product(s) of the reactions. 20 marks (b) (i) Comment upon the structure and stability of cyclopropylmethyl carbocation. (ii) Which one of the above compound is more acidic and why ? (iii) Write the product(s) in the above reaction. 15 marks (c) (i) Indicating the stereochemistry of the product, complete the above reaction with mechanism. 10 marks (ii) Write down the products in the above reactions. 5 marks
हिंदी में प्रश्न पढ़ें
(a) त्रिविम रासायनिक संकल्पना को सम्मिलित करते हुए क्षार की उपस्थिति में यौगिक A और B के उन्मूलन व्यवहार की तुलना करें । अभिक्रिया के उत्पादों को भी लिखें । 20 अंक (b) (i) साइक्लोप्रोपाइलमेथिल कार्बोकैटायन की संरचना और स्थायित्व पर टिप्पणी करें । (ii) निम्नलिखित यौगिकों में कौन सा ज़्यादा अम्लीय है और क्यों ? (iii) निम्नलिखित अभिक्रिया में उत्पाद/उत्पादों को लिखें : 15 अंक (c) (i) उत्पाद की त्रिविम रसायन को दर्शाते हुए निम्नलिखित अभिक्रिया को क्रियाविधि देते हुए पूर्ण करें : 10 अंक (ii) निम्नलिखित अभिक्रियाओं के उत्पादों को लिखें : 5 अंक
Directive word: Compare
This question asks you to compare. 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
Compare demands systematic juxtaposition of stereochemical outcomes across all sub-parts. Allocate ~40% time to part (a) given its 20 marks, focusing on anti-periplanar vs syn-elimination stereoelectronic requirements; ~30% to part (b) covering bicyclobutonium ion resonance and acidity comparison; ~30% to part (c) for stereospecific addition mechanisms. Structure: introduce stereochemical principles, then address each sub-part sequentially with clear mechanistic diagrams, concluding with synthetic utility significance.
Key points expected
- Part (a): Comparison of E2 elimination stereochemistry—anti-periplanar requirement for cyclohexyl systems vs syn-elimination possibilities in rigid bicyclic frameworks; identification of Hofmann vs Zaitsev products based on substrate geometry
- Part (a): Correct prediction of major/minor alkene products with E/Z stereochemistry specified for each compound
- Part (b)(i): Description of cyclopropylmethyl carbocation as non-classical ion with bicyclobutonium structure; resonance stabilization via Walsh orbitals and homoaromaticity
- Part (b)(ii): Acidity comparison based on carbanion stability and s-character of conjugate base; cyclopropyl ring effects on pKa
- Part (b)(iii): Ring expansion products via cyclobutyl/cyclopropylmethyl rearrangement pathways
- Part (c)(i): Stereospecific mechanism (SN2 or addition) with correct stereochemical outcome—retention/inversion or syn/anti addition clearly indicated with wedge-dash notation
- Part (c)(ii): Prediction of regioisomeric and stereoisomeric products in multi-step transformations
Evaluation rubric
| Dimension | Weight | Max marks | Excellent | Average | Poor |
|---|---|---|---|---|---|
| Concept correctness | 25% | 12.5 | Demonstrates mastery of stereoelectronic principles: for (a) correctly identifies anti-periplanar requirement in chair cyclohexane vs constraints in bicyclic systems; for (b) accurately describes bicyclobutonium ion non-classical structure with orbital overlap; for (c) applies Cram's rule or Baldwin's rules appropriately with correct stereochemical predictions | States basic E2/SN2 stereochemistry but confuses anti/syn requirements; describes cyclopropylmethyl carbocation as classical without resonance stabilization; predicts products without stereochemical justification | Fundamental errors: claims E2 allows syn-elimination freely; describes carbocation as simple primary without rearrangement; ignores stereochemistry entirely in product prediction |
| Mechanism / equation | 25% | 12.5 | Complete curved-arrow mechanisms for all transformations: for (a) shows base abstraction with simultaneous leaving group departure; for (b) illustrates ring expansion via 1,2-shift with bicyclobutonium intermediate; for (c) details concerted or stepwise mechanism with stereochemical control element explicitly shown | Mechanisms present but missing key details: omitting arrow from base in E2; showing classical carbocation without rearrangement arrow; incomplete electron flow in (c) | Incorrect mechanisms: SN1-type elimination for (a); no rearrangement shown for (b); wrong facial selectivity in (c) without explanation |
| Numerical accuracy | 10% | 5 | Correct pKa values cited for acidity comparison in (b)(ii); accurate bond angles (60° cyclopropane, 104° bicyclobutonium) and strain energies referenced; proper IUPAC naming with E/Z and R/S descriptors for all products | Approximate values given without precision; minor errors in stereochemical designation (confusing R/S or E/Z) | No numerical data; incorrect pKa ordering; systematic errors in stereochemical nomenclature |
| Diagram / structure | 25% | 12.5 | Clear 3D representations: chair conformations for (a) with axial/equatorial substituents; orbital drawing for bicyclobutonium ion showing Walsh orbital overlap; wedge-dash structures for (c) with stereochemistry unambiguous; all diagrams properly labeled with atom numbering | 2D structures where 3D needed; missing conformational analysis in (a); flat cyclopropane rings without perspective; inconsistent wedge-dash usage | No diagrams or chemically incorrect structures; impossible geometries; failure to show stereochemistry in products |
| Application context | 15% | 7.5 | Connects to synthetic relevance: mentions Corey-Winter olefination for (a); cites use of cyclopropylmethyl systems in terpene biosynthesis and pharmaceutical synthesis (e.g., prostaglandins) for (b); references Sharpless asymmetric epoxidation or related stereoselective methods for (c) | Brief mention of synthetic importance without specific examples; generic statement about stereochemistry in drug design | No application context; purely theoretical treatment without real-world relevance |
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