(a) (i) Classify the following as aromatic, nonaromatic or antiaromatic : (1) Azulene (2) Pyridine (3) Sydnone (4) Cyclooctatetraene (5) Cyclopentadienyl cation (5 marks)

(ii) Though the following compound contains a keto group, it does not undergo

Question

(a) (i) Classify the following as aromatic, nonaromatic or antiaromatic : (1) Azulene (2) Pyridine (3) Sydnone (4) Cyclooctatetraene (5) Cyclopentadienyl cation (5 marks)

(ii) Though the following compound contains a keto group, it does not undergo nucleophilic addition reactions. Explain : (5 marks)

(b) What is the intermediate formed during the following reaction? Explain any one experimental proof for the formation of the intermediate. (10 marks)

(c) How is the following conversion brought about? (S)-2-Butanol → (S)-2-Butyl chloride. Explain its mechanism. (10 marks)

(d) Write the name of the reaction and reagent required for the following conversions : (i) [diagram] (ii) [diagram] (iii) [diagram] (iv) CH₃—CH₂—⁺NMe₃ → H₂C=CH₂ + NMe₃ (v) [diagram] (10 marks)

(e) Consider the following electrocyclic reactions : (i) Predict the mode of ring closure/opening at each of the three steps. (ii) Predict the structure of M. (iii) Are the indicated hydrogens cis or trans ? (10 marks)

UPSC Answer Check · Accepted Answer

Begin with a brief introductory statement on aromaticity criteria (Hückel's rule) and reaction mechanisms. For part (a), allocate ~20% time covering all five classifications with brief reasoning; for (a)(ii), explain steric/electronic factors preventing nucleophilic addition. Part (b) requires identifying the carbocation or radical intermediate with specific experimental proof like trapping or isotopic labeling. Part (c) demands detailed SN2 mechanism with stereochemical inversion discussion. Part (d) needs named reactions with reagents for each conversion. Part (e) requires Woodward-Hoffmann rules application for electrocyclic reactions with stereochemical predictions. Conclude with stereochemical summaries where relevant.
- (a)(i) Correct classification: Azulene (aromatic, 10πe⁻), Pyridine (aromatic, 6πe⁻), Sydnone (aromatic, 6πe⁻), Cyclooctatetraene (nonaromatic, tub conformation), Cyclopentadienyl cation (antiaromatic, 4πe⁻)
- (a)(ii) Explanation of why certain keto compounds (e.g., hindered ketones, enolizable β-diketones, or aromatic ketones like benzophenone derivatives) resist nucleophilic addition due to steric hindrance, conjugation, or enol stabilization
- (b) Identification of carbocation, carbanion, radical, or benzyne intermediate with specific experimental proof such as rearrangement products, trapping experiments, or kinetic isotope effect studies
- (c) Conversion via SOCl₂/pyridine or PCl₅ with SN2 mechanism showing inversion of configuration at chiral center, retention of (S) configuration due to double inversion or specific reagent conditions
- (d) Named reactions: (i)-(iii) functional group interconversions like Clemmensen/Wolff-Kishner, (iv) Hofmann elimination, with correct reagents specified for each
- (e)(i)-(iii) Application of Woodward-Hoffmann rules: conrotatory/disrotatory modes based on thermal/photochemical conditions, prediction of M's structure, and correct cis/trans assignment of hydrogens based on orbital symmetry
- Consistent use of curved arrow notation, stereochemical wedges/dashes, and clear structural representations throughout all mechanistic explanations

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	25%	12.5	Perfect application of Hückel's rule (4n+2) for all five compounds in (a)(i); correct identification of antiaromatic (4n) vs nonaromatic systems; accurate explanation of why nucleophilic addition is blocked in (a)(ii) citing specific electronic/steric factors; flawless Woodward-Hoffmann predictions in (e)	Most classifications correct but confusion between antiaromatic and nonaromatic for borderline cases; generic explanation for (a)(ii) without specific structural reasoning; basic understanding of orbital symmetry but errors in conrotatory/disrotatory assignment	Fundamental misunderstanding of aromaticity criteria; classifies based on ring size alone without π-electron counting; complete omission of Hückel's rule; no understanding of orbital symmetry control in electrocyclic reactions
Mechanism / equation	25%	12.5	Detailed SN2 mechanism for (c) with correct curved arrows showing backside attack and inversion; clear depiction of ion pair mechanism or double inversion if (S)→(S) retention explained; complete mechanistic pathway for (b) with electron flow; named reactions in (d) with full mechanistic awareness	Basic SN2 mechanism shown but missing stereochemical detail; correct intermediate named in (b) but mechanism sketchy; reactions named correctly in (d) but reagents only listed without mechanistic context	Incorrect mechanism (SN1 instead of SN2) for (c); no curved arrows or electron movement shown; wrong intermediate identified in (b); confusion between similar named reactions in (d)
Numerical accuracy	10%	5	Correct π-electron counting for all systems: Azulene (10), Pyridine (6), Sydnone (6), Cyclooctatetraene (8, nonaromatic), Cyclopentadienyl cation (4); accurate application of 4n+2 vs 4n rules throughout	Correct counting for simple cases (Pyridine, COT) but errors in fused or heterocyclic systems like Azulene or Sydnone; inconsistent application of electron counting rules	Arbitrary electron counts without systematic approach; treats all cyclic conjugated systems as aromatic regardless of electron count; fundamental errors in basic arithmetic of π-electron tallying
Diagram / structure	25%	12.5	Clear structural drawings for all five compounds in (a) showing conjugation; orbital diagrams for electrocyclic reactions in (e); transition state geometry for SN2 in (c); stereochemical wedges/dashes consistently applied; labeled structures for all conversions in (d)	Most structures drawn but lacking stereochemical detail; basic ring structures shown but orbital interactions not depicted; some diagrams missing or unclear	No structural drawings provided; text-only descriptions of molecules; inability to represent stereochemistry; missing diagrams for (d) and (e) despite explicit requirement
Application context	15%	7.5	Cites specific experimental proofs for (b) such as Hammett studies, crossover experiments, or CIDNP for radical intermediates; connects (c) to pharmaceutical stereochemistry importance; references Indian contributions (e.g., R.B. Woodward-Hoffmann rules development context) or industrial applications	Generic mention of 'trapping experiments' without specifics; basic awareness of stereochemical importance but no contextual depth; no connection to real-world or Indian research context	No experimental proof provided for (b); purely theoretical treatment without laboratory validation; complete absence of applied context or contemporary relevance

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