3.(a) Would you expect the above conversion to require heat or light? Explain using molecular orbital diagram. (15 marks)

3.(b) 3-Phenyl-4-pentenal is synthesised in the following manner:
(i) Identify the type of pericyclic reaction involved in the

Question

3.(a) Would you expect the above conversion to require heat or light? Explain using molecular orbital diagram. (15 marks)

3.(b) 3-Phenyl-4-pentenal is synthesised in the following manner:
(i) Identify the type of pericyclic reaction involved in the formation of unsaturated intermediate.
(ii) What elements are lost when the intermediate is converted to 3-phenyl-4-pentenal?
(iii) Identify the carbon atom in the starting acrylic acid that becomes the aldehyde carbon in the pentenal.
(iv) Write the steps involved for this transformation. (15 marks)

3.(c) Explain the regioselectivity of a Hoffmann elimination reaction with the help of Newman projection formula. (10 marks)

3.(d) Write down the structure(s) of the product(s) obtained in the above reactions. Provide suitable justification and propose the mechanisms.
(i) H₃CO + △
(ii) OCH₃ + △ (10 marks)

UPSC Answer Check · Accepted Answer

Explain the photochemical/thermal requirements for each pericyclic transformation using orbital symmetry principles. For part (a), construct HOMO-LUMO diagrams to justify heat vs light conditions. For part (b), identify the [3,3]-sigmatropic rearrangement (Claisen/Cope type), trace carbon atom mapping from acrylic acid to aldehyde, and show retro-ene or oxidation steps. For part (c), use Newman projections along Cα-Cβ bond to demonstrate anti-periplanar geometry preference for less substituted alkene formation. For part (d), analyze thermal [2+2] cycloadditions or electrocyclic reactions of methoxy-substituted systems. Allocate approximately 30% time to (a), 30% to (b), 20% to (c), and 20% to (d), with diagrams constituting roughly 40% of total response.
- Part (a): Correct identification of conrotatory/disrotatory mode based on 4n/4n+2 π-electron system; construction of HOMO under thermal vs photochemical conditions showing symmetry-allowed pathway
- Part (b)(i): Recognition of [3,3]-sigmatropic rearrangement (oxy-Cope or Claisen variant) as the pericyclic step forming the unsaturated intermediate
- Part (b)(ii)-(iii): Identification of CO₂ or formaldehyde loss; precise carbon mapping from C-1 or C-3 of acrylic acid derivative to aldehyde carbon via isotopic labeling logic
- Part (c): Newman projection showing anti-periplanar β-hydrogen from less substituted carbon (Hofmann rule); explanation of steric vs electronic factors favoring terminal alkene
- Part (d): Structure determination of cycloaddition/electrocyclic products; thermal allowedness based on orbital symmetry; methoxy substituent effects on regioselectivity

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	22%	11	Correctly applies Woodward-Hoffmann rules across all parts: identifies 4n vs 4n+2 systems for (a), recognizes [3,3]-sigmatropic nature for (b), states Hofmann rule with correct reasoning for (c), and predicts allowed thermal reactions for (d); no confusion between conrotatory/disrotatory or suprafacial/antarafacial modes	Identifies most pericyclic types correctly but misapplies orbital symmetry in one part (e.g., conflates thermal and photochemical conditions) or states Hofmann/Saytzeff rules without clear mechanistic justification	Fundamental errors in pericyclic classification (calls [3,3] a cycloaddition), invents incorrect orbital symmetry arguments, or reverses Hofmann/Saytzeff selectivity without explanation
Mechanism / equation	24%	12	Complete stepwise mechanisms for all transformations: arrow-pushing in sigmatropic rearrangement showing six-membered cyclic transition state; clear ene reaction or oxidative cleavage for (b)(iv); E2 mechanism with proper base and leaving group depiction for (c); concerted pericyclic mechanisms with stereochemical outcome for (d)	Shows most mechanisms but misses transition state geometry or omits key intermediates; correct products but incomplete arrow formalism in one or two parts	Missing mechanisms entirely or shows incorrect ionic mechanisms for pericyclic reactions; radical pathways proposed without justification; no E2 depiction for Hofmann elimination
Numerical accuracy	10%	5	Correct electron counting (4n/4n+2) for orbital symmetry analysis; accurate carbon numbering and atom mapping in (b)(iii); proper stereochemical descriptors (E/Z, R/S) where applicable in product structures	Minor errors in electron counting or carbon mapping that don't fundamentally alter conclusions; one incorrect stereochemical assignment	Major errors in electron count leading to wrong allowedness prediction; complete failure to map carbon atoms; inconsistent stereochemical labeling
Diagram / structure	24%	12	Clear MO diagrams for (a) showing HOMO symmetry with proper shading; accurate Newman projection for (c) with correct dihedral angles (180° anti-periplanar); all structures drawn with proper stereochemistry including phenyl and methoxy substituent orientations; transition state depictions for sigmatropic rearrangement	Most diagrams present but lacking detail (e.g., Newman projection without explicit H atoms, MO diagram without symmetry labels); one missing structure or unclear stereochemical representation	Absent or illegible diagrams; incorrect Newman projection geometry (syn-periplanar shown); no MO diagrams; structures missing double bonds or stereocenters
Application context	20%	10	Cites relevant named reactions (Claisen rearrangement, Hofmann elimination, Cope rearrangement); connects to synthetic utility (industrial synthesis of citral, vitamin A precursors); mentions Indian contributions (R.B. Woodward's collaboration or Indian synthetic organic chemistry); discusses stereoelectronic control in natural product synthesis	Names 1-2 reactions correctly but limited synthetic context; generic mention of pericyclic reactions in synthesis without specific examples	No named reactions identified; no connection to real-world synthesis; irrelevant or fabricated applications

Q3

Directive word: Explain

How this answer will be evaluated

Approach

Key points expected

Evaluation rubric

Practice this exact question

More from Chemistry 2023 Paper II