UPSC Civil Services (Mains), Chemistry optional · 2023 · Paper II
Q1(d). Photochemical electrocyclic closure of two cyclonona-1,3-dienes
Question
The above compounds both undergo photo-induced electrocyclic reactions. What are the structures and stereochemistry of the products? [structures of two cyclonona-1,3-dienes (A) and (B) given in the paper]
Answer in brief
Both close by the 4π disrotatory mode to bicyclo[5.2.0]non-8-ene: (A), the Z,E diene, gives the trans-fused product; (B), the Z,Z diene, gives the cis-fused product.
Key points
- Woodward–Hoffmann rule, 4n electrons: thermal conrotatory, photochemical disrotatory.
- Excited-state HOMO ψ₃: end lobes in phase on one face: disrotatory.
- (B) Z,Z: both terminal H outside; disrotation: same face: cis.
- (A) Z,E: one H inside, one outside; disrotation: opposite faces: trans.
- Thermal conrotatory closure would reverse this: (A) cis, (B) trans.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Topic notes: pericyclic reactions, with every Civil Services, IFoS and Geo-Scientist PYQ →
Coverselectrocyclic reactiondisrotatoryWoodward–Hoffmann rulesphotochemical ring closurebicyclo[5.2.0]non-8-ene
All parts of Question 1
- 1(a)(i) Acidity of cyclopentadiene5 marks
- 1(a)(ii) Hydrogen exchange in cyclopropyl and cyclopropenyl ketones5 marks
- 1(b)(i) IUPAC name with stereodescriptors5 marks
- 1(b)(ii) Dimerisation ability of three triarylmethyl-type radicals5 marks
- 1(c) Methyl iodide with sodium azide: DMF against methanol10 marks
- 1(d) Photochemical electrocyclic closure of two cyclonona-1,3-dienes10 marks
- 1(e)(i) 2-Chloro-6-methylcyclohexanone with methoxide5 marks
- 1(e)(ii) Name reaction that gives nitrogen as a by-product5 marks