Q1(e). Electrocyclic steps of cyclononatriene
Consider the following electrocyclic reactions: [cyclononatriene: step I at 100 °C, step II with hν giving M, step III at 25 °C; scheme given in the paper] (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?
Answer in brief
(i) Step I disrotatory, step II conrotatory, step III disrotatory. (ii) M is (1E,3Z,5Z)-cyclonona-1,3,5-triene. (iii) First product: H cis; final product: H trans.
Key points
- Woodward–Hoffmann rule, 6π system: thermal disrotatory, photochemical conrotatory.
- Step I: Z,Z,Z-triene, HOMO ψ3; disrotatory closure puts both H on one face: cis.
- Step II: excited-state HOMO ψ4; conrotatory opening turns one end outward: E,Z,Z-triene M.
- Step III: M closes thermally (disrotatory); its E end puts the two H on opposite faces: trans.
- Rate: strained trans C=C in a nine-membered ring; step III runs at 25 °C.
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 reactionWoodward–Hoffmann rulesdisrotatoryconrotatorycyclononatriene6π electrocyclisation
All parts of Question 1
- 1(a)(i) Aromatic, non-aromatic or antiaromatic5 marks
- 1(a)(ii) Diphenylcyclopropenone resists nucleophilic addition5 marks
- 1(b) Intermediate in the amination of bromobenzene10 marks
- 1(c) (S)-2-Butanol to (S)-2-butyl chloride10 marks
- 1(d) Name and reagent for five conversions10 marks
- 1(e) Electrocyclic steps of cyclononatriene10 marks