Indian Forest Service (Mains), Chemistry optional · 2024 · Paper II
Q1(g). Photochemical alkyl migration
Question
Explain how the drawn chiral R* group undergoes a photochemical [1,5] sigmatropic shift with inversion of configuration. [structure given in the paper]
Answer in brief
For a six-electron [1,5] rearrangement, the photochemical suprafacial pathway requires inversion at the migrating carbon.
Key points
- The cyclic array contains four π electrons from the diene and two from the migrating σ bond: six electrons in total.
- A thermal suprafacial [1,5] alkyl shift is allowed with retention at the migrating centre. Electronic excitation changes the orbital-symmetry requirement.
- The photochemical suprafacial path achieves constructive terminal overlap by using the opposite lobe at the migrating carbon, corresponding to inversion.
- The new C–R* bond forms on the same face of the diene, but the spatial configuration of R* is inverted.
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.
Covers[1,5] sigmatropic shiftphotochemical sigmatropic rearrangementWoodward-Hoffmann rulesinversion at migrating carbonFMO approach
All parts of Question 1
- 1(a) Annulenes and aromaticity5 marks
- 1(b) Secondary kinetic isotope effect5 marks
- 1(c) Five aromaticity decisions5 marks
- 1(d) Amination through a benzyne5 marks
- 1(e) Intramolecular aldol condensation5 marks
- 1(f) Pulegone to a ring-contracted ester5 marks
- 1(g) Photochemical alkyl migration5 marks
- 1(h) Hydrazine reductions5 marks