Indian Forest Service (Mains), Chemistry optional · 2025 · Paper II
Q1(f). Medium-dependent pinacol rearrangement
Question
Give products of Me₂C(OH)–C(OH)Ph₂ with cold concentrated sulfuric acid, and with acetic acid containing sulfuric acid.
Answer in brief
Cold concentrated H₂SO₄ gives MeCOC(Me)Ph₂; AcOH/H₂SO₄ gives PhCOC(Me)₂Ph.
Key points
- In concentrated acid, loss of water from the diphenyl-substituted carbon gives the strongly stabilised diphenyl carbocation.
- Methyl migration from the adjacent OH-bearing carbon accompanies C=O formation, giving CH₃COC(CH₃)(Ph)₂.
- In the acetic-acid medium, phenyl participation assists departure of the other OH; phenyl migrates from CPh₂ to CMe₂, and the former CPh₂ carbon becomes the carbonyl.
- The product is PhCOC(CH₃)₂Ph; the acid medium changes which OH leaves and which group migrates.
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: molecular rearrangements, with every Civil Services, IFoS and Geo-Scientist PYQ →
Coverspinacol–pinacolone rearrangementmigratory aptitudemethyl vs phenyl migrationcarbocation stability
All parts of Question 1
- 1(a) Aromaticity of small rings5 marks
- 1(b) Nitrene transfer to an alkene5 marks
- 1(c) Electrophilic substitution of heteroarenes5 marks
- 1(d) Elimination next to an existing alkene5 marks
- 1(e) Oxymercuration of norbornene5 marks
- 1(f) Medium-dependent pinacol rearrangement5 marks
- 1(g) Two thermal electrocyclic steps5 marks
- 1(h) Intramolecular Claisen condensation5 marks