Q1(d). Acid-catalysed rearrangement of two phenyl-substituted 1,2-diols
Write down the structure(s) of the product(s) obtained in the following reactions. Provide suitable justification and propose the mechanisms: (i) and (ii), each diol heated with H⁺. [structures of 2-methyl-1,1-diphenylpropane-1,2-diol and 2,3-diphenylbutane-2,3-diol given in the paper]
Answer in brief
Both diols give the same ketone, 3,3-diphenylbutan-2-one, CH₃CO–C(C₆H₅)₂CH₃, by the pinacol–pinacolone rearrangement: (i) through a methyl shift, (ii) through a phenyl shift.
Key points
- Protonated OH leaves as H₂O: the slow step, giving a carbocation.
- (i) Unsymmetrical diol: OH on Ph₂C leaves (more stable cation).
- So only CH₃ can migrate: cation stability outranks migratory aptitude.
- (ii) Symmetrical diol: one cation; migratory aptitude decides: Ph > CH₃.
- Phenyl migrates through a bridged phenonium transition state.
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 aptitude1,2-shiftcarbocationphenonium ion3,3-diphenylbutan-2-one
All parts of Question 1
- 1(a) Ionisation of 3,4-dichloro-1,2,3,4-tetramethylcyclobutene in SbF₅–SO₂10 marks
- 1(b) (4-Nitrophenyl)oxirane with methoxide in methanol10 marks
- 1(c) The reaction that is not a condensation, and its mechanism10 marks
- 1(d) Acid-catalysed rearrangement of two phenyl-substituted 1,2-diols10 marks
- 1(e) Heating cyclopentadiene10 marks