Indian Forest Service (Mains), Chemistry optional · 2016 · Paper II
Q1(g). 6,6-Dimethylfulvene, phenyllithium and acetaldehyde
Question
Write the products formed in the following reaction sequence:
[structure] → (PhLi, Dry ether) A → ((i) MeCHO, (ii) H₃O⁺) B
Answer in brief
A = lithium (2-phenylpropan-2-yl)cyclopentadienide ; B = the secondary carbinol CH₃CH(OH)-C₅H₄-C(CH₃)₂Ph. If B is warmed with acid it loses water: the alternative answer after dehydration is 6-methyl-3-(2-phenylpropan-2-yl)fulvene.
Key points
- PhLi delivers Ph⁻ to the electrophilic C-6; the π pair moves into the ring. A = lithium (2-phenylpropan-2-yl)cyclopentadienide, an aromatic 6π anion.
- A is a carbon nucleophile (like an enolate). It adds to the C=O of acetaldehyde at the ring carbon remote from the bulky cumyl group (1,3-relation); H₃O⁺ gives the secondary alcohol.
- B = 1-[3-(2-phenylpropan-2-yl)cyclopenta-1,3-dien-1-yl]ethan-1-ol, C₁₆H₂₀O; the ring double bonds shift by [1,5]-H shifts, so double-bond isomers interconvert.
- With warm acid or base, B loses water to give a new 6-methylfulvene carrying the cumyl group on the ring (Thiele-type condensation).
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.
Coversfulvenearomatic cyclopentadienideelectrophilic C-6secondary alcohol
All parts of Question 1
- 1(a) Aromatic, antiaromatic or non-aromatic5 marks
- 1(b) Crossover experiments5 marks
- 1(c) Quinuclidine against triethylamine with ethyl iodide5 marks
- 1(d) Order of carbocation stability5 marks
- 1(e) Basicity of pyrrole, triethylamine and pyridine5 marks
- 1(f) Saytzeff and Hofmann eliminations5 marks
- 1(g) 6,6-Dimethylfulvene, phenyllithium and acetaldehyde5 marks
- 1(h) Pinacol rearrangement with ring expansion5 marks