Indian Forest Service (Mains), Chemistry optional · 2023 · Paper II
Q1(c). Benzyne capture by tetracyclone
Question
Identify A, B and C from o-bromofluorobenzene/Mg in THF, trapping with tetraphenylcyclopentadienone and heating. [reaction scheme given in the paper]
Answer in brief
A is benzyne; B is its bridged [4+2] adduct with tetraphenylcyclopentadienone; C is 1,2,3,4-tetraphenylnaphthalene.
Key points
- Magnesium first forms the arylmagnesium bromide at the C–Br site. Elimination of magnesium halides across the neighbouring C–F position generates benzyne.
- The cyclopentadienone acts as a diene and benzyne as the dienophile. Two new C–C bonds form the bicyclic carbonyl-bridged adduct B.
- Heating extrudes CO from the carbonyl bridge. Formation of a second aromatic ring drives production of the tetraphenyl-substituted naphthalene.
- The carbonyl carbon is the carbon lost as CO; the four phenyl groups remain attached to the four diene-derived carbons.
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.
CoversbenzynetetracycloneDiels–Alder trapping1,2,3,4-tetraphenylnaphthaleneCO extrusion
All parts of Question 1
- 1(a) Aromaticity of heterocycles5 marks
- 1(b) Evidence for a tetrahedral intermediate5 marks
- 1(c) Benzyne capture by tetracyclone5 marks
- 1(d) Para-alkyl effects on benzyl solvolysis5 marks
- 1(e) Dehydrobromination through an ion pair5 marks
- 1(f) Why nucleophilic shifts are common5 marks
- 1(g) A double oxy-Cope rearrangement5 marks
- 1(h) Substituent effects in Cannizzaro reaction5 marks