UPSC Civil Services (Mains), Chemistry optional · 2022 · Paper II
Q1(e). Heating cyclopentadiene
Question
Predict the product(s) formed on heating the cyclopentadiene and provide a suitable justification to your answer.
Answer in brief
Cyclopentadiene dimerises by a Diels–Alder [4 + 2] cycloaddition to endo-dicyclopentadiene (kinetic product); above about 170 °C the dimer cracks back to the monomer (retro-Diels–Alder).
Key points
- Ring holds the diene in the s-cis form: very reactive diene.
- Second molecule is the dienophile: [4 + 2], six electrons, thermally allowed.
- Endo transition state is lower: secondary overlap with the spare C=C.
- Dimerisation is exothermic but loses entropy: reversed at high temperature.
- Monomer is made fresh by cracking the dimer; it distils at 41 °C.
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: pericyclic reactions, with every Civil Services, IFoS and Geo-Scientist PYQ →
CoversDiels–Alder reactiondicyclopentadieneendo ruleretro-Diels–Alder[4+2] cycloadditionsecondary orbital overlap
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