UPSC Combined Geo-Scientist
Chemist
- 3 descriptive papers
- 200 marks each
- 3 h per paper
Mains Paper III covers analytical and organic chemistry.
Electrocyclic reactions: every question, 2016 to 2025
42 parts worth 233 marks have been set on this unit in UPSC Geo-Scientist Chemistry Mains Paper III from 2016 to 2025. Each one opens on its own page with the question, the answer in brief and the key points.
Molecular orbital symmetry, frontier orbitals of ethylene, 1,3-butadiene, 1,3,5-hexatriene, allyl system, FMO approach, pericyclic reactions, Woodward-Hoffmann correlation diagram method and perturbation molecular orbital (PMO) approach for the explanation of pericyclic reactions under thermal and photochemical conditions. Simple cases of Norrish type-I and type-II reactions. Conrotatory and disrotatory motions of (4n) and (4n+2) polyenes with emphasis on [2+2] and [4+2] cycloadditions, sigmatropic rearrangements: shift of H and carbon moieties, Claisen, Cope, Sommelet-Hauser rearrangement.
Notes: Pericyclic reactions (full notes) · short notes
2025
- Q6(f) Cope rearrangement of a 1,5-diene4 marks Answered
- Q6(g) Diels–Alder reaction: product and reactants4 marks Answered
- Q7(b)(i) 6π electrocyclic ring closure: thermal and photochemical modes5 marks Answered
- Q8(c)(i) [1,5]-Hydrogen shift, then ring opening of a cyclobutene5 marks Question only
- Q8(c)(ii) Oxy-Cope rearrangement of a bicyclic vinyl carbinol5 marks Question only
- Q10(a)(i) para-Claisen rearrangement of an allyl aryl ether5 marks Question only
- Q10(a)(ii) Electrocyclic ring closure and Diels–Alder cycloaddition5 marks Question only
2024
- Q6(g) [1,5]-Carbon shift and Cope rearrangement4 marks Answered
- Q7(b) Oxepin: 6π electrocyclic ring closure, then Diels–Alder addition5 marks Answered
- Q7(c) Chloromethylation, then Sommelet–Hauser rearrangement5 marks Answered
- Q8(b) Photolysis of two ketones: Norrish Type II and Type I5 marks Question only
2023
- Q6(f) Dimerisation of cyclopentadiene: endo rule by frontier orbitals4 marks Answered
- Q7(c) Photochemical ring opening of 8-methyl-1-decalone: Norrish Type II5 marks Answered
- Q10(b) 4π ring opening, then 6π ring closure: the intermediate II5 marks Answered
- Q10(c) Cope rearrangement of cis-2,3-divinyloxirane5 marks Answered
2022
- Q6(e) Cyclopentadiene with maleic anhydride: endo and exo adducts4 marks Answered
- Q8(c) A diene and an enone in UV light: [2 + 2] cycloaddition5 marks Question only
- Q8(d) Photochemical 6π electrocyclic ring closure of an octatriene5 marks Question only
- Q8(e) Cope rearrangement of a 1,5-diene with CN and CO₂Et5 marks Question only
- Q10(c) Thermal [1,3] shift: hydrogen cannot migrate, carbon can5 marks Answered
2021
- Q6(b) Quaternisation, then Sommelet–Hauser rearrangement4 marks Answered
- Q7(b) Maleic anhydride with anthracene, furan and pyrrole12 marks Answered
- Q8(b) Eucarvone and light: 4π electrocyclic ring closure8 marks Question only
- Q9(c) Chorismate to prephenate; a steroidal diene opened by light10 marks Question only
- Q10(b) Dimer of cyclopentadiene; a [1,3] shift of carbon with inversion10 marks Answered
2020
- Q6(f) Diene reactivity in Diels–Alder reactions4 marks Answered
- Q6(g) Norrish II reactivity4 marks Answered
- Q9(a) Photochemical reaction of a ketone in methanol10 marks Question only
- Q9(b) Heating an anion X5 marks Question only
- Q9(c) Identify P, Q and R in a thermal sequence at 180 °C5 marks Question only
- Q10(b) Bulky substituents and diene conformation5 marks Answered
2019
- Q6(e) Fused cyclobutene opening4 marks Answered
- Q9(d) Thermal or photochemical? Woodward–Hoffmann prediction5 marks Question only
- Q10(d) Frost circles and aromaticity10 marks Answered
2018
- Q6(f) Sequential electrocyclic reactions4 marks Answered
- Q6(g) Benzyne trapped by isoindole4 marks Answered
- Q7(c)(ii) Successive Claisen rearrangements5 marks Answered
- Q9(c) Electrocyclic product by Woodward–Hoffmann rules5 marks Question only
2017
- Q6(b) Benzene and cyclooctatetraene orbitals5 marks Answered
- Q9(a) Hexatriene frontier orbitals10 marks Answered
2016
- Q7(f) Frontier orbital nodes4 marks Answered
- Q10(a) Electrocyclic stereochemistry5 marks Answered
UPSC Combined Geo-Scientist (Main) Chemistry Paper III: Model Answers 2021 to 2025
- All ten questions of each paper, with the question printed above its answer
- Instruments drawn as labelled diagrams, numericals worked with units
- Topic-wise weightage and a topic-wise index