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VKC Notes · Pericyclic reactions · Civil Services P-II · IFoS P-II · Geo-Scientist P-III

8.1 Pericyclic Reactions

★★★ The most asked organic topic of all three exams, 2021–2025: 59 parts, 425 marks (Civil Services 19 parts / 185 m · IFoS 19 / 120 m · Geo-Scientist 21 / 120 m), in every paper of every year.

Syllabus lines. Civil Services and IFoS: “Pericyclic reactions: classification and examples; Woodward–Hoffmann rules: electrocyclic reactions, cycloaddition reactions [2+2 and 4+2] and sigmatropic shifts [1,3; 3,3 and 1,5], FMO approach”. Geo-Scientist: “frontier orbitals of ethylene, 1,3-butadiene, 1,3,5-hexatriene, allyl system, FMO approach, Woodward–Hoffmann correlation diagram method and PMO approach … under thermal and photochemical conditions; 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 and Norrish reactions are in other notes.

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Geo 2023 Q6(f) = asked there; tap to open the solved answerSN = line kept in the short notes

8.1.1 Classes and the one rule ★★★

A pericyclic reaction makes and breaks bonds in one step, through a cyclic array of overlapping orbitals: no intermediate, little solvent effect, highly stereospecific, and switched on by heat or by light but not by both in the same way.SN

  • Electrocyclic: a conjugated polyene closes to a ring (one new σ bond, one π bond lost), or the ring opens.SN
  • Cycloaddition [m + n]: two π systems join through two new σ bonds (Diels–Alder [4 + 2], [2 + 2]).SN
  • Sigmatropic [i,j]: a σ bond moves along a π system; i and j count atoms from the old bond to the new one on each fragment.SN
  • Also: group transfer (ene reaction), cheletropic (SO₂ + butadiene).
ReactionElectronsHeat (Δ)Light (hν)
Electrocyclic4n (diene ⇌ cyclobutene)conrotatorydisrotatory
4n + 2 (triene ⇌ cyclohexadiene)disrotatoryconrotatory
Cycloaddition4n + 2 ([4 + 2])supra–suprasupra–antara
4n ([2 + 2])supra–antara (only ketenes)supra–supra
Sigmatropic4n + 2 ([1,5], [3,3])supra–supraantara
4n ([1,3], [1,7])antara, or supra with inversion at Csupra
(8.1.1)

The one rule (Woodward–Hoffmann, general form, eq. 8.1.1): a ground-state pericyclic reaction is allowed if the number of (4q + 2)s and (4r)a components is odd.SN Light reverses every entry of the table. Equivalent picture: a 4n + 2 Hückel (or 4n Möbius) transition state is aromatic and allowed by heat.

8.1.2 π MOs and the frontier-orbital method ★★★ asked as “draw the HOMO”

  • A chain of k p orbitals gives k MOs; ψₙ has n − 1 nodes. Fill two electrons per MO from the bottom.SN
  • Ethene: HOMO π (ψ₁), LUMO π* (ψ₂). Butadiene: HOMO ψ₂, ends opposite phase; LUMO ψ₃. Hexatriene: HOMO ψ₃, ends same phase; LUMO ψ₄. Allyl: ψ₂ is non-bonding, node through the middle carbon.SN
  • Electrocyclic: look at the HOMO of the open polyene; turn the ends so that lobes of the same phase meet. Cycloaddition: HOMO of one partner with LUMO of the other; both ends must match. Sigmatropic: treat the migrating group and the π fragment (allyl or pentadienyl) as two radicals.SN
  • Under light one electron is promoted: the old LUMO becomes the HOMO (SOMO), its end phases are reversed, and so is the answer.
Subtle point · the correlation-diagram method (Geo-Scientist syllabus)

Pick the symmetry element kept all along the reaction: a C₂ axis for conrotatory motion, a mirror plane for disrotatory. Label every MO of the reactant and of the product S or A with respect to it, and join MOs of the same label in order of energy. If every occupied reactant MO correlates with an occupied product MO, the reaction is thermally allowed; if a bonding MO correlates with an antibonding one, it is thermally forbidden and photochemically allowed. For butadiene → cyclobutene this gives con allowed by heat (C₂), dis by light (m). The PMO (aromatic transition state) method gives the same answers: 4n + 2 electrons in a Hückel array, or 4n in a Möbius array, is aromatic and thermally allowed.SN

4π: butadiene, thermal HOMO ψ2ends opposite phaseCONrotatory6π: hexatriene, thermal HOMO ψ3ends same phaseDISrotatorylight puts one electron in the next MO: the end phases flip, so does the rotation
Fig. 8.1.1 Thermal HOMOs decide the rotation.
Draw it in 60 s: draw four p orbitals, shade top-top-bottom-bottom → arrows both clockwise: CON. Draw six, shade top-top-bottom-bottom-top-top → arrows opposite: DIS.

8.1.3 Electrocyclic reactions ★★★ most asked

Starting polyeneHeatLight
(2E,4E)-hexa-2,4-diene (4π)con → trans-3,4-dimethylcyclobutenedis → cis
(2E,4Z)-hexa-2,4-diene (4π)con → cisdis → trans
(2E,4Z,6E)-octa-2,4,6-triene (6π)dis → cis-5,6-dimethylcyclohexa-1,3-dienecon → trans
(2E,4Z,6Z)-octa-2,4,6-triene (6π)dis → transcon → cis
(2E,4E)-hexa-2,4-dieneΔconrotatoryhν, disrotatory trans-3,4-dimethylcyclobutene cis (meso) (2E,4Z,6E)-octa-2,4,6-trieneΔdisrotatory cis-5,6-dimethylcyclohexa-1,3-diene(hν, conrotatory → trans)
Fig. 8.1.2 Electrocyclic ring closure with curly arrows: the π electrons move round the ring, one new σ bond forms between the terminal carbons, and the rotation (con or dis) fixes cis or trans.
Draw it in 60 s: draw the diene as a U (methyls outside) → two arrows: π 2=3 into 3–4, π 4=5 into the gap 2…5 → square with C=C opposite the new bond; con: one Me wedge, one hash (trans); dis: both hashed (cis).

Memory hook: E,E ends → con gives trans, dis gives cis; one Z end flips the answer.SN The same rules run backwards for ring opening: thermal opening of a cyclobutene is conrotatory, of a cyclohexadiene disrotatory.SN IFoS 2024 Q3(c)(ii) IFoS 2021 Q2(a) Geo 2025 Q7(b)(i) Geo 2022 Q8(d) CSE 2025 Q7(c)(ii)

  • Which way it runs: 4π favours the open diene (cyclobutene is strained) unless the ring is fused or substituted; 6π favours the closed cyclohexadiene (one π bond becomes a stronger σ bond).SN
  • Relays: a stereocentre or C=C geometry that cannot change in one allowed step is changed by a ring closure–ring opening pair, one thermal and one photochemical CSE 2021 Q1(e) 10m IFoS 2022 Q4(e) IFoS 2023 Q3(f).SN
  • Fused cyclobutenes open conrotatory to a cis,cis,trans medium-ring triene, which then closes disrotatory to a trans-fused bicycle IFoS 2025 Q1(g) Geo 2023 Q10(b).
  • Oxepin ⇌ benzene oxide (6π, disrotatory), trapped by a dienophile Geo 2024 Q7(b) Geo 2025 Q10(a)(ii). Methoxy trienes close and hydrolyse to enones CSE 2024 Q3(b)(i). Eucarvone in light: 4π disrotatory closure to a bicyclo[3.2.0] ketone Geo 2021 Q8(b).

Solved PYQ CSE 2024 Q1(e) 10m

Cyclononatriene: step I at 100 °C, step II with hν to M, step III at 25 °C. Modes, M and the H stereochemistry.

  1. Step I: Z,Z,Z-triene, 6π, heat → disrotatory closure; the two ring-fusion H end on one face: cis-fused.
  2. Step II: light, 6π → conrotatory opening; one end turns outward: M = (1E,3Z,5Z)-cyclonona-1,3,5-triene.
  3. Step III: M closes thermally, disrotatory; its E end puts the H on opposite faces: trans-fused. It runs at 25 °C because the trans C=C in a nine-membered ring is strained.

Solved PYQ CSE 2023 Q1(d) 10m

Two cyclonona-1,3-dienes in light: products and stereochemistry.

  1. 4π in light: disrotatory (excited HOMO ψ₃ has the same phase at both ends).
  2. Z,Z-diene: both terminal H outside; disrotation sends them to the same face: cis-fused bicyclo[5.2.0]non-8-ene.
  3. Z,E-diene: one H inside, one outside: trans-fused. Heating would give the opposite pair.
Subtle point · vitamin D and sunlight CSE 2022 Q3(b)(i) 10m Geo 2021 Q9(c)

Ergosterol (and 7-dehydrocholesterol in skin) has a cyclohexa-1,3-diene in ring B. UV light opens it by a 6π conrotatory electrocyclic reaction to precalciferol (previtamin D), a hexatriene. A thermal [1,7]-H shift, antarafacial, then gives vitamin D. Heat cannot do the first step (thermal 6π opening would be disrotatory and is uphill), so without sunlight no vitamin D is made.SN

8.1.4 Diels–Alder reaction ★★★ every year

dieneHOMO ψ2dienophileLUMO π*C=O (endo)both ends in phase:suprafacial, allowed by heatdotted: secondary overlap→ endo transition state lower
Fig. 8.1.3 Diels–Alder: HOMO(diene) + LUMO(dienophile), and the endo extra.
Draw it in 60 s: four p orbitals shaded top-top-bottom-bottom above two orbitals → red dashes join the matching end lobes → a blue dotted line from C-2 to the C=O orbital: secondary overlap.
diene (s-cis)dienophileΔ[4s + 2s] cyclohexeneone step, cyclic 6-electron TS:3 π bonds out, 2 σ + 1 π in butadiene+ maleic anhydride (cis)Δ cis-fused adduct onlystereospecific: cis stays ciscyclopentadiene gives the endoadduct (secondary orbital overlap) 2-methoxybuta-1,3-diene+ acroleinΔ “para” (1,4) product2-substituted diene → “para” (1,4)1-substituted diene → “ortho” (1,2)
Fig. 8.1.4 Diels–Alder drawn with three cyclic arrows; cis dienophile gives the cis-fused adduct; a 2-substituted diene gives the “para” product.
Draw it in 60 s: hexagon with the right side open: diene C=C top-left and bottom-left, dienophile on the right → three arrows round the ring, each from a π bond to the next bond position → cyclohexene, C=C where the diene single bond was.
 (cyclohexene)(8.1.2)
  1. [π4s + π2s], thermally allowed: diene HOMO ψ₂ and dienophile LUMO π* match at both ends, both partners suprafacial.SN
  2. s-cis diene needed. Cyclopentadiene is locked s-cis and is the most reactive diene; an inner (Z) 1-substituent or 2,3-di-tert-butyl groups block the s-cis form IFoS 2022 Q3(b) CSE 2025 Q7(b)(i).SN
  3. Electron demand: donors on the diene raise its HOMO, acceptors on the dienophile lower its LUMO; the closer they come, the faster the reaction. Maleic anhydride, acrolein, acrylates, quinones, DMAD are good dienophiles.SN
  4. Stereospecific (cis principle): a cis dienophile gives cis substituents in the ring; the “outside” groups of an E,E diene end cis to each other.SN
  5. Endo rule: the dienophile's C=O lies under the diene in the transition state; secondary orbital overlap lowers its energy, so the endo adduct is the kinetic product, although exo is more stable.SN
  6. Regiochemistry (“ortho–para rule”): a 1-substituted diene gives the 1,2 (“ortho”) adduct, a 2-substituted diene the 1,4 (“para”) adduct; the largest HOMO and LUMO coefficients bond first.SN

Solved PYQ Geo 2022 Q6(e) 4m CSE 2021 Q4(a)(ii) 10m IFoS 2024 Q3(c)(i) IFoS 2021 Q4(a)(i)

Cyclopentadiene + maleic anhydride: two products possible, one major. Why?

  1. Concerted [4 + 2], suprafacial on both partners: two diastereomers, endo and exo, both C₉H₈O₃.
  2. Endo transition state: the anhydride C=O π orbitals overlap C-2 and C-3 of the diene (secondary overlap, no bond made): lower Eₐ.
  3. Endo adduct major (kinetic). Exo is less crowded and more stable; it accumulates only on long strong heating via retro-Diels–Alder.

Solved PYQ CSE 2023 Q3(d) 10m IFoS 2023 Q4(c) CSE 2025 Q7(b)(ii)

2-Methoxy- and 1-methoxybuta-1,3-diene with acrolein.

  1. OMe raises the diene HOMO, CHO lowers the LUMO: normal electron demand.
  2. 2-OMe diene → 4-methoxycyclohex-3-ene-1-carbaldehyde, the “para” adduct. With acrylonitrile and then H₃O⁺ the enol ether hydrolyses to 4-oxocyclohexane-1-carbonitrile.
  3. 1-OMe diene → 2-methoxycyclohex-3-ene-1-carbaldehyde, the “ortho” adduct, OMe and CHO cis (endo).

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