Pericyclic Reactions: electrocyclic, cycloaddition and sigmatropic rearrangements MCQs With Answer

Pericyclic Reactions: electrocyclic, cycloaddition and sigmatropic rearrangements MCQs With Answer

This quiz set focuses on pericyclic reactions—concerted processes that proceed through cyclic transition states—and targets M.Pharm students preparing for MPC 202T Advanced Organic Chemistry II. Questions emphasize the Woodward–Hoffmann rules, orbital symmetry, stereochemical outcomes (conrotatory vs disrotatory), thermal vs photochemical control, and classic examples like Diels–Alder, 1,3-dipolar cycloadditions, Cope and Claisen rearrangements and [1,5]-sigmatropic shifts. Each question tests mechanistic understanding, regiochemistry, and factors influencing selectivity and feasibility. These MCQs are designed to deepen conceptual mastery rather than rote memorization, helping students predict reaction courses and interpret experimental stereochemical results in advanced organic and medicinal chemistry contexts.

Q1. Which statement best describes a pericyclic reaction?

• A reaction that proceeds through discrete ionic intermediates.
• A concerted process that occurs via a cyclic redistribution of bonding electrons in a single transition state.
• A radical chain reaction initiated by light.
• A stepwise substitution reaction involving carbocation formation.

Correct Answer: A concerted process that occurs via a cyclic redistribution of bonding electrons in a single transition state.

Q2. According to Woodward–Hoffmann rules, how does a thermal 4n+2 π-electron electrocyclic reaction proceed stereochemically?

• Thermal 4n+2 proceeds disrotatory.
• Thermal 4n+2 proceeds conrotatory.
• Thermal 4n+2 is symmetry-forbidden.
• Thermal 4n+2 proceeds via stepwise radical mechanism.

Correct Answer: Thermal 4n+2 proceeds disrotatory.

Q3. In the electrocyclic ring closure of (E,E)-1,3,5-hexatriene under thermal conditions, what stereochemical outcome at the new sigma bond is expected?

• Formation of cis-fused cyclohexadiene via disrotatory closure.
• Formation of trans-fused cyclohexadiene via conrotatory closure.
• Mixture of stereoisomers due to stepwise mechanism.
• No reaction because thermal closure is forbidden.

Correct Answer: Formation of cis-fused cyclohexadiene via disrotatory closure.

Q4. The Diels–Alder reaction is classified as which type of pericyclic reaction?

• Electrocyclic reaction.
• Sigmatropic rearrangement.
• 4π-electrocyclization.
• Cycloaddition (specifically [4+2] cycloaddition).

Correct Answer: Cycloaddition (specifically [4+2] cycloaddition).

Q5. Which factor commonly favors the endo product in a Diels–Alder reaction?

• Maximization of steric repulsion between substituents.
• Secondary orbital interactions between the diene and dienophile substituents.
• Higher reaction temperature always favors endo.
• Solvent polarity that stabilizes charged intermediates.

Correct Answer: Secondary orbital interactions between the diene and dienophile substituents.

Q6. Which of the following best describes a [3,3]-sigmatropic rearrangement?

• Migration of a sigma bond over three atoms with a concerted six-electron cyclic transition state, e.g., Cope or Claisen rearrangement.
• Stepwise ionic substitution with carbocation intermediate.
• Interconversion of stereocenters via free radical chain.
• Photochemical cleavage of a sigma bond followed by recombination.

Correct Answer: Migration of a sigma bond over three atoms with a concerted six-electron cyclic transition state, e.g., Cope or Claisen rearrangement.

Q7. Which orbital symmetry description corresponds to a thermally allowed [1,5]-hydrogen sigmatropic shift (suprafacial) for a 6π electron system?

• Suprafacial [1,5]-H shift is thermally forbidden for 6π systems.
• Suprafacial [1,5]-H shift is thermally allowed by conservation of orbital symmetry.
• Antarafacial [1,5]-H shift is thermally allowed while suprafacial is forbidden.
• Both suprafacial and antarafacial pathways are equally allowed thermally.

Correct Answer: Suprafacial [1,5]-H shift is thermally allowed by conservation of orbital symmetry.

Q8. In a photochemical electrocyclic reaction of a 4π system, what stereochemical mode is predicted by Woodward–Hoffmann?

• Photochemical 4π proceeds disrotatory.
• Photochemical 4π proceeds conrotatory.
• Photochemical 4π is symmetry-forbidden.
• Photochemical 4π gives a stepwise diradical product.

Correct Answer: Photochemical 4π proceeds disrotatory.

Q9. Which statement about antarafacial sigmatropic shifts is correct?

• Antarafacial shifts are geometrically easy for short chains like 1,3 shifts.
• Antarafacial shifts require inversion of stereochemistry and are often geometrically disfavored in small systems.
• Antarafacial shifts are always thermally allowed for any π-electron count.
• Antarafacial shifts proceed via ionic intermediates.

Correct Answer: Antarafacial shifts require inversion of stereochemistry and are often geometrically disfavored in small systems.

Q10. The Cope rearrangement is an example of which mechanistic feature?

• A [2,3]-sigmatropic rearrangement proceeding stepwise.
• A concerted [3,3]-sigmatropic rearrangement with a chair- or boat-like transition state.
• A [1,5]-hydride migration requiring photochemical activation only.
• An intermolecular cycloaddition between two alkenes.

Correct Answer: A concerted [3,3]-sigmatropic rearrangement with a chair- or boat-like transition state.

Q11. Which orbital interaction concept is most useful for predicting outcomes of cycloaddition reactions?

• HSAB (Hard–Soft Acid–Base) theory.
• Frontier Molecular Orbital (FMO) theory, especially HOMO-LUMO interactions.
• VSEPR theory of molecular shapes.
• Marcus theory of electron transfer.

Correct Answer: Frontier Molecular Orbital (FMO) theory, especially HOMO-LUMO interactions.

Q12. In a normal electron demand Diels–Alder reaction, which orbital interaction is dominant?

• Diene LUMO with dienophile HOMO.
• Diene HOMO with dienophile LUMO.
• Both HOMO–HOMO overlap is dominant.
• Neither orbital interactions matter; it’s purely steric.

Correct Answer: Diene HOMO with dienophile LUMO.

Q13. Which experimental observation would indicate a concerted pericyclic mechanism rather than a stepwise radical or ionic mechanism?

• Isolation of a discrete carbocation intermediate.
• Retention of stereochemistry and stereospecific conversion of stereodefined substrates to single stereoisomeric products.
• Formation of a complex mixture of scrambled stereoisomers irrespective of starting stereochemistry.
• Requirement of radical initiator for reaction to proceed.

Correct Answer: Retention of stereochemistry and stereospecific conversion of stereodefined substrates to single stereoisomeric products.

Q14. For the Claisen rearrangement of allyl vinyl ether, which product and mechanistic feature are expected?

• Formation of an allylic alcohol via an ionic substitution mechanism.
• Formation of a γ,δ-unsaturated carbonyl via a concerted [3,3]-sigmatropic rearrangement with chair-like transition state preference.
• No rearrangement under thermal conditions; requires photochemistry.
• Intermolecular Diels–Alder adduct formation predominates.

Correct Answer: Formation of a γ,δ-unsaturated carbonyl via a concerted [3,3]-sigmatropic rearrangement with chair-like transition state preference.

Q15. Which feature makes a 1,3-dipolar cycloaddition concerted and symmetry-allowed?

• It always involves ionic intermediates stabilized by solvent.
• Complementary symmetry and phase match between the HOMO of the dipole and LUMO of the dipolarophile (or vice versa) enabling a concerted cyclic transition state.
• It proceeds via thermal radical polymerization steps.
• It requires a metal catalyst to proceed concertedly.

Correct Answer: Complementary symmetry and phase match between the HOMO of the dipole and LUMO of the dipolarophile (or vice versa) enabling a concerted cyclic transition state.

Q16. Which of the following predicts whether a pericyclic reaction proceeds suprafacially or antarafacially?

• The number of atoms involved and orbital topology as dictated by Woodward–Hoffmann rules and geometric constraints of the substrate.
• The acidity of the solvent alone.
• The presence of heavy metals in catalytic amounts only.
• The concentration of the reactants exclusively.

Correct Answer: The number of atoms involved and orbital topology as dictated by Woodward–Hoffmann rules and geometric constraints of the substrate.

Q17. Which pericyclic reaction is most likely to be accelerated by photochemical excitation of the substrate?

• A thermally allowed 6π electrocyclization that is also thermally facile.
• An electrocyclic reaction that is thermally forbidden but photochemically allowed, e.g., 4π conrotatory vs disrotatory switch.
• Any [3,3]-sigmatropic rearrangement, as they all require light.
• Diels–Alder reactions never respond to photochemical activation.

Correct Answer: An electrocyclic reaction that is thermally forbidden but photochemically allowed, e.g., 4π conrotatory vs disrotatory switch.

Q18. A thermal [2+2] cycloaddition between two simple alkenes is generally:

• Thermally allowed and proceeds concertedly for most unconstrained alkenes.
• Thermally forbidden by orbital symmetry and usually requires photochemical activation or diradical intermediates.
• Favored in polar protic solvents only.
• Always reversible and gives aromatic products.

Correct Answer: Thermally forbidden by orbital symmetry and usually requires photochemical activation or diradical intermediates.

Q19. In a suprafacial [1,3]-sigmatropic shift of a hydrogen atom, which statement is true for a 4π electron system under thermal conditions?

• Suprafacial [1,3]-H shifts are thermally allowed for 4π systems.
• Suprafacial [1,3]-H shifts are thermally forbidden; antarafacial would be allowed but usually geometrically impractical.
• Both suprafacial and antarafacial shifts are equally facile thermally.
• The shift proceeds via a discrete carbocation intermediate invariably.

Correct Answer: Suprafacial [1,3]-H shifts are thermally forbidden; antarafacial would be allowed but usually geometrically impractical.

Q20. Which experimental technique can most directly provide evidence of a concerted pericyclic transition state aromaticity (Hückel or Möbius type) in a reaction mechanism?

• Isolation of stable charged intermediates by NMR.
• Computational analysis of transition state using DFT to show aromatic-like cyclic electron delocalization and energy profile consistent with concerted pathway.
• Measuring melting point of starting material.
• Adding radical trap and observing no change in rate to prove stepwise ionic mechanism.

Correct Answer: Computational analysis of transition state using DFT to show aromatic-like cyclic electron delocalization and energy profile consistent with concerted pathway.

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

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