"Are You a Pericyclic Pro? Test Yourself with These MCQs on Pericyclic reactions"

"Are You a Pericyclic Pro? Test Yourself with These MCQs on Pericyclic reactions"

1. Which of the following is a pericyclic reaction?

A. Friedel-Crafts alkylation 

B. Hofmann degradation 

C. Diels-Alder reaction 

D. Wittig reaction

Answer: C. Diels-Alder reaction

2. Which of the following statements about pericyclic reactions is true?

A. They only involve cyclic transition states. 

B. They are always exothermic. 

C. They can be either thermally or photochemically induced. 

D. They are always concerted reactions.

Answer: C. They can be either thermally or photochemically induced.

3. Which of the following pericyclic reactions is an example of an electrocyclic reaction?

A. Diels-Alder reaction 

B. Sigmatropic rearrangement

C. Cycloaddition reaction 

D. Cycloreversion reaction

Answer: D. Cycloreversion reaction

4. Which of the following pericyclic reactions is an example of a cycloaddition reaction?

A. Sigmatropic rearrangement 

B. Electrocyclic reaction

C. Diels-Alder reaction 

D. Cycloreversion reaction

Answer: C. Diels-Alder reaction

5. What is the Woodward-Hoffmann rule?

A. It predicts the stereochemistry of pericyclic reactions.

B. It predicts the regiochemistry of pericyclic reactions.

C. It relates the symmetry of the reactants to the symmetry of the transition state.

D. It describes the mechanism of pericyclic reactions.

Answer: C. It relates the symmetry of the reactants to the symmetry of the transition state.

6. Which of the following is not a type of pericyclic reaction?

A. Electrocyclic reaction 

B. Cycloaddition reaction

C. Cycloreversion reaction 

D. Substitution reaction

Answer: D. Substitution reaction

7. Which of the following is true of the Diels-Alder reaction?

A. It is a cycloaddition reaction. 

B. It always forms a six-membered ring.

C. It is always an exothermic reaction. 

D. It cannot be catalyzed by transition metals.

Answer: A. It is a cycloaddition reaction.

8. Which of the following is not a factor that influences the rate of a pericyclic reaction?

A. The reaction temperature 

B. The concentration of the reactants

C. The reaction solvent 

D. The molecular geometry of the reactants

Answer: B. The concentration of the reactants

9. Which of the following pericyclic reactions is an example of a sigmatropic rearrangement?

A. Diels-Alder reaction 

B. Electrocyclic reaction

C. Cycloaddition reaction 

D. Claisen rearrangement

Answer: D. Claisen rearrangement

10. Which of the following pericyclic reactions is an example of a cycloreversion reaction?

A. Claisen rearrangement 

B. Cope rearrangement

C. Retro-Diels-Alder reaction 

D. Diels-Alder reaction

Answer: C. Retro-Diels-Alder reaction

11. Which of the following is not a requirement for a pericyclic reaction to occur?

A. The reaction must be concerted. 

B. The reaction must have a cyclic transition state.

C. The reaction must be exothermic. 

D. The reaction must obey the Woodward-Hoffmann rules.

Answer: C. The reaction must be exothermic.

12. Which of the following pericyclic reactions is an example of a [1,5] sigmatropic rearrangement?

A. Cope rearrangement 

B. Claisen rearrangement

C. Carroll rearrangement 

D. Brook rearrangement

Answer: A. Cope rearrangement

13. Which of the following pericyclic reactions is an example of a [3,3] sigmatropic rearrangement?

A. Claisen rearrangement 

B. Brook rearrangement

C. Enone-ene reaction 

D. Ireland-Claisen rearrangement

Answer: A. Claisen rearrangement

14. Which of the following is not a type of pericyclic reaction?

A. Rearrangement reaction 

B. Cycloaddition reaction

C. Cycloreversion reaction 

D. Electrocyclic reaction

Answer: A. Rearrangement reaction

15. Which of the following is not a requirement for a pericyclic reaction to be thermally allowed?

A. The reaction must obey the Woodward-Hoffmann rules.

B. The reaction must have a cyclic transition state.

C. The reactants must have the correct symmetry.

D. The reaction must have a large negative entropy of activation.

Answer: D. The reaction must have a large negative entropy of activation.

16. Which of the following pericyclic reactions is an example of an electrocyclic reaction? 

A. Diels-Alder reaction 

B. Cope rearrangement 

C. Retro-ene reaction 

D. Ireland-Claisen rearrangement

Answer: B. Cope rearrangement

17. Which of the following pericyclic reactions is an example of a photochemical reaction?

A. Cope rearrangement 

B. Claisen rearrangement

C. Diels-Alder reaction 

D. Sigmatropic rearrangement

Answer: C. Diels-Alder reaction

18. Which of the following pericyclic reactions is an example of a [3,2] sigmatropic rearrangement?

A. Claisen rearrangement 

B. Enone-ene reaction

C. Ireland-Claisen rearrangement 

D. Brook rearrangement

Answer: C. Ireland-Claisen rearrangement

19. Which of the following statements about sigmatropic rearrangements is true?

A. They always involve the migration of a carbocation.

B. They can only occur through the formation of a cyclic transition state.

C. They can occur with either retention or inversion of configuration.

D. They always occur with a change in the number of pi electrons.

Answer: C. They can occur with either retention or inversion of configuration.

20. Which of the following pericyclic reactions is an example of a [1,3] sigmatropic rearrangement?

A. Cope rearrangement 

B. Claisen rearrangement

C. Carroll rearrangement 

D. Brook rearrangement

Answer: C. Carroll rearrangement

21. Which of the following is not a factor that can affect the selectivity of a pericyclic reaction?

A. The reaction temperature 

B. The reaction solvent

C. The identity of the catalyst 

D. The geometry of the reactants

Answer: C. The identity of the catalyst

22. Which of the following pericyclic reactions is an example of a cycloreversion reaction?

A. Claisen rearrangement 

B. Cope rearrangement

C. Retro-Diels-Alder reaction 

D. Electrocyclic reaction

Answer: C. Retro-Diels-Alder reaction

23. Which of the following pericyclic reactions is an example of a [2+2] cycloaddition reaction?

A. Diels-Alder reaction 

B. 1,3-dipolar cycloaddition reaction

C. [2+2] photocycloaddition reaction 

D. [2+2] thermal cycloaddition reaction

Answer: D. [2+2] thermal cycloaddition reaction

24. Which of the following pericyclic reactions is an example of a [4+2] cycloaddition reaction?

A. Diels-Alder reaction 

B. 1,3-dipolar cycloaddition reaction

C. [2+2] photocycloaddition reaction 

D. [2+2] thermal cycloaddition reaction

Answer: A. Diels-Alder reaction

25. Which of the following statements about electrocyclic reactions is true?

A. They always involve the formation of a cyclic transition state.

B. They can only occur with fully conjugated systems.

C. They can occur with either a thermal or photochemical initiation.

D. They always result in the breaking of a sigma bond.

Answer: C. They can occur with either a thermal or photochemical initiation.

26. Which of the following pericyclic reactions is an example of a [1,2] sigmatropic rearrangement? 

A. Cope rearrangement 

B. Claisen rearrangement 

C. Carroll rearrangement 

D. Brook rearrangement

Answer: B. Claisen rearrangement

27. Which of the following statements about Woodward-Hoffmann rules is true?

A. They are a set of empirical rules that predict the outcome of pericyclic reactions.

B. They are based on the analysis of quantum mechanical calculations of pericyclic reactions.

C. They apply only to thermally allowed pericyclic reactions.

D. They have no practical applications in organic chemistry.

Answer: A. They are a set of empirical rules that predict the outcome of pericyclic reactions.

28. Which of the following pericyclic reactions is an example of a [1,6] sigmatropic rearrangement?

A. Cope rearrangement 

B. Claisen rearrangement 

C. Carroll rearrangement 

D. Brook rearrangement

Answer: D. Brook rearrangement

29. Which of the following pericyclic reactions is an example of a [2+2] photocycloaddition reaction?

A. Diels-Alder reaction 

B. 1,3-dipolar cycloaddition reaction

C. [2+2] thermal cycloaddition reaction 

D. [4+4] photocycloaddition reaction

Answer: C. [2+2] thermal cycloaddition reaction

30. Which of the following pericyclic reactions is an example of a thermal [1,5] sigmatropic rearrangement? 

A. Cope rearrangement 

B. Claisen rearrangement 

C. Carroll rearrangement 

D. Brook rearrangement

Answer: C. Carroll rearrangement

31. Which of the following approaches is used to predict the outcome of pericyclic reactions?

A. PMO 

B. FMO 

C. Both A and B 

D. None of the above

Answer: C. Both A and B

32. In the PMO approach, what do the coefficients of the molecular orbitals represent?

A. The energy of the orbitals 

B. The electron density of the orbitals

C. The symmetry of the orbitals 

D. The nodal planes of the orbitals

Answer: B. The electron density of the orbitals

33. Which of the following pericyclic reactions is symmetry allowed?

A. Electrocyclic ring opening 

B. Cope rearrangement

C. [2+2] cycloaddition 

D. [3,3] sigmatropic rearrangement

Answer: D. [3,3] sigmatropic rearrangement

34. In the FMO approach, what do the energies of the molecular orbitals determine?

A. The electron density of the orbitals 

B. The stability of the molecule

C. The symmetry of the orbitals 

D. The nodal planes of the orbitals

Answer: B. The stability of the molecule

35. Which of the following pericyclic reactions is symmetry forbidden?

A. Electrocyclic ring closure 

B. Cope rearrangement

C. [2+2] cycloaddition 

D. [3,3] sigmatropic rearrangement

Answer: C. [2+2] cycloaddition

36. Which of the following is not one of the Woodward-Hoffmann rules?

A. Conservation of orbital symmetry

B. The frontier orbitals of the reactants control the outcome of the reaction

C. The transition state should be as low in energy as possible

D. The reaction should be allowed by the conservation of angular momentum

Answer: C. The transition state should be as low in energy as possible

37. In a suprafacial reaction, what happens to the two groups involved in the reaction?

A. They remain on the same side of the molecule 

B. They switch sides of the molecule

C. One group moves to the opposite side of the molecule 

D. Both groups are removed from the molecule

Answer: A. They remain on the same side of the molecule

38. Which of the following pericyclic reactions is an example of an antarafacial process?

A. [1,5] sigmatropic rearrangement 

B. [3,3] sigmatropic rearrangement

C. Electrocyclic ring opening 

D. Cope rearrangement

Answer: B. [3,3] sigmatropic rearrangement

39. Which of the following pericyclic reactions is an example of a suprafacial process?

A. [1,5] sigmatropic rearrangement 

B. [3,3] sigmatropic rearrangement

C. Electrocyclic ring closure 

D. Cope rearrangement

Answer: D. Cope rearrangement

40. In a symmetry allowed pericyclic reaction, what is conserved?

A. Angular momentum 

B. Spin 

C. Orbital symmetry 

D. Molecular weight

Answer: C. Orbital symmetry

 

41. In which type of pericyclic reaction does the reaction proceed through a cyclic transition state?

A. Electrocyclic reaction 

B. Cycloaddition reaction 

C. Sigmatropic rearrangement 

D. All of the above

Answer: D. All of the above

42. Which of the following pericyclic reactions is an example of a thermal reaction?

A. Diels-Alder reaction 

B. Photochemical cycloaddition

C. [1,5] sigmatropic rearrangement 

D. Electrocyclic ring closure

Answer: A. Diels-Alder reaction

43. Which of the following pericyclic reactions involves a concerted reaction pathway?

A. Electrocyclic ring opening 

B. Diels-Alder reaction 

C. Cope rearrangement 

D. All of the above

Answer: D. All of the above

44. In a Diels-Alder reaction, which of the following orbitals must be in phase?

A. HOMO of diene and LUMO of dienophile 

B. LUMO of diene and HOMO of dienophile

C. HOMO of diene and HOMO of dienophile 

D. LUMO of diene and LUMO of dienophile

Answer: A. HOMO of diene and LUMO of dienophile

45. Which of the following is a characteristic of a concerted pericyclic reaction?

A. The reaction occurs in multiple steps

B. The reaction mechanism involves the formation of a carbocation intermediate

C. The reaction is not stereospecific

D. The reaction is exothermic

Answer: D. The reaction is exothermic

46. Which of the following is an example of a [4+2] cycloaddition reaction?

A. Diels-Alder reaction 

B. [1,3] dipolar cycloaddition

C. [3,2] sigmatropic rearrangement 

D. Cope rearrangement

Answer: A. Diels-Alder reaction

47. In a sigmatropic rearrangement, which of the following groups is conserved?

A. The size of the molecule 

B. The stereochemistry of the molecule

C. The electronic configuration of the molecule 

D. The functional group of the molecule

Answer: A. The size of the molecule

48. Which of the following is an example of an electrocyclic reaction?

A. Cope rearrangement 

B. [3,3] sigmatropic rearrangement

C. Electrocyclic ring opening 

D. [1,5] sigmatropic rearrangement

Answer: C. Electrocyclic ring opening

49. Which of the following is an example of a photochemical reaction?

A. Diels-Alder reaction 

B. [1,3] dipolar cycloaddition

C. [3,2] sigmatropic rearrangement 

D. None of the above

Answer: B. [1,3] dipolar cycloaddition

50. Which of the following is a characteristic of a symmetry-forbidden pericyclic reaction?

A. The reaction occurs via a cyclic transition state

B. The reaction is stereospecific

C. The reaction does not follow the Woodward-Hoffmann rules

D. The reaction involves the conservation of angular momentum

Answer: C. The reaction does not follow the Woodward-Hoffmann rules

51. Which of the following is true regarding nodes in pericyclic reactions?

A. Nodes are regions of high electron density in the reaction center

B. Nodes are regions of low electron density in the reaction center

C. Nodes are regions of maximum overlap between orbitals

D. Nodes are regions of destructive interference between orbitals

Answer: D. Nodes are regions of destructive interference between orbitals.

52. Which of the following statements is true regarding aromatic and antiaromatic molecules in pericyclic reactions?

A. Aromatic molecules always undergo pericyclic reactions with low activation energy

B. Anti-aromatic molecules always undergo pericyclic reactions with low activation energy

C. Aromatic molecules follow the Woodward-Hoffmann rules, while antiaromatic molecules do not

D. Anti-aromatic molecules follow the Woodward-Hoffmann rules, while aromatic molecules do not

Answer: C. Aromatic molecules follow the Woodward-Hoffmann rules, while antiaromatic molecules do not.

53. Which of the following is an example of a thermal pericyclic reaction?

A. Diels-Alder reaction 

B. [1,5] sigmatropic rearrangement

C. Photochemical [2+2] cycloaddition 

D. [1,3] dipolar cycloaddition

Answer: B. [1,5] sigmatropic rearrangement.

54. Which of the following is true regarding photochemical pericyclic reactions?

A. Photochemical reactions always occur with high stereoselectivity

B. Photochemical reactions always have high activation energy barriers

C. Photochemical reactions always involve the breaking of a bond

D. Photochemical reactions can be used for the synthesis of complex molecules with high efficiency

Answer: D. Photochemical reactions can be used for the synthesis of complex molecules with high efficiency.

55. Which of the following statements is true regarding photochemical pericyclic reactions?

A. Photochemical reactions always require light of a specific wavelength to occur

B. Photochemical reactions always involve the formation of a cyclic transition state

C. Photochemical reactions are always exothermic

D. Photochemical reactions can only occur in the gas phase

Answer: A. Photochemical reactions always require light of a specific wavelength to occur.

Understanding Pericyclic Reactions in terms of Symmetry-allowed and Symmetry-forbidden and Molecular orbital theory

 

Understanding Pericyclic Reactions in terms of Symmetry-allowed and Symmetry-forbidden and Molecular orbital theory:

Pericyclic reactions are a class of organic reactions that involve a cyclic rearrangement of electrons within a conjugated system of atoms. These reactions are highly stereospecific and are governed by the principles of orbital symmetry and molecular orbital theory.

The following key points are raised in MOT;

§  Molecular orbital can be described by the linear combination of atomic orbitals (LCAO). In a π molecular orbital, each electron that previously occupied a p atomic orbital surrounding an individual carbon nucleus now surrounds the entire part of the molecule that is included in the interacting p orbitals.

§  A p orbital has opposing phases for its two lobes. A covalent bond is created when two in-phase atomic orbitals come into contact. A node forms between two nuclei when two out-of-phase atomic orbitals come into contact with one another.

§  The same principles that govern how electrons fill atomic orbitals—the aufbau principle, the Pauli exclusion principle, and Hund's rule—apply to how they fill molecular orbitals: Only two electrons can occupy a given molecular orbital, and an electron enters the accessible molecular orbital with the lowest energy.

Figure displays an explanation of ethene's molecular orbitals. (One phase of the two lobes of a p orbital is represented by a blue lobe, while the other phase is represented by a pink lobe).  Ethene has one π bond, which results in two p atomic orbitals and two π molecular orbitals. A bonding π molecular orbital is produced by the in phase interaction of the two p atomic orbitals and is denoted by Ψ₁ (Ψ is the Greek letter psi). The isolated p atomic orbitals have more energy than the bonded molecular orbital. Ethene's two p atomic orbitals are capable of out-of-phase interactions as well. An antibonding π* molecular orbital Ψ₂, which has a higher energy than the p atomic orbitals, is produced by the interaction of out-of-phase orbitals. The atomic orbitals interact additively to produce the bonding molecule orbital, whereas they interact subtractively to produce the antibonding molecular orbital. In other words, the interaction between in-phase and out-of-phase orbitals pulls atoms away whereas the interaction between in-phase and in-phase orbitals binds atoms together. The two electrons in ethene are located in the bonding molecular orbital because two electrons can occupy a molecular orbital because electrons live in the available molecular orbitals with the lowest energy. All molecules with a single carbon-carbon double bond are represented by this molecular orbital diagram.

Due to its two conjugated bonds, 1,3-butadiene contains four p atomic orbitals. There are four possible linear combinations for four atomic orbitals. There are thus four molecular orbitals: Ψ₁, Ψ₂, Ψ₃ and Ψ₄ and Oscillations are retained, as you can see: Four molecular orbitals are created when four atomic orbitals are combined. The other half are antibonding molecular orbitals (Ψ₃ and Ψ₄), and half are bonding molecular orbitals (Ψ₁ and Ψ₂). Two electrons are in Ψ₁ and two electrons are in Ψ₂ because the four electrons will live in the available molecular orbitals with the lowest energy. Keep in mind that despite having varying energies, all molecular orbitals are legitimate and can coexist. All compounds with two conjugated carbon-carbon double bonds are represented by this molecular orbital image.

For instance, Ψ₁ has three bonding interactions and zero nodes between the nuclei, Ψ₂ has two bonding interactions and one node between the nuclei, Ψ₃ has one bonding interaction and two nodes between the nuclei, and has zero bonding interactions and three nodes between the nuclei. In 1,3-butadiene, the highest occupied molecular orbital (HOMO) is Ψ₂ and the lowest unoccupied molecular orbital (LUMO) is Ψ₃. Light of the right wavelength will boost an electron from a molecule's ground-state HOMO to its LUMO if the molecule absorbs the light from Ψ₂ to Ψ₃. At that point, the molecule is stimulated. The HOMO is Ψ₃ and the LUMO is Ψ₄ in the excited state. In a photochemical reaction, the reactant is in an excited state as opposed to the ground state in a thermal reaction.

 You should be aware that a molecular orbital is bonding if there are more bonding interactions than there are nodes between the nuclei, and it is antibonding if there are less bonding contacts than there are nodes between the nuclei.

A molecular orbital description of 1,3,5-hexatriene is shown in figure given below;

The concept of symmetry is central to the understanding of pericyclic reactions, as it can determine whether a reaction is allowed or forbidden.

Symmetry-allowed reactions

In terms of molecular orbital, if two p-orbitals are in phase or both have same lobes they have symmetry allowed. Symmetry-allowed reactions involve a change in the symmetry of the molecular orbitals in the reactants as they undergo a cyclic rearrangement of electrons. This change in symmetry must be consistent with the symmetry of the cyclic transition state. When the symmetry of the molecular orbitals and the transition state are the same, the reaction is allowed.

For example, the Diels-Alder reaction involves the reaction of a diene and a dienophile to form a cyclic product. The reaction is allowed when the HOMO of the diene and the LUMO of the dienophile have the same symmetry as the cyclic transition state. This allows for a smooth flow of electrons through the system, leading to the formation of the cyclic product.

Symmetry-forbidden reactions:

In terms of molecular orbital, if two p-orbitals are out of phase or both have different lobes generating a nodal plane then, they have symmetry forbidden. Symmetry-forbidden reactions involve a change in the symmetry of the molecular orbitals that is not consistent with the symmetry of the cyclic transition state. These reactions are generally disallowed and do not occur under normal conditions.

For example, the electrocyclic ring closure of 1,3,5-hexatriene involves a change in the symmetry of the molecular orbitals that is not consistent with the symmetry of the transition state. This results in a symmetry-forbidden reaction that does not occur under normal conditions.

Methods for explaining pericyclic reactions:

There are several methods that can be used to explain the principles of orbital symmetry and the behavior of pericyclic reactions. These methods include:

Molecular orbital theory:

This theory describes the behavior of electrons in a molecule using a set of mathematical functions called molecular orbitals. Molecular orbital theory can be used to predict the outcome of pericyclic reactions by analyzing the symmetry and energy levels of the molecular orbitals involved.

Frontier orbital theory:

This theory focuses on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the reactants in a pericyclic reaction. Frontier orbital theory can be used to predict the regio- and stereoselectivity of a reaction by analyzing the overlap of the HOMO and LUMO orbitals.

Woodward-Hoffmann rules:

These rules provide a set of guidelines for predicting the stereochemistry of pericyclic reactions based on the symmetry of the reactants and the transition state.

Conclusion:

In conclusion, energy of the molecular orbital increases, the number of bonding interactions decreases and the number of nodes between the nuclei increases. Understanding the principles of symmetry-allowed and symmetry-forbidden reactions is crucial for understanding the behavior of pericyclic reactions. Molecular orbital theory, frontier orbital theory, and Woodward-Hoffmann rules are important tools for predicting the outcome of pericyclic reactions and designing new reactions with specific stereochemical outcomes. By utilizing these methods, chemists can unlock new possibilities for organic synthesis and materials science.