Pericyclic Reactions MCQ Quiz in বাংলা - Objective Question with Answer for Pericyclic Reactions - বিনামূল্যে ডাউনলোড করুন [PDF]

Concept:-

Sigmatropic Reactions:-

• A Sigmatropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one sigma bond changed to another sigma bond.
• They are kind of pericyclic reactions one sigma- bond is changed to another by uncatalyzed intermolecular reactions. in these reactions by the dislocation of pie-bonds, the carbon-carbon bond rearrangements are done.

1,5-Hydrogen shift ([1,5-H] shift): 

• The 1,5-Hydrogen shift involves the migration of a hydrogen atom to an atom four bonds further away in a single mechanism step.
• A 1,5-H shift is an example of a sigmatropic reaction.
• In a sigmatropic reaction a σ bond 'migrates' across a conjugated system to a new site.

Cycloaddition Reaction:

• The cycloaddition reaction is a chemical reaction in which two or more unsaturated molecules combine with each other to form a cyclic compound, with a reduction of bond multiplicity.
• In other words, cycloaddition reaction indicates the addition of two or more π-bonds to form a cyclic adduct with the formation of bonds at the end of the pi bonds.

Explanation:-

• The reaction pathway is shown below:

• From the above reaction, we can see that the first step of the reaction involves a [1,5]H-shift reaction.
• In the next step, it undergoes a [4+2]pi cycloaddition reaction to give the final product.

Conclusion:-

Hence, the given reaction proceeds via​ a [1, 5]-H shift followed by [4 + 2] cycloaddition

Concept:

Cycloaddition reaction:

• The cycloaddition reaction indicates the addition of the two unsaturated compounds to form a cyclic compound with the formation of a sigma () bond at the end of the  component.
• An example of [2+2] cycloaddition reaction is shown below:

1,3 dipolar cycloaddition reaction:

• The 1,3-dipolar cycloaddition is a chemical reaction between a 1,3-dipole and a dipolarophile to form a five-membered ring.
• An example is given below:

Explanation:-

• The reaction pathway is shown below:

• From the above reaction, it is shown that in the first step of the reaction, ZnCl₂ catalyzes the decarboxylation reaction that results in a 1,3 dipolar intermediate.
• In the next step, the 1,3 dipolar intermediate undergoes a cycloaddition reaction with cyclopentadiene to give the final product. 

Conclusion:-

Hence, the major product formed in the given reaction is

Concept:

​Carbene:

• Carbenes are neutral substances with six valence shell electrons on the carbon atom.
• Carbenes are divalent compounds with two valence electrons distributed between two nonbonding orbitals and the carbon atom is subsequently bonded to two same or different atoms.

[2,3] Sigmatropic Rearrangement:

• [2,3] Sigmatropic rearrangement is a thermal isomerization reaction involving 6 electrons and a five-membered cyclic transition state as shown below in the general form.
• An example of [2,3] Sigmatropic rearrangement is shown below:

• According to the Woodward-Hoffmann rules the rearrangement should occur in a suprafacial manner with retention of the configuration of the migrating group.

Explanation:-

• The first step of the reaction involves the formation of a carbene intermediate upon heating N2(CO2Et)₂ with Cu salt.

​

• In the next step, the lone pair of electrons of the S atom interacts with the generated carbene to give an intermediate product.
• In the next step, the intermediate product undergoes a [2,3] Sigmatropic rearrangement reaction to give the final product.
• The reaction pathway is shown below:

Conclusion:-

• Hence, answer 1 is the correct answer.

Concept: 

The mechanism of the reaction follows Norrish Type-I and Type-II reaction.

Norrish Type-I reaction involves photochemical cleavage of -unsaturated ketone/aldehyde group where as Norrish Type-II proceeds via photochemical abstraction of -hydrogen.

Example:

Norrish Type-I:

Norrish Type-II

Explanation:

In this reaction photolysis of C-O bond occurs followed by removal of -hydrogen and then cyclisation.

Conclusion:

Therefore the correct option is option (4).

Concept:

Electrocyclic Reaction:

Electrocyclic Reactions involves ring opening and ring-closing by the formation or breaking of new  bond from  bond through the concerted mechanism.

• Electrocyclic Reactions have two modes which are commonly known as disrotatory and conrotatory mode.

1. Disrotatory Mode - In this type of rotation both atomic orbitals of the end group are present in opposite directions. This results in Trans Geometry of the molecule.​
2. Conrotatory Mode-  In this type of rotation both atomic orbitals are present in the counterclockwise direction. This results in Cris Geometry of the molecule.

• Under the photochemical condition for a system with 4n number of pi electrons, ring opening and ring-closing reaction is given below:

Explanation:

It is 4n+2 system with light so the symmetry of product after sigma bond rearrangement would be antarafacial.

Concept:

Cycloaddition Reaction:

• The cycloaddition reaction is a chemical reaction in which two or more unsaturated molecules combine with each other to form a cyclic compound, with a reduction of bond multiplicity.
• In other words, cycloaddition reaction indicates the addition of two or more π-bonds to form a cyclic adduct with the formation of bonds at the end of the pi bonds.

Explanation: 

• The reaction is a [4+2]π cycloaddition reaction followed by a cycloreversion reaction, in which 5-oxo-3-phenyl-4,5-dihydro-1,2,3-oxadiazol-3-ium-4-ide acts as a dipolarophile. whereas the (1Z,5Z)-cycloocta-1,5-diene acts as a dienophile. The dipolarophile reacts in a cycloaddition reaction with 1,3-dipoles.

• The reaction proceeds through the following mechanism

Conclusion:

Hence, the following transformation involves a series of ​​cycloaddition and cycloreversion reactions.

Explanation:

The reagent alkyl lithium favors 1,2-addition , the product formed undergo cope rearrangement.

The correct answer is Option 1.

Explanation:

The conrotatory eight-electron electrocyclization is followed by the disrotatory six-electron electrocyclization.

Conclusion:

Therefore, 8π conrotation followed by 6π disrotatorion will give the desired product.

Concept:

Cycloaddition reactions, are a type of chemical reaction where two π-systems combine to form a new ring structure. The behavior of these reactions under thermal conditions is explained by the Woodward-Hoffmann rules, which are based on the conservation of orbital symmetry.

• Woodward-Hoffmann Rules: These rules predict whether a pericyclic reaction will proceed via a thermal or photochemical process based on the symmetry properties of the molecular orbitals involved. For thermal reactions, a reaction pathway is favorable if it involves suprafacial interaction of orbitals on the same face of the π-system, or antarafacial interaction where one component reacts from the opposite side.

• Suprafacial and Antarafacial: Suprafacial reactions involve interactions where the orbitals on the same face of the π-systems come together. In contrast, antarafacial reactions involve interactions where the orbitals on opposite faces of the π-systems come together. The former is more common in reactions involving fewer π-electrons due to lesser strain and more favorable orbital overlap.

  • Diene: The cyclic compound with an 8π electron system acts as the diene. It provides its Highest Occupied Molecular Orbital (HOMO) for interaction.

  • Dienophile: The azodicarboxylate compound with a 2π system acts as the dienophile. It provides its Lowest Unoccupied Molecular Orbital (LUMO) for interaction with the HOMO of the diene.

Explanation:

Mechanism:

• As shown in the diagram, the diene has an 8π electron system, which provides the HOMO (ψ₄). The dienophile, with a 2π system, provides the LUMO (ψ₂). The HOMO of the diene interacts with the LUMO of the dienophile through a suprafacial interaction.

• The resulting product is structure A, where the cycloaddition occurs between the conjugated π-systems of both the diene and the dienophile. The 8π + 2π interaction leads to the formation of a six-membered ring in the product.

​Conclusion:

The major cycloaddition product is A, and the orbital interaction involves an 8π_s + 2π_s interaction, as depicted in option 1.

Concept:

The transformation involves a photochemical reaction that can proceed via different pathways depending on the isomer. The key terms to understand here are:

• Threo and Erythro Isomers: These are stereoisomers that differ in the relative configuration of substituents on adjacent carbon atoms. In the threo isomer, the two substituents are on opposite sides, while in the erythro isomer, the substituents are on the same side.

• Norrish Type-I Reaction: This is a photochemical reaction where a carbonyl compound undergoes α-cleavage, breaking the bond between the α-carbon and the carbonyl carbon, leading to the formation of a radical pair.

• Norrish Type-II Reaction: This reaction involves γ-hydrogen abstraction by the excited carbonyl compound, leading to the formation of a cyclic intermediate, followed by cleavage to produce an olefin and a ketone.

Explanation: 

• • Formation of alkene indicates the occurrence of Norrish type II reaction by abstraction of γ-hydrogen followed by breaking of bond between α and  carbon atoms.

• Mechanism:
  • Bond rotation in step I occur because"Hydrogen abstraction in norrish type II reaction occur from the same plane".
  • In step II, γ-hydrogen abstraction(Norrish Type II) occurs followed by bond breaking leading to final alkene as a product.

Conclusion:

The correct statement is that the erythro isomer gives the product via Norrish Type-II reaction.