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:-

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.
• The original sigmatropic rearrangement occurred when an aryl allyl ether was heated without solvent and an ortho-allyl phenol resulted. This is the Claisen rearrangement. The first step in this reaction is a pericyclic reaction of a type that you will learn to call a [3,3]-sigmatropic
  rearrangement.

Claisen rearrangement:

• In a claisen rearrangement reaction, the allyl vinyl ether is converted into a gamma, delta-unsaturated carbonyl compound on heating or in the presence of a Lewis acid.
• An example is shown below:

Explanation:-

• The Chemical reaction involved in the above transformation can be illustrated as

 
So, this reaction is Claisen rearrangement and oxidative decarboxylation.

Conclusion:-

• Hence, for the following biogenetic conversion of chorismic acid (A) to 4-hydroxyphenyl pyruvic acid (C), the correct statement is

X is Claisen rearrangement; Y is oxidative decarboxylation.

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:

Electrocyclic Reactions

• An electrocyclic reaction is a type of pericyclic reaction where a conjugated polyene undergoes a ring closure to form a cyclic product, or a cyclic compound undergoes ring opening to form a conjugated polyene.
• These reactions are generally intramolecular and can be reversible.
• Electrocyclic reactions can be either photochemically or thermally induced, leading to the formation or breaking of a sigma bond.

EXPLANATION:

• In the given statements:
  • Option 1: "It is an inter-molecular reaction." - This is untrue because an electrocyclic reaction is typically an intramolecular reaction, not an intermolecular one.
  • Option 2: "A reactant converts into cyclic product." - This is true as the reaction involves the formation of a cyclic product from a conjugated polyene.
  • Option 3: "It is a reversible reaction." - This is true since electrocyclic reactions can often be reversed, leading back to the original conjugated polyene.
  • Option 4: "When it reverses, a sigma-bond in cyclic compound breaks to form a conjugated pi-system." - This is true as the reverse process involves breaking a sigma bond to regenerate the conjugated pi system.

Therefore, the untrue statement about an electrocyclic reaction is: "It is an inter-molecular reaction."