Pericyclic Reactions MCQ Quiz in मल्याळम - Objective Question with Answer for Pericyclic Reactions - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

The correct option is option 2.

Concept :-    [2+2] cycloaddition

[2+2] cycloaddition is a type of pericyclic chemical reaction, where a four-membered ring is formed via the coupling of two unsaturated molecules known as alkenes.

The reaction involves a concerted movement of electrons where two π bonds break and two new σ bonds form simultaneously. The reaction name [2+2] refers to two π electrons from each alkene (total 4) being involved in the ring formation.

Staudinger ketene cycloaddition is the organic pericyclic reaction. The thermal [2+2] cycloaddition reaction of ketenes with carbon-carbon, carbon-oxygen, and carbon-nitrogen double bonds is referred to as the Staudinger ketene cycloaddition. Staudinger ketene-imine cycloaddition also known as Staudinger Ketene Cycloaddition.

Explanation:-

Mechanism of Staudinger ketene cycloaddition :

The Mechanism Staudinger cycloaddition reaction of ketenes with alkenes is believed to occur via a concerted nonsynchronous mechanism, where the approach of the reacting partners is orthogonal. Consequently, the bulkier substituent of the ketene will end up on the sterically more crowded face of the cyclobutanone product.

Conclusion:-

This reaction goes by Staudinger ketene cycloaddition and  resulted product is option 2

Concept:

The 1,7-sigmatropic rearrangement, often referred to as a [1,7]-sigmatropic rearrangement, is a type of pericyclic reaction in organic chemistry. In this rearrangement, a sigma (σ) bond between two carbon atoms separated by six intervening carbon atoms (hence, a 1,7-bond) shifts, resulting in the migration of substituents and the formation of a new sigma bond.

Explanation:

⇒The reactant undergoes 1,7-sigmatropic rearrangement in the presence of heat as follows-
 

⇒Stereochemistry of the product-

On C-3 and C-5 atoms the higher priority groups are both on the same side of the double bond. Therefore, alkenes on position 3 and 5 are Zusammen i.e. Z.

On C-7 the higher priority groups are on the opposite side of each other of the double bond. Therefore, alkene on position 7 is Entgegen i.e. E.

Conclusion:

Hence, the stereochemistry of the double bonds in the product is 3Z, 5Z, 7E

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 (\(\sigma \)) bond at the end of the \(\pi\) 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 \(\alpha,\beta\)-unsaturated ketone/aldehyde group where as Norrish Type-II proceeds via photochemical abstraction of \(\gamma\)-hydrogen.

Example:

Norrish Type-I:

Norrish Type-II

Explanation:

In this reaction photolysis of C-O bond occurs followed by removal of \(\gamma\)-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 \(\sigma \) bond from \(\pi\) 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.

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.