Pericyclic Reactions MCQ Quiz - Objective Question with Answer for Pericyclic Reactions - Download Free PDF

CONCEPT:

Electrocyclic Reactions and Transition States

• Electrocyclic reactions involve the interconversion between π systems (like dienes) and σ bonds (in cyclic compounds) through a conrotatory or disrotatory process, depending on the number of π electrons and thermal/photochemical conditions.
• Thermal reactions involving 6 π-electrons (like in this case) proceed via a conrotatory mechanism according to the Woodward–Hoffmann rules.
• The geometry of the transition state (chair-like vs. boat-like) is crucial, as it determines whether the product will be in a cis or trans configuration.
• Chair-like transition states favor trans-stereochemistry, while boat-like transition states allow cis-stereochemistry to be retained.

EXPLANATION:

• In the reaction shown, the starting compound undergoes a thermal electrocyclic ring closure.
• The stereochemistry of the hydrogens on the terminal ends is such that they are on the same face (both below the plane), which necessitates a cis product.
• To retain the required stereochemistry in the product (compound B), a boat-like transition state must be adopted, as the chair form cannot maintain cis hydrogens due to its fixed geometry.
• Therefore, the major product A is formed via a boat-like transition state to maintain the correct stereochemistry.

Hence, the correct statement is: Option 3 – A is formed as major product via a boat-like transition state.

CONCEPT:

Electrocyclic Reactions – Woodward-Hoffmann Rules

• Electrocyclic reactions involve the conversion of π bonds to σ bonds (or vice versa) in a cyclic system under thermal or photochemical conditions.
• The stereochemistry (conrotatory or disrotatory) depends on:
  • The number of π electrons (n)
  • The reaction condition: thermal or photochemical
• According to the Woodward–Hoffmann rules:
  • For thermal reactions:
    • n = 4q + 2 → disrotatory
    • n = 4q → conrotatory
  • For photochemical reactions:
    • n = 4q + 2 → conrotatory
    • n = 4q → disrotatory

EXPLANATION:

ODC rule 

O means: opposite           D means: Disrotatory               C means : Cis

S means :same                C means : conrotatory              T means : trans

• The given molecule is a cyclobutene derivative undergoing ring-opening to a conjugated diene.
• It involves 4 π electrons (2 double bonds formed)
• According to Woodward–Hoffmann rules:
  • Under thermal conditions, a system with 4 π electrons undergoes conrotatory ring opening.

Therefore, the reaction occurs under thermal conditions via a conrotatory ring-opening.

Concept:

Pericyclic Reactions

• Pericyclic reactions are a class of reactions that involve the concerted, cyclic rearrangement of electrons. These reactions typically involve a transition state that has a closed ring of electrons moving simultaneously around the ring.
• These reactions can be classified into different types, including electrocyclic reactions, cycloadditions, and sigmatropic rearrangements, depending on the specific mechanism and the nature of the electron flow.
• In the given reaction, the reaction mechanism involves the concerted movement of electrons in a cyclic manner, which is characteristic of pericyclic reactions.

Explanation:

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• In the given reaction, the hydroxyl group (-OH) on the aromatic ring undergoes a transformation at elevated temperatures (200°C), which is indicative of a pericyclic reaction. This reaction likely involves a [3,5]-sigmatropic rearrangement, a type of pericyclic reaction, leading to the formation of a new product.
• The reaction proceeds with the concerted movement of electrons in a cyclic fashion, making it a typical example of a pericyclic reaction.

Therefore, the class of the reaction is a pericyclic reaction.

Concept:

Electrocyclic reaction

• An electrocyclic reaction is the concerted cyclization of a conjugated π-electron system by converting one π-bond to a ring forming σ-bond. The reverse reaction may be called electrocyclic ring opening.
• Electrocyclic reactions are a type of pericyclic reaction that involves the formation or breaking of a ring in a molecule. The process involves a cyclic reorganization of the π-system's electrons.
• Electrocyclic ring-opening reactions occur under thermal or photochemical conditions and can be either conrotatory or disrotatory based on the number of π-electrons involved and the conditions under which the reaction occurs:
• In thermal reactions, 4n π-electrons lead to a disrotatory motion (the terminal atoms rotate in opposite directions), whereas 4n+2 π-electrons lead to a conrotatory motion (the terminal atoms rotate in the same direction).
• The rules are reversed under photochemical conditions: 4n π-electrons now lead to a conrotatory motion and 4n+2 π-electrons lead to a disrotatory motion.

Explanation:​

Therefore, the correct option is 1.

The correct answer is Option 1.

Explanation:

Step 1: 4π Ring Opening leading to the formation of diene system.

Step 2: Diels Alder Reaction: [4+2] cycloaddition reaction.

Conclusion:

The process involves in the given transformation is: 4π ring opening followed by [4+2] cycloaddition.

Concept:

• Cycloaddition reactions are light or heat assisted combination of two alkene systems. 
• The product has two extra strong  bonds. This is the driving force of reaction
• These reactions follow concerted mechanism
• [2+2] cycloaddition is one such class of cycloaddition reaction
• Allenes contain consecutive double bonds joined by sp hybridized C. Allenes also shows cycloaddition reactions.

Explanation:

The given allene system will show [2+2] cycloaddition with another alkene system on thermal assistance.

The less hindered double bond of allene will participate more in the reaction and will give major product.

Since, the reaction follows the concerted mechanism, stereochemistry of substituent is not affected. Therefore, the major product will have same orientation as in reactant, that is, the product will be trans for the given reaction.

Conclusion:

Therefore, the major product of given reaction is:

Concept:

• Cycloaddition reactions are combination of two or more unsaturated conjugated systems.
• The combination results in formation of another cyclic system with 2 lesser  bond but 2 more  bond.
• Formation of new  bonds is the driving force of the reaction.
• The number of participating  electrons is used to define the type of cycloaddition occurring between two conjugate systems. for example, cycloaddition between a diene and simplest alkene (dienophile) is called as [4 + 2] cycloaddition reaction.

Explanation:

• Benzyne is very reactive species and  shares its

  electron of non-linear triple bond.

• The given conjugated unsaturated system undergoes cycloaddition with benzyne as follows:

8 electrons of bicyclic conjugate system are involved in cyclization with 2 electrons of benzyne system.

Conclusion:

Hence, given transformation is an example of [8π + 2π] cycloaddition reaction.

Concept:

• Diels-Alder reaction is a type of pericyclic reaction between an alkene (called dienophile) and a diene. 
• The reaction proceeds through concerted mechanism.
• It is a syn cycloaddition reaction and thus 'locked 'cis isomer favors the reactivity with dienophile.

Explanation:

Out of A and B, Compound A is less stable and more reactive. The higher reactivity towards the dienophile can be explained by following reason

• Compound A has smaller ring and thus it acts as more rigid cis diene. While compound B is 7-membered aromatic ring. Bigger rings are more flexible. Therefore, compound B will have lower reactivity towards the dienophile.

Compound A is reactive, the activation energy will be lower for the formation of P_A.

Conclusion:

Therefore, the correct energy profile of given reactions is:

Concept:​

• Pericyclic Reaction-  Organic Reactions take place in the presence of heat or light in the cyclic compounds. The compounds or molecules taking part in it have a transition state with a cyclic geometry is called pericyclic reaction.
• Electrocyclic Reactions have two modes which are commonly known as disrotatory and conrotatory structures.
• Disrotatory Mode - In this type of rotation both atomic orbitals of the end group are present in opposite directions. This results in the Trans Geometry of the molecule.
• Conrotatory Mode-  In this type of rotation both atomic orbitals are present in the counterclockwise direction. This results in the Cis Geometry of the molecule.

​

Concept:

Diels Alder:

• Diels alder reactions occur between a conjugated diene and an alkene, usually called the dienophile.
• The Diels alder reaction is stereospecific. If there is stereochemistry in the dienophile, i.e the alkene, then the product will maintain the stereochemistry.
• Thus, cis and trans dienophiles give different diastereomers of the product.

Regioselectivity of the reaction:

• The simplest way to determine which product will be formed is to draw an ionic stepwise mechanism for the reaction to determine which end of the alkene will react with which end of the dienophile.
• Decide where the diene will act as a nucleophile and where the dienophile will act as an electrophile.
• This position indicates the largest coefficients of the HOMO and LUMO must lie.
• The possible combinations are given below:

Explanation:

• When we consider the second orientation, we get the following nucleophile and electrophile.
• So, the favourable orientation is:
• The product formed is as follows:

Hence, the correct option is: 

CONCEPT:

Electrocyclic Reactions: Rotation and Stereochemistry

• O (Opposite direction): The substituents rotate in opposite directions.
• S (Same direction): The substituents rotate in the same direction.
• D (Disrotatory rotation): The substituents rotate in opposite directions relative to each other.
• C (Conrotatory rotation): The substituents rotate in the same direction.
• C (Cis stereochemistry): The resulting product has cis configuration.
• T (Trans stereochemistry): The resulting product has trans configuration.

Explanation:- 

• Sigmatropic Reactions:- 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.

​

[3,3] sigmatropic reaction.

Concept: The Diels–Alder reaction is the reaction between a conjugated diene and an alkene (dienophile) to form unsaturated six-membered rings. Since the reaction involves the formation of a cyclic product via a cyclic transition state, it is also referred to as a "cycloaddition".

Explanation: In Diels alder reaction the diene component must be able to assume an s-cis conformation. 

This is a 1,4 addition.

ODC rule 

O means: opposite           D means: Disrotatory               C means : Cis

S means :same                C means : conrotatory              T means : trans

The given compound is 4 system, in the presence of heat, will give con rotation. both the methyl groups are in opposite directions.

According to OCT rule. The product will show trans configuration.

Cis dienophiles give us cis products, and trans dienophiles give us trans products.

1. Electron withdrawing groups on the dienophile facilitate reaction.

2. Electron donating groups on the diene facilitate reaction.

Conclusion:-

So the correct answer is option 4

Explanation:-

The O-N bond is weak compared to C-O bond due to the presence of lone pair- lone pair repulsion. So it will break.

Conclusion:-

so the final product is option 1

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: 

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).