What is the pericyclic reaction? -Introduction class notes-chemarticle

This is the introduction class of pericyclic reaction in organic chemistry for CSIR NET, GATE, IIT-JAM, M.Sc Entrance, BARC, TIFR, NTPC, DRDO.


Hello everyone, welcome to the chem article. Today we will discuss pericyclic reactions. We briefly discuss what is a pericyclic reaction. This is the first class on pericyclic in organic chemistry. This is the introduction class of pericyclic reaction in organic chemistry for CSIR NET, GATE, IIT-JAM, M.Sc Entrance, BARC, TIFR, NTPC, DRDO, etc all examinations. We are provided pericyclic reaction class notes pdf for beginners. 

What is the pericyclic reaction? -Introduction class notes-chemarticle

What is the pericyclic reaction?

The pericyclic reaction is a process of product formation that occurs by cyclic transition state. This reaction process to the cyclic transition state occurs by one or more bond formation or bond-breaking within the cyclic process. It is the pericyclic reaction definition

Example of pericyclic reaction 

One of the versatile examples of pericyclic reaction is the Diels-Alder cycloaddition reaction. This reaction is one of the most common and useful reactions for organic synthesis. Cycloaddition reaction, sigmatropic rearrangements, and electrocyclic reaction are all processes of pericyclic reaction. That means all pericyclic reactions occur in this path. 

Pericyclic reaction properties

All pericyclic reactions follow these types of properties. 

  • All pericyclic reactions take place through a cyclic transition state. That means pericyclic reactions have a cyclic transition state.
  • Pericyclic reactions are reversible.
  • No formation of any intermediate Only occurs in the transition state.
  • All pericyclic reactions are usually arrangement or addition reactions.
  • Pericyclic reactions are generally stereospecific. That means they have cis or trans configuration/ E or Z configuration. 
  • Pericyclic reactions are not affected by any catalyst
  • The pericyclic reaction only occurs in presence of heat or hv.
  • Here pericyclic reaction no need for any reagents. This reaction occurs in the presence of heat or hv.
  • This pericyclic reaction does not involve nucleophile or electrophile.

Cyclic transition state for the pericyclic reaction 

In this following cycloaddition reaction, one is diene and another is dienophile, two are attached with cyclic transition state and formation of a cyclic product. In the cycloaddition reaction formation of two sigma bonds and arrangement of three pi bonds. Or we can say that six pi electrons are arranged for this pericyclic reaction.

Cyclic transition state for the pericyclic reaction
Diels-Alder reaction: cycloaddition reaction form cyclic transition state. In case (4 pi electrons + 2 pi electrons system) cycloaddition reaction. 

The pericyclic reaction is reversible

All pericyclic reaction products are back to the formation of reactants. We can say that all pericyclic reactions are reversible. We have the same example of a cycloaddition reaction.

Pericyclic reaction molecular orbital

Symmetry of orbitals

We have to consider that the molecular orbitals Involving electrons of an atom are symbolized by wave function ψ. For every molecular orbital to have different types of a nodal plane we have to show the nodal plane 0, 1, 2….n. we can use the function that is wave ψ. We can see in this picture the wave function  ψ1  ψ2 ψ3 for different types of nodes. Increasing the nodal plane corresponds to increasing the energy of the symmetric orbitals. In case ψ1 orbital has zero nodes, ψ2 orbital symmetry has one node, ψ3 orbital has two nodes. Correspondence to stability is a decrease with increasing the energy of the orbital. Or we can say that increasing the node between orbital asymmetries corresponds to increasing the energy and decreasing the stability of the orbital. 

Ethylene molecular orbitals structure| pericyclic reaction
Ethylene molecular orbitals structure| pericyclic reaction

We have to consider +ve charge is shading and the -ve charge is no shading. The molecular orbital is complete by attaching the atomic orbitals. In the case of ethylene two carbon atom orbitals are attached and formation of an ethylene molecular orbital.

Molecular Orbital for Butadiene

We have to show that in the Butadiene molecule, a total of four types of molecular orbital are there. All molecular configurations have four different types of wave function and correspond to different types of nodes. In case two configurations are bonding orbital(pi) and another two configurations are antibonding orbital( pi*). 

Butadiene molecular orbitals structure| pericyclic reaction
Butadiene molecular orbitals structure| pericyclic reaction

Corrospondly there are two different types of terminus atomic orbital. One is ψeven and the other is ψodd. In case ψeven ( ψ2  ψ4) their terminus orbitals are opposite phase or opposite sign and ψodd ( ψ1  ψ3) their terminus orbitals are same phase or same sign. 

This butadiene molecule has two pi bonds or four pi electrons

Naturally, four electrons are occupied by the ψ1 and  ψ2 orbital which are in a lower energy state compared to  ψ3 and  ψ4. We call that  ψ2 is HOMO ( Highest Occupied Molecular Orbital) due to this orbital having one pair of electrons and  ψ3 is LUMO ( Lowest Unoccupied Molecular Orbital) due to there being no electrons. This whole process occurs in the presence of heat.

We are considering this whole situation in the presence of hv. We have to see that one HOMO electron transition occurs, which means an electron transition between  ψ2 to  ψ3. That is because in the presence of hv we call that  ψ3 is HOMO(Highest Occupied Molecular Orbital) and  ψ4 is LUMO (Lowest Unoccupied Molecular Orbital).

Importance notes

The important notes of Pericyclic reaction for the considering phase of an orbital is that: Same phase orbitals overlap to each other and formation of a bond and difference phase orbitals repulsion to each other and formation of antibonding situations.

Pericyclic reaction molecular orbital| phase and symmetry of orbital
Pericyclic reaction molecular orbital| phase and symmetry of orbital

Considering the relative phase, the Symmetry orbitals are involved in this pericyclic reaction, these data provided by various explanations that Predict the product and details about what is the stereochemistry of types of product. Using the symmetry element, Woodward and Hoffmann two scientists formulated a set of rules. 

Predict the product thermally or photochemically and their stereochemistry would follow, deeply introduced by Woodward and Hoffmann

The relative phase is the symmetry of all molecular orbitals involved in the transition of reactants to product formation. This process when you use this mechanism is understandable. This process is very much simplified by Frontier Orbital Approach

In case HOMO orbitals have an outer valence electron of the atom. And LUMO has a vacant electronic orbital.

In case we can see easily the HOMO orbitals are potential electron donors and on the other hand, the LUMO orbitals are potential electron acceptors

Types of Pericyclic Reaction

All pericyclic reactions exist in three processes, which are below.

  1. Electrocyclic reaction
  2. Cycloaddition reaction 
  3. Sigmatropic reaction

And other pericyclic reactions are Ene reaction or Group transfer reaction which proceeds in this procedure.


We Briefly discuss the pericyclic reaction session. Summarising the point of view of the pericyclic reaction mechanism, pericyclic reaction definition,  pericyclic reaction types, pericyclic reaction molecular orbitals, etc. We are also provided with the pericyclic reaction class note PDF

Frequently asked question

Q.1. How many intermediates are found in Pericyclic reactions? Or Which intermediate formed during the pericyclic reaction?

Ans: The pericyclic reaction has no intermediates form. Due to the reaction, the only formation of a cyclic transition state, which may be six members, five members, four members, etc depends on diene or dienophile.

Q.2. Why are Pericyclic reactions stereospecific?

Ans: The whole pericyclic reaction occurs in the presence of thermal or photochemical conditions. Mainly pericyclic reactions involve diene groups that have one or more than double bonds. Therefore their molecular orbital configuration, HOMO to LUMO electronic transition, occurs in the presence of thermal or photochemical conditions. According to two same phase orbitals forming a bonding, in diene, the terminus molecular orbital is different phase orbits, all rotating by con-rotation or dis-rotation. Due to con-Rotation or dis-Rotation formation of stereospecific products. That's why we can say the pericyclic reaction is Stereospecific.

Q.3. Is pericyclic reaction reversible?

Ans: Yes, the pericyclic reaction reversible is possible due to the forward reaction one bond formation and another bond is brack. In the same situation for backward reaction one bond is brack and another bond is formed respectively. This whole reaction occurs in the presence of thermal or photochemical conditions.

Q.4. Which is an example of a pericyclic reaction?

Ans: An example of pericyclic reaction is 1,3-dipolar cycloaddition reaction(Four pi electrons system + two pi electrons system). In the case of 1,3-dipolarophile ( two pi electrons system) which is attacked in 1,3-dipole (e.g- Diazomethane) and formation of five-member transition rings. Finally, the resultant value is a five-member ring product for a 1,3-dipolar cycloaddition reaction.