"Unveiling Pericyclic Reactions with Energy Correlation Diagrams of Ethylene and 1, 3-Butadiene"

 

"Unveiling Pericyclic Reactions with Energy Correlation Diagrams of Ethylene and 1, 3-Butadiene"

Pericyclic reactions are a type of organic chemical reaction that involve a cyclic rearrangement of electrons in a molecular system. These reactions are typically characterized by their concerted nature, meaning that all the atoms involved in the reaction move simultaneously. Correlation diagrams show energy diagrams of the original electron orbitals and the resulting molecular orbitals.

The energy correlation diagrams can provide valuable insights into the molecular orbitals, properties of hydrocarbons and behavior of the molecules in pericyclic reactions. 

One example of a pericyclic reaction that involves ethylene is the cycloaddition reaction. In this reaction, two molecules of ethylene react to form a cyclobutane ring. The reaction occurs through a concerted process, involving the formation of four new carbon-carbon bonds and the breaking of two double bonds. 

Ethylene-Molecular orbital-Symmetry

Out of all symmetry elements, ethylene molecule possesses two;

• Mirror plane σ
• Axis of rotation C_n

Symmetry about a mirror plane (σ) bisects the molecular orbital in such a way that lobes of the same color or sign are reflected, and, therefore, reflections on either side of the plane are identical. It is perpendicular to the plane of the atoms.

Symmetry about a twofold axis (C₂) passing at right angles in the same plane, and through the center of the framework of the atoms forming the molecular orbital is said to be present if the rotation of the molecule around the axis by 180 (360/2) results in a molecular orbital identical with the original. the symmetry operations applied on ethylene molecule is given below;

We know that ethylene molecule follows [2+2] cycloaddition. It might be driven under thermal conditions or photochemical conditions. But in both cases, the stereochemistry of product will be different.

[2+2] Thermal Cycloadditions

[2+2] Photochemical Cycloadditions:

We know that HOMO belongs to highest occupied molecular orbital; whereas LUMO belongs to lowest unoccupied molecular orbital. There is another term known as SOMO referring towards single occupied molecular orbital.

So, there are two cases mentioned below;

• When both alkenes interact, one of them consist of symmetric HOMO and the other's LUMO is anti-symmetric. The stereochemistry will be retained.
•  When both alkenes overlapped, both SOMO and LUMO are anti-symmetric. The stereochemistry will be reverted. 

In the case of 1,3-butadiene, pericyclic reactions can involve the cycloaddition or cycloreversion of the molecule. In the cycloaddition reaction, two molecules of butadiene react to form a cyclohexene ring. This reaction occurs through the overlap of the π orbitals of the butadiene molecules, with the resulting product having a more stable cyclic system.

The structure of butadiene is given below;

Energy correlation diagrams for electro-cyclization of butadiene is shown for both conrotatory and disrotatory pathway.

Energy correlation diagrams for electro-cyclization of butadiene is shown for both Thermal and photochemical conditions.

We can conclude the following very important points for linear conjugated π systems:

• The wave function ψ_n will have (n – 1) nodes.
• When n is odd, ψ_n will be symmetric with mirror plane and asymmetric with C₂.
• When n is even, ψ_n will be symmetric with C₂ and asymmetric with mirror plane.

Woodward Hoffman rules utilized for the electrocyclization of the hydrocarbon.

Conclusion:

In conclusion, the energy correlation diagrams of ethylene and butadiene provide a useful tool in predicting the behavior of these molecules in pericyclic reactions. These diagrams allow us to determine the relative energies of the molecular orbitals and predict the outcome of these reactions based on the concerted rearrangement of electrons in the system.