Chapter 20: Problem 28

Write the frontier molecular orbital analysis for the cycloaddition of butadiene with butadiene when both interact in a suprafacial manner. Is this reaction allowed?

Short Answer

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Why or why not? Answer: No, the suprafacial cycloaddition reaction between butadiene and butadiene is not allowed due to the difference in symmetry between the interacting molecular orbitals (symmetric HOMO and asymmetric LUMO), which violates the Woodward-Hoffmann rules, stating that only orbitals with the same symmetry can productively interact.

Step by step solution

Understand Suprafacial Interaction and Cycloaddition Reactions

In suprafacial interactions, the reacting ends of a molecule are on the same side. In the case of butadiene, suprafacial interaction would mean that both ends of the molecule interact from the same side. Cycloaddition reactions involve the addition of two pi bonds to form a new ring structure, typically forming sigma bonds in the process.

Identify HOMO and LUMO in both Butadiene Molecules

In order to perform the FMO analysis, we need to first identify the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) of both butadiene molecules. Butadiene has two pi bonds and can be represented as a 4 pi electron system. It has three molecular orbitals: \(\psi_1\), \(\psi_2\) and \(\psi_3\). In the ground state, the HOMO is \(\psi_2\) (which is bonding and symmetric), and the LUMO is \(\psi_3\) (which is antibonding and asymmetric).

Check Orbital Symmetry for Reaction

During a cycloaddition reaction, the interacting molecular orbitals should have the same symmetry. In this case, the reaction being analyzed is between two butadienes, so we need to check if their HOMO and LUMO orbitals have the same symmetry. The HOMO (\(\psi_2\)) of one butadiene molecule is symmetric and would interact with the LUMO (\(\psi_3\)) of another butadiene molecule, which is asymmetric.

Determine if the Reaction is Allowed

Since the molecular orbitals of the two butadiene molecules involved in the cycloaddition have different symmetry (symmetric HOMO and asymmetric LUMO), the reaction is not allowed according to the Woodward-Hoffmann rules, which state that only orbitals with the same symmetry can productively interact. The Frontier Molecular Orbital analysis of the suprafacial cycloaddition between butadiene and butadiene shows that the reaction is not allowed due to the difference in symmetry between the interacting molecular orbitals.

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Write the frontier molecular orbital analysis for the cycloaddition of butadiene with butadiene when both interact in a suprafacial manner. Is this reaction allowed?

Short Answer

Step by step solution

Understand Suprafacial Interaction and Cycloaddition Reactions

Identify HOMO and LUMO in both Butadiene Molecules

Check Orbital Symmetry for Reaction

Determine if the Reaction is Allowed

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