Concerted Mechanism

Definition

A concerted mechanism refers to a reaction that occurs in a single, continuous step without the formation of any discrete intermediates. In a concerted mechanism, the bonds that are being formed and broken happen simultaneously, leading to the product in a single, coordinated process.

5 Must Know Facts For Your Next Test

1. Concerted mechanisms are often associated with pericyclic reactions, such as the Diels-Alder cycloaddition, electrocyclic reactions, and sigmatropic rearrangements.
2. The concertedness of a reaction can be determined by the presence or absence of intermediates, as well as the synchronicity of the bond-forming and bond-breaking processes.
3. Concerted mechanisms are typically more efficient and have lower activation energies compared to stepwise mechanisms, as they avoid the formation of high-energy intermediates.
4. The stereochemistry of the products in a concerted mechanism is often retained, as the reaction occurs in a single, continuous step.
5. The deuterium isotope effect can be used to probe the concertedness of a reaction, as the synchronous bond-breaking and bond-forming processes in a concerted mechanism can lead to a larger isotope effect.

Review Questions

• Explain how the concerted mechanism is characterized in the context of the Diels-Alder cycloaddition reaction.
  • The Diels-Alder cycloaddition reaction is a classic example of a concerted mechanism. In this reaction, the diene and the dienophile undergo a simultaneous formation of two new carbon-carbon bonds, resulting in the formation of a cyclic product. The concertedness of this reaction is evident from the lack of discrete intermediates and the synchronous bond-forming and bond-breaking processes. The stereochemistry of the reactants is also typically retained in the product, further indicating the concerted nature of the Diels-Alder reaction.
• Discuss how the concerted mechanism is related to the E2 reaction and the deuterium isotope effect.
  • In the E2 reaction, the base-induced elimination of a leaving group occurs through a concerted mechanism. The simultaneous removal of the leaving group and the abstraction of the α-hydrogen result in the formation of the alkene product. The concertedness of this reaction can be probed using the deuterium isotope effect, as the synchronous bond-breaking and bond-forming processes in the concerted mechanism can lead to a larger isotope effect compared to a stepwise mechanism. The observation of a significant deuterium isotope effect in an E2 reaction is often indicative of the concerted nature of the elimination process.
• Analyze the role of frontier molecular orbitals in determining the concertedness of a pericyclic reaction, such as an electrocyclic reaction.
  • The concertedness of pericyclic reactions, like electrocyclic reactions, is closely linked to the interaction of frontier molecular orbitals. In a concerted electrocyclic reaction, the HOMO of one reactant interacts with the LUMO of the other reactant in a specific, symmetry-allowed manner. This orbital interaction allows for the simultaneous formation and breaking of bonds, resulting in the concerted mechanism. The stereochemistry of the products is also influenced by the frontier orbital interactions, as the concerted nature of the reaction preserves the relative orientation of the reacting groups. Understanding the frontier orbital interactions is crucial in predicting and explaining the concerted nature of pericyclic reactions.

Related terms

Stepwise Mechanism:

A stepwise mechanism involves the formation of discrete intermediates, where the reaction proceeds through multiple individual steps.

Synchronous: In a concerted mechanism, the bond-forming and bond-breaking processes are synchronous, meaning they occur at the same time.

Frontier Molecular Orbitals:

The interaction of frontier molecular orbitals, such as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), is crucial in determining the concertedness of a reaction.