The Diels Alder reaction is a concerted reaction. Define concerted. O A reaction where the diene must be in the s-cis conformation to react. O A reaction where the product contains a cyclic ring. O A reaction where all changes in bonding (bond making and bond breaking) occur simultaneously O Another term for an endothermic reaction.

Answer: A reaction where all changes in bonding (bond making and bond breaking) occur simultaneously.

Explanation: In chemistry, a concerted reaction is one where all bond-breaking and bond-forming processes occur in a single step without intermediates. The Diels-Alder reaction is a classic example of a concerted reaction, where the π bonds in the diene and dienophile rearrange simultaneously to form a new cyclic compound.

Steps:

1. Identify the Reaction Type: The Diels-Alder reaction is a pericyclic reaction, specifically a [4+2] cycloaddition.

1. Understand Concerted Mechanism: In a concerted mechanism, all bond changes occur in one step. There are no intermediates formed during the reaction.

1. Apply to Diels-Alder: In the Diels-Alder reaction, the diene and dienophile interact directly, forming new σ bonds while π bonds are broken and formed simultaneously.

1. Select the Correct Definition: Among the given options, the one that describes a concerted reaction is where all changes in bonding occur simultaneously.