Question

What does it mean if the Diels Alder reaction is a concerted reaction?

Answer 1

It means that all the bonds are formed and broken simultaneously.

Explanation:

Let's begin with something more commonly encountered: Compare the reaction between `"S"_N1` and `"S"_N2`.

`color(green){"S"_N1" Mechanism"}`

`color(green){"S"_N2" Mechanism"}`

The `"S"_N2` reaction is concerted. No intermediates are formed and there is only a single transition state.

The Diels-Alder reaction also involves no intermediates. An example of Diels-Alder reaction between 1,3-butadiene and ethene is shown below.

All the electrons move simultaneously. The old bonds are broken at the same time as the new bonds are formed.