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

Mar 22, 2016

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

#### Explanation:

Let's begin with something more commonly encountered: Compare the reaction between ${\text{S}}_{N} 1$ and ${\text{S}}_{N} 2$.

$\textcolor{g r e e n}{\text{S"_N1" Mechanism}}$

$\textcolor{g r e e n}{\text{S"_N2" Mechanism}}$

The ${\text{S}}_{N} 2$ 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.