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

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Answer 1 · 2016-03-22T09:07:43

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

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

$color(green){"S"_N1" Mechanism"}$
www.ochempal.org

$color(green){"S"_N2" Mechanism"}$
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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.

chemwiki.ucdavis.edu

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