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

1 Answer
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 "S"_N1 and "S"_N2.

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

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