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.


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.