Question: Rank the following dienes in order of increasing reactivity in a Diels–Alder reaction.

a.

b.

c.

Diels-Alder reaction is the reaction between a 1,3-diene and a dienophile in a concerted pathway to give a six-membered cyclic ring. The reaction is a thermal reaction that occurs in the presence of heat.The general reaction scheme is shown below.

Diels-Alder reaction

There are some specific rules that govern the process of the Diels-Alder reaction, and one of the most important rules has to be that the diene should be in the s-cis conformation.

Thus, the dienes having rotatable double bonds can be converted to the s-cis conformation and are then made to perform the reaction, but the dienes in which the movement of the double bond is restricted, are unreactive.

The three dienes are given below.

Diene in s-trans conformation

In this case, although the diene is in s-trans conformation, the double bonds are rotatable, and thus, the diene is moderately reactive.

Diene in s-cis conformation

The diene is present in s-cis conformation, and hence, is highly reactive.

Diene in s-trans conformation with the restricted movement of bonds

The diene is s-trans, and the rotation of the double bonds is restricted. Hence, the diene is unreactive.

Thus, the dienes in the order of increasing reactivity are given below.

Order of reactivity of dienes