Select the most reactive dienophile from the ones shown below.
In the Diels-alder reaction, the diene and dienophile react with each other to form the new product. In this, the diene is the one that has high electron density and is bonded to electron-donating groups while the dienophile is the one that has less electron density as it is bonded to electron-withdrawing groups which will pull the electron density towards itself and create a stronger positive charge on carbon. The higher electron-withdrawing group leads to the greater reactivity of dienophile.
Out of four molecules, 'A' is bonded to methyl groups which donate the electron density as it exerts the +I effect. Thus, this will not be a good choice for the dienophile.
Now, the 'B' molecule has one carbonyl group and one methyl group. The carbonyl group is electron-withdrawing while methyl is electron-donating. So, it better dienophile as compared to 'A'.
The 'C' is bonded to two -CN group which is an electron-withdrawing group. Since both will pull the electron density, the reactivity of the dienophile increases much.
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