Which one of the following is not true concerning diels-alder reactions?
a. the reaction is stereospecific.
b. the reaction mechanism has only one step.
c. the reaction mechanism involves a resonance stabilized carbocation.
d. the diene must be a conjugated diene.

Option c is incorrect

Explanation:

Diels-Alder reaction is a 4+2 cycloaddition reaction .

The reaction occurs between a diene and a dieneophile .

Generally In the Diels-alder reaction HOMO of the diene and LUMO of the dienophile react with proper orbital symmetry. But vice-versa can also be used.

HOMO-Highest occupied molecular orbital

LUMO-Lowest unoccupied molecular orbital

The primary driving force for the diels alder reaction is is the conversion of 2pi bonds into 2 sigma bonds. The sigma bonds are energetically more favorable than pi bonds.

The diels alder reaction happens through orbital interaction and hence the substituents on either the diene or dienophile do not change there stereochemistry in the product so  it is a stereospecific reaction.Since predominantly only isomer would be produced. so a is correct.

The Diels alder reaction is a concerted( more  bonds form at a time) reaction which means it is just a one step reaction. so statement b is correct.

The option c is incorrect as diels alder reaction occurs through orbital interaction in a pericyclic manner. Since Diels alder reactions are pericyclic in nature and occur through orbital symmetry they do not involve polar intermediates like carbocation or radicals.

The dienes must be conjugated as on account of conjugation stability of a diene also increases. Also since we know that due to conjugation the energy of LUMO decreases and that of HOMO increases and so HOMO is more reactive and generally we involve HOMO of the diene and LUMO of the dienophile. So conjugated dienes are important.

So for a bond formation to take place in a  Diels-Alder reaction HOMO and LUMO with proper symmetry must overlap.