Consider whether formation of ionic rather than diradical intermediates would affect the argument in favor of a two-step mechanism for the Diels-Alder reaction. What information does the fact that typical Diels-Alder additions occur in the vapor state give about free-radical vs. ionic reaction mechanisms? Reactions

The Diels-Alder reaction is a [4+2] cycloaddition reaction, involving a diene (a molecule containing two double bonds) and a dienophile (a molecule containing one double bond). The reaction results in the formation of a six-membered ring with a new double bond, by creating two new sigma bonds and breaking a pi bond. Generally, a one-step (concerted) mechanism is proposed for this reaction, which involves the formation of diradical intermediates.

To consider whether the formation of ionic intermediates would affect the argument for a two-step mechanism in the Diels-Alder reaction, let us compare the ionic and diradical intermediates concept. In the ionic mechanism, one of the reactants would gain a positive charge while the other would gain a negative charge, leading to the formation of an ionic intermediate. These charged species could then combine and lose their charges to form the final product. However, the formation of ionic intermediates depends on the polarities of the reactant molecules. Thus, the formation of ionic intermediates in the Diels-Alder reaction would require diene or dienophile species with high polarity. This would limit the scope of the reaction or make it more controlled by solvent polarity.

Given that the Diels-Alder reactions typically occur in the vapor state (a non-polar environment), it implies that the reaction is less affected by the polarities of the reaction species, and ionic intermediates are unlikely to form. The information about the reactions occurring in the vapor state favors the diradical based reaction mechanism (free-radical), as it doesn't involve drastic changes in charge distribution.

A free-radical reaction mechanism would involve the formation of diradical intermediates (with unpaired electrons) briefly in the course of a one-step, concerted Diels-Alder reaction. The fact that the reactions occur in the vapor state supports this mechanism, as it indicates that the reaction pathway is unlikely to be significantly affected by the polarities of the reactants, i.e., it doesn't involve significant charge transfer. In conclusion, the formation of ionic intermediates, instead of diradical intermediates, would make the Diels-Alder reaction more dependent on the polarities of the reactants and the solvent, restricting its general applicability. Furthermore, the fact that the reaction typically occurs in the vapor state supports the diradical based (free-radical) reaction mechanism, as it implies that the Diels-Alder reaction does not rely on drastic changes in charge distribution and is less affected by polarities. Thus, the formation of ionic intermediates could affect the argument in favor of a two-step mechanism for the Diels-Alder reaction, but the vapor state information supports the free-radical mechanism.