The Roles of Changes in Bonding vs. Packing Fraction in the Pressure‐Induced Acceleration of the Diels‐Alder Reaction

The rates of Diels‐Alder reactions increase with pressure, giving rise to large negative activation volumes that are similar to reaction volumes. These negative volumes are generally ascribed to contraction occuring during bond formation, with the further inference that the reactions are concerted because the parity of –Δ V ≠ and –Δ V ≠ implies that both new bonds are forming in the transition state, rather than one bond at a time. We hold that this is difficult to reconcile with the Bell‐Evans‐Polanyi‐Hammond postulate, and from calculations of intrinsic molar volumes we have reached different conclusions. (1) The intrinsic contraction arising from bond formation is very small. In the product it is only 11–16% of Δ V , and in the transition state, only 3–5% of Δ V ≠ . (2) Almost all the shrinkage in the system that occurs during reaction results from the loss of empty space surrounding the molecules, and not from the reaction itself. (3) Therefore the activation and reaction volume data are not useful as criteria of the Diels‐Alder mechanism.