Spatiotemporal hypothesis

Intramolecular hydride transfer in equation 4 proceeds with an enzymelike EM of 6.5 X 106 M. In other words, the intramolecular reaction is 6.5 X 106 times faster than the intermolecular counterpart at 1 M concentration (11). Davis et al. (II) argued that relief of strain cannot explain the fast rate because (a) the equilibrium constant in equation 4 is close to unity and (b) force-field calculations show that hydroxy ketone is only 1.7 kcal/mol more strained than the corresponding diketone, which lacks nonbonded H/C=0 interactions. The extremely fast nucleophilic attack on the carbonyl is, however, expected from our spatiotemporal hypothesis. Because the mobile hydrogen is held rigidly only 2.35 A away from the carbonyl carbon, well under the suspected critical distance of 2.8 A (6), the conditions for an enzyme-like acceleration are met. 

One final point with regard to comparing the critical-distance and entropy theories is the following. Words like true and false , correct and incorrect , and valid and invalid have been avoided. Such descriptives have no place in discussions of chemical models that are, above all, fictitious. Models—one must never forget—are to be used, not believed. Thus, I do not claim the spatiotemporal hypothesis represents the truth I merely claim that it is a valuable aid for thinking, especially in cases where entropic arguments fail to help. 

---