Lecture 2 - Peculiar Rate Laws, Bond Dissociation Energies, and Relative Reactivities

Curious kinetic orders can be mechanistically informative. Fractional kinetic orders suggest dissociation of a dominant aggregate to give a smaller reactive species. An apparent negative kinetic order, due to competition with a second-order process, leads to spontaneous deracemization of chiral crystals. Changes in bond dissociation energies can be due to differences in bonds or in radicals. Although it is often said that the order of alkyl radical stability is tertiary > secondary > primary, careful analysis suggests that the order of bond dissociation energies may be due to differences in the alkanes rather than in the radicals. Hammond helped organic chemists begin to think systematically about predicting relative reaction rates by suggesting that the transition states of more exothermic reactions should lie closer to the starting materials in structure and energy.