No abstract

Absolute rate constants and Arrhenius parameters for hydrogen abstractions (from carbon) by the t-butoxyl radical ((t) BuO.) are reported for several hydrocarbons and tertiary amines in solution. Combined with data already in the literature, an analysis of all the available data reveals that most hydrogen abstractions (from carbon) by (t) BuO. are entropy controlled (i.e., TdeltaS > deltaH, in solution at room temperature). For substrates with C-H bond dissociation energies (BDEs) > 92 kcal/mol, the activation energy for hydrogen abstraction decreases with decreasing BDE in accord with the Evans-Polanyi equation, with alpha approximately 0.3. For substrates with C-H BDEs in the range from 79 to 92 kcal/mol, the activation energy does not vary significantly with C-H BDE. The implications of these results in the context of the use of (t) BuO. as a chemical model for reactive oxygen-centered radicals is discussed.

The hydroxyl radical (HO*) is a highly reactive oxygen-centered radical whose bimolecular rate constants for reaction with organic compounds (hydrogen atom abstraction) approach the diffusion-controlled limit in aqueous solution. The results reported herein show that hydroxyl radical is considerably less reactive in dipolar, aprotic solvents such as acetonitrile. This diminished reactivity is explained on the basis of a polarized transition state for hydrogen abstraction, in which the oxygen of the hydroxyl radical becomes highly negative and can serve as a hydrogen bond acceptor. Because acetonitrile cannot participate as a hydrogen bond donor, the transition state cannot be stabilized by hydrogen bonding, and the reaction rate is lower; the opposite is true when water is the solvent. This hypothesis explains hydroxyl radical reactivity both in solution and in the gas phase and may be the basis for a "containment strategy" used by Nature when hydroxyl radical is produced endogenously.

No abstract

Cyclopropylcarbinyl --> homoallyl and related rearrangements of radical ions (a) are frequently used as mechanistic "probes" to detect the occurrence of single electron transfer in chemical and biochemical processes, (b) provide the basis for mechanism-based drug design, and (c) are important tools in organic synthesis. Unfortunately, these rearrangements are poorly understood, especially with respect to the effect of substrate structure on reactivity. Frequently, researchers assume that the same factors which govern the reactivity of neutral free radicals also pertain to radical ions. The results reported herein demonstrate that in some cases structure-reactivity trends in radical ion rearrangements are very different from neutral radicals. For radical ions, delocalizations of both charge and spin are important factors governing their reactivity.