Free radicals electronic configuration

Most stable ground-state molecules contain closed-shell electron configurations with a completely filled valence shell in which all molecular orbitals are doubly occupied or empty. Radicals, on the other hand, have an odd number of electrons and are therefore paramagnetic species. Electron paramagnetic resonance (EPR), sometimes called electron spin resonance (ESR), is a spectroscopic technique used to study species with one or more unpaired electrons, such as those found in free radicals, triplets (in the solid phase) and some inorganic complexes of transition-metal ions. 

While the vast majority of molecules may be described in terms of closed-shell electron configurations, that is, all electrons being paired, there are several important classes of molecules with one or more unpaired electrons. So-called free radicals are certainly the most recognizable. One way to treat open-shell molecules is by strict analogy with the treatment of closed-shell molecules, that is, to insist that electrons are either paired or are unpaired. 

Reductive decyanations of 2-cyanotetrahydropyran derivatives with sodium in ammonia yield predominantly axially protonated products. The observations are consistent with the reductive decyanation proceeding via the pyramidal, axial radical which accepts a second electron to give a configurationally stable carbanion, which in turn abstracts a proton from ammonia with retention of configuration (Rych-novsky, S. D. Powers, J. P. LePage, T. J., J. Am. Chem. Soc., 1992, 114, 8375-8384). Provide an explanation for the axial preference of the intermediate free radical on the basis of orbital interactions. Hint The title of the paper by Rychnovsky et al. is Conformation and Reactivity of Anomeric Radicals. ... 

Homolytic oxidations involve free radical intermediates and are catalyzed by first-row transition metals characterized by one-electron oxidation-reduction steps, eg. Com/Con, Mnln/Mnn. The hydrocarbon substrate is generally not coordinated to the metal and is oxidized outside the coordination sphere. Consequently, the oxidation is not very selective and does not often preserve the stereochemical configuration of the substrate. 

The reactivity of carbon-centered free radicals results from dieir drive to achieve an octet electronic configuration, which they do by two principal reaction processes. The first is atom transfer. This process is one in which an atom with one electron is transferred from a closed-shell molecule (fully paired, valence octets) to the free radical. 

Reactive oxygen species (ROS) are molecules that contain an oxygen atom and that are highly reactive as a result of the presence of a free radical, or a configuration of the oxygen atom whereby there are more electrons than usual. Examples of the first class include the hydroxyl radical (OH), and the superoxide radical (02 ). whereas the peroxide (022-) and hypochlorite... 

The most common type of chain-growth polymerization is free-radical polymerization. An initiator or a photochemical reaction produces a free radical that attaches itself to a monomer molecule, creating a group with odd-electron configuration (reactive center) at which monomer molecules are added until two such centers react with one another or, more rarely, a center is deactivated by some other process. This is a mechanism much like that of ordinary chain reactions (see Chapter 9 the term "chain" in chain growth refers to that kind of mechanisms, not to the growing molecular chain of repeating units in the polymer.)... 

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