Reactivity of Oxygen Radicals [HO•, RO•, HOO•, ROO-, and RC(O)O•]

Oxygen radicals are defined as those molecules that contain an oxygen atom with an unpaired, nonbonding electron (e.g., HO·). Although triplet dioxygen (·O2·) and superoxide ion (O2 - ·) come under this definition, their nonradical chemistry dominates their reactivity, which is discussed in Chapters 6 (·O2·) and 7 (O2-·). The hydroxyl radical (HO·) is the most reactive member of the family of oxygen radicals [HO·, RO·, ·O·, HOO·, ROO·, and RC(O)O·], and is the focus of most oxygen radical research. In the gas phase the dramatic example of oxygen radical reactivity with hydrocarbon substrates is combustion, which is initiated by HO· (or RO· or MO·) and propagated by ·O2· and ·O·.

Redox Thermodynamics for Oxygen Species (O3, O2, HOO·, O2-, HOOH, HOO-, O, O-, and HO-); Effects of Media and pH

Biological systems activate ground-state dioxygen (3O2) for controlled energy transduction and chemical syntheses via electron-transfer and hydrogen-atomtransfer reduction to O2-, HOO·, and HOOH. These reduction products are further activated with metalloproteins to accomplish oxygen atom-transfer chemistry. Conversely, green plants via photosystem II facilitate the oxidation of chemistry.

Introduction: Why oxygen chemstry?

The fundamental premise of chemistry is that all matter consists of molecules. The physical and chemical properties of matter are those of the constituent molecules, and the transformation of matter into different materials (compounds) is the result of their reactions to form new molecules. A molecule consists of two or more atoms held in a relatively fixed array via valence-electron orbital overlap (covalent bonds; chemical bonds). In the nineteenth century chemists focused on the remarkable diversity of molecules produced by living organisms, which have in common the presence of tetravalent carbon atoms. As a result the unique versatility of carbon for the design and synthesis of new molecules was discovered, and the subdiscipline of organic chemistry (the science of carbon-containing molecules) has become the dominant part of the discipline. Clearly, the results from a focus on carbon-based chemistry have been immensely useful to science and to society. Although most molecules in biological systems [and produced by living organisms (particularly aerobic systems)] contain oxygen atoms as well as carbon and hydrogen (e.g., proteins, nucleic acids, carbohydrates, lipids, hormones, and vitamins), there has been a long tradition in all of chemistry to treat oxygen atoms as “neutral counterweights” for the “important,” character-determining elements (C, H, Al, Si, Fe, I) of the molecule. Thus, chemists have tended to take the most important element (oxygen) for granted. The chemistry curriculum devotes one or two year-courses to the chemistry of carbon (“Organic Chemistry”), but only a brief chapter on oxygen is included in the first-year and the inorganic courses. However, if the multitude of hydrocarbon molecules is from the incorporation of oxygen atoms in single-carbon molecules argues against the assignment of a “neutral character” for oxygen atoms [e.g., Cn(graphite), CH4(g), CH3OH(1), CH2(O)(1), HC(O)OH(1), (HO)2C(O)(aq), CO(g), CO2(g)]. Just as the focus of nineteenth century chemists on carbon-containing molecules has produced revolutionary advances in chemical understanding, and yielded the technology to synthesize and produce useful chemicals, polymers, and medicinals; I believe that a similar focus on oxygen chemistry is appropriate and will have analogous rewards for chemistry, biochemistry, and the chemical process technologies.

Reactivity of Hydrogen Peroxide, Alkyl Hydroperoxides, and Peracids

The reactivity of hydroperoxides is primarily dependent upon their unique bond energies (e.g., H-OOH, 90 kcal; HO-OH, 51 kcal; H-OOBu-t, 91 kcal, HO-OBu-t, 47 kcal), which allow low-energy rearrangements to give (HO·) and (·O·).

Nature of chemical bonds for oxygen in its compounds

The bonding for oxygen atoms in heteratomic molecules is viewed as essentially covalent [e.g., MeOH, Me2C=O, MeCH(O), and MeC(O)OOH] and similar to that for carbon, nitrogen, and chlorine atoms. In contrast, a Lewis acid-base formalism often is used for metal-oxygen compounds with ionic interactions by dianionic oxo groups (e.g., [Ba2+O2-], [Fe6+(O2)4]2- , [Mn7+(O2-)4]-, and [(Cu+)2O2-]).

Reactivity of superoxide ion

Reduction of dioxygen by electron transfer yields superoxide ion (O2-.), which has its negative charge and electronic spin density delocalized between the two oxygens. As such it has limited radical character [H-OO bond energy ΔGBF, 72 kcal]2 and is a weak Bransted base in water . . . HOO· → H+ + O2-. Kdiss, 2.0 × 10-5 (7.1) . . . The dynamics for the hydrolysis and disproportionation of O2-. in aqueous solutions have been characterized by pulse radiolysis. For all conditions the rate-limiting step is second order in O2-. concentration, and the maximum rate occurs at a pH that is equivalent to the pKa for HOO· (it decreases monotonically with further decreases in the hydrogen ion concentration).

Reactivity of dioxygen and its activation for selective dioxygenation, mono-oxygenation, dehydrogenation, and auto-oxidation of organic substrates and metals (corrosion)

Ground-state dioxygen has two unpaired electrons (·O2·), which makes it a biradical with a triplet electronic state (see Table 3-1). Its radical character is limited because the H-OO· bond is weak [-ΔGBF/ 51 kcal (Chapter 3)], and the triplet state precludes direct reaction with singlet-state substrate molecules with saturated σ bonding. Perhaps the most important (but nonproductive) reaction chemistry for 3O2 is its reversible binding by the metalloproteins: hemoglobin, myoglobin, hemerythrin, and hemocyanin. Nature developed such systems to obviate the limited solubility of O2 in water (~1 mM at 1 atm O2), which restricts the energy flux from oxidative metabolism in aerobic organisms.

Reactivity of Oxy-anions [HO-, RO-, HOO- (ROO-), and O2-] as Nucleophiles and One-Electron Reducing Agents

The preceding chapter describes the primary reaction chemistry of superoxide ion (O2-. to be that of (1) a Brønsted base (proton transfer from substrate), (2) a nucleophile (via displacement or addition), (3) a one-electron reductant, and (4) a dehydrogenase of secondary-amine groups. The chemistry is characteristic of all oxy anions [HO- (RO-), HOO- (ROO-), and O2-.], but the relative reactivity for each is determined by its pKa and one-electron oxidation potential, which are strongly affected by the anionic solvation energy of the solvent matrix. The present chapter will focus on the reactivity of hydroxide ion (HO-), but the principles apply to all oxy anions and permit assessments of their relative reactivity. The reactivity of hydroxide ion (and that of other oxy anions) is interpreted in terms of two unifying principles: (1) the redox potential of the YO- / YO· (Y = H, R, HO, RO, and O) couple (in a specific reaction) is controlled by the solvation energy of the YO- anion and the bond energy of the R-OY product (RX + YO- → R-OY + X-), and (2) the nucleophilic displacement and addition reactions of YO- occur via an inner-sphere single-electron shift. The electron is the ultimate base and one-electron reductant, which, upon introduction into a solvent, is transiently solvated before it is “leveled” (reacts) to give the conjugate base (anion reductant) of the solvent.