Pathways and rates of redox reactions in environmental media are often approached from the perspective of major oxidation states.  Intermediate oxidation states and their speciation can govern which pathways are predominant, which products are formed, and how fast transformations take place.  When permanganate is reduced by alkenes, a five-atom, Mn(V)-containing ring is formed.  The manner in which this ring breaks influences identities and yields of alcohol, aldehyde, and carboxylic acid organic products and amounts of two additional intermediate oxidation states, Mn(III) and Mn(IV), that are formed.  Oxidation states outside the normal range can be important as well.  Dissolution of Cr(III) oxyhydroxide solids, for example, can be dramatically accelerated through generation of short-lived yet highly soluble Cr(II) species.  Overlooked nonmetal oxidation states exist as well.  Potassium phosphite, a form of P(III), is widely employed in agriculture as a fungicide.  The phosphite tautomer, which lacks a P-H bond, is far more susceptible towards oxidation than the hydrogen phosphonate tautomer, which contains such a bond.  The latter is more predominant in simple electrolyte solutions, but the tautomer ratio can be substantially altered by metal ion coordination, formation of adducts with other oxyanion species, and possibly by adsorption.  Hence, it isn't sufficient just to know that unusual oxidation states exist.  We need improved analytical methods to sense them, and improved experimental designs to reveal their role in reactivity.

Overlooked oxidation states of chemical elements in treatment plants and the environment: Interconnections between speciation and reactivity

253rd National Meeting (2017)

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