oxidation

Examples of oxidation in the following topics:

• Oxidation Numbers of Metals in Coordination Compounds

  • Transition metals typically form several oxidation states and therefore have several oxidation numbers.
  • This oxidation number is an indicator of the degree of oxidation (loss of electrons) of an atom in a chemical compound.
  • O2- and S2- have oxidation numbers of -2.
  • In a molecule or compound, the oxidation number is the sum of the oxidation numbers of its constituent atoms.
  • The oxidation number of H is +1 (H+ has an oxidation number of +1).

• Oxides

  • Metal oxides typically contain an anion of oxygen in the oxidation state of −2.
  • Most of the Earth's crust consists of solid oxides, the result of elements being oxidized by the oxygen in air or water.
  • Although most metal oxides are polymeric, some oxides are monomeric molecules.
  • Those attacked only by acids are basic oxides; those attacked only by bases are acidic oxides.
  • Metals tend to form basic oxides, non-metals tend to form acidic oxides, and amphoteric oxides are formed by elements near the boundary between metals and non-metals (metalloids).

• Oxidation

  • The carbon atom of a carbonyl group has a relatively high oxidation state.
  • Useful tests for aldehydes, Tollens' test, Benedict's test & Fehling's test, take advantage of this ease of oxidation by using Ag(+) and Cu(2+) as oxidizing agents (oxidants).
  • The Fehling and Benedict tests use cupric cation as the oxidant.
  • This deep blue reagent is reduced to cuprous oxide, which precipitates as a red to yellow solid.
  • All these cation oxidations must be conducted under alkaline conditions.

• Oxidations & Reductions

  • A parallel and independent method of characterizing organic reactions is by oxidation-reduction terminology.
  • Carbon atoms may have any oxidation state from –4 (e.g.
  • Fortunately, we need not determine the absolute oxidation state of each carbon atom in a molecule, but only the change in oxidation state of those carbons involved in a chemical transformation.
  • Carbon atoms colored blue are reduced, and those colored red are oxidized.
  • Peracid epoxidation and addition of bromine oxidize both carbon atoms, so these are termed oxidation reactions.

• Oxidation States

  • An atom's increase in oxidation state through a chemical reaction is called oxidation, and it involves a loss of electrons; an decrease in an atom's oxidation state is called reduction, and it involves the gain of electrons.
  • The oxidation state of a free element (uncombined element) is zero.
  • For example, Cl- has an oxidation state of -1.
  • When present in most compounds, hydrogen has an oxidation state of +1 and oxygen an oxidation state of −2.
  • This helps determine the oxidation state of any one element in a given molecule or ion, assuming that we know the common oxidation states of all of the other elements.

• Manganese

  • The most common oxidation states of the metal manganese are +2, +3, +4, +6, and +7; the +2 oxidation state is the most stable.
  • Manganese compounds where manganese is in oxidation state of 7+ are powerful oxidizing agents.
  • Compounds with oxidation states 5+ (blue) and 6+ (green) are strong oxidizing agents.
  • The 3+ oxidation state is seen in compounds like manganese(III) acetate; these are very powerful oxidizing agents.
  • Predict the oxidation or reduction propensity of a manganese species given its formula or oxidation state.

• Oxidation of Phenols

  • Phenols are rather easily oxidized despite the absence of a hydrogen atom on the hydroxyl bearing carbon.
  • The redox equilibria between the dihydroxybenzenes hydroquinone and catechol and their quinone oxidation states are so facile that milder oxidants than chromate (Jones reagent) are generally preferred.
  • One such oxidant is Fremy's salt, shown below.
  • Although chromic acid oxidation of phenols having an unsubstituted para-position gives some p-quinone product, the reaction is complex and is not synthetically useful.
  • The solvent of choice for these oxidations is usually methanol or dimethylformamide (DMF).

• Nitrification

  • Nitrobacter plays an important role in the nitrogen cycle by oxidizing nitrite into nitrate in soil.
  • Nitrification is the net result of two distinct processes: oxidation of ammonium to nitrite (NO2−) by nitrosifying or ammonia-oxidizing bacteria and oxidation of nitrite (NO2−) to nitrate (NO3−) by the nitrite-oxidizing bacteria.
  • Nitrification is a process of nitrogen compound oxidation (effectively, loss of electrons from the nitrogen atom to the oxygen atoms):
  • Biochemically, ammonium oxidation occurs by the stepwise oxidation of ammonium to hydroxylamine (NH2OH) by the enzyme ammonium monooxygenase in the cytoplasm, followed by the oxidation of hydroxylamine to nitrite by the enzyme hydroxylamine oxidoreductase in the periplasm.
  • Oxygen is required in ammonium and nitrite oxidation, meaning that both nitrosifying and nitrite-oxidizing bacteria are aerobes.

• Chromium

  • Chromium exhibits a wide range of possible oxidation states, where the +3 state is the most stable energetically.
  • It is dehydrated by heating to form the green chromium(III) oxide (Cr2O3), which is the stable oxide with a crystal structure identical to that of corundum.
  • Chromium(VI) compounds are powerful oxidants at low or neutral pH.
  • Both the chromate and dichromate anions are strong oxidizing reagents at low pH.
  • The oxidation state +5 is only realized in few compounds but are intermediates in many reactions involving oxidations by chromate.

• Oxidation of Reduced Sulfur Compounds

  • Sulfur oxidation involves the oxidation of reduced sulfur compounds, inorganic sulfur, and thiosulfate to form sulfuric acid.
  • Sulfur oxidation involves the oxidation of reduced sulfur compounds such as sulfide (H2S), inorganic sulfur (S0), and thiosulfate (S2O2−3) to form sulfuric acid (H2SO4).
  • An example of a sulfur-oxidizing bacterium is Paracoccus.
  • In addition to aerobic sulfur oxidation, some organisms (e.g.
  • Marine autotrophic Beggiatoa species are able to oxidize intracellular sulfur to sulfate.