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Periodate ( ) is an anion composed of iodine and oxygen. It is one of a number of of iodine and is the highest in the series, with iodine existing in oxidation state +7. Unlike other perhalogenates, such as perchlorate, it can exist in two forms: metaperiodate and orthoperiodate . In this regard it is comparable to the tellurate ion from the adjacent group. It can combine with a number of to form periodates, which may also be regarded as the salts of periodic acid.
Periodates were discovered by Heinrich Gustav Magnus and C. F. Ammermüller; who first synthesised periodic acid in 1833.
Modern industrial scale production involves the electrochemical oxidation of iodates, on a lead dioxide () anode, with the following standard electrode potential:
Metaperiodates are typically prepared by the dehydration of sodium hydrogen periodate with nitric acid,M. Schmeisser (1963). 012126601X, Academic Press. 012126601X or by dehydrating orthoperiodic acid by heating it to 100 °C under vacuum.
They can also be generated directly from iodates by treatment with other strong oxidizing agents such as :
The ortho- and metaperiodate forms also exist in equilibrium.
For this reason orthoperiodate is sometimes referred to as the dihydrate of metaperiodate, written ; however, this description is not strictly accurate as X-ray crystallography of shows 5 equivalent groups.
At extremes of pH additional species can form. Under basic conditions a dehydration reaction can take place to form the diperiodate (sometimes referred to as mesoperiodate).
Under strongly acid conditions periodic acid can be protonated to give the orthoperiodonium cation.
Exact structures vary depending on counter ions, however on average orthoperiodates adopt a slightly deformed octahedral geometry with X-ray diffraction showing I–O bond lengths of 1.89 Å. Metaperiodates adopt a distorted tetrahedral geometry with an average I–O distance of 1.78 Å.
Alkenes can also be oxidised and cleaved in the Lemieux–Johnson oxidation. This uses a catalytic loading of osmium tetroxide which is regenerated in situ by the periodate. The overall process is equivalent to that of ozonolysis.
Cleavage reactions proceed via a cyclic intermediate called a periodate ester. The formation of this may be affected by pH and temperature but is most strongly affected by the geometry of the substrate, with Cis trans-diols reacting significantly faster than Cis trans-diols. The reactions are exothermic and are typically performed at 0 °C. As periodate salts are only readily soluble in water reactions are generally performed in aqueous media. Where solubility is an issue periodic acid may be used, as this is soluble in alcohols; phase transfer catalysts are also effective in biphasic reaction mixtures. In extreme cases the periodate may be exchanged for lead tetraacetate which reacts in a similar manner and is soluble in organic solvents (Criegee oxidation).
Periodate cleavage is often utilized in molecular biochemistry for the purposes of modifying saccharide rings, as many five- and six-membered sugars have vicinal . Historically it was also used to determine the structure of monosaccharides.
Periodate cleavage may be performed on an industrial scale to form dialdehyde starch which has uses in Paper machine.
Several staining agents use in microscopy are based around periodate (e.g. periodic acid–Schiff stain and Jones' stain).
Periodates have also been used as oxidising agents for use in pyrotechnics. In 2013 the US Army announced that it would replace the environmentally harmful chemicals barium nitrate and potassium perchlorate with sodium metaperiodate for use in their tracer ammunition.
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