The metaperiodate ion
The orthoperiodate ion
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|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
Periodate // is an anion composed of iodine and oxygen. It is one of a number of oxyanions of iodine and is the highest in the series, with iodine existing in oxidation state VII. Unlike other perhalogenates, such as perchlorate, it can exist in two forms: metaperiodate IO4− and orthoperiodate IO65−. In this regard it is comparable to the tellurate ion from the adjacent period. It can combine with a number of counter ions to form periodates, which may also be regarded as the salts of periodic acid.
Classically, periodate was most commonly produced in the form of sodium hydrogen periodate (Na3H2IO6). This is commercially available, but can also be produced by the oxidation of iodates with chlorine and sodium hydroxide. Or, similarly, from iodides by oxidation with bromine and sodium hydroxide:
- NaIO3 + Cl2 + 4 NaOH → Na3H2IO6 + 2 NaCl + H2O
- NaI + 4 Br2 + 10 NaOH → Na3H2IO6 + 8 NaBr + 4 H2O
- Na3H2IO6 + 2 HNO3 → NaIO4 + 2 NaNO3 + 2 H2O
- H5IO6 → HIO4 + 2 H2O
They can also be generated directly from iodates by treatment with other strong oxidizing agents such as hypochlorites:
Forms and interconversion
- H5IO6 ↔ H4IO6− + H+, pKa = 3.29
- H4IO6− ↔ H3IO62− + H+, pKa = 8.31
- H3IO62− ↔ H2IO63− + H+, pKa = 11.60
The ortho- and metaperiodate forms also exist in equilibrium.
- H4IO6− ↔ IO4− + 2 H2O, K = 29
For this reason orthoperiodate is sometimes referred to as the dihydrate of metaperiodate, written IO4−·2H2O, however this description is not strictly accurate as X-ray crystallography of H5IO6 shows 5 equivalent I-OH 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).
- 2 H3IO62− ↔ H2I2O104− + 2 H2O, K = 820
Under strongly acid conditions periodic acid can be protonated to give the orthoperiodonium cation.
- H6IO6+ ↔ H5IO6− + H+, pKa = -0.8
Structure and bonding
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 1.89Å. Metaperiodates adopt a distorted tetrahedral geometry with an average I-O distance of 1.78Å.
Periodates can cleave carbon-carbon bonds on a variety of 1,2-difunctionalised alkanes, with the most common example being diol cleavage. This reaction was first reported by Léon Malaprade in 1934; and is sometimes referred to as the Malaprade reaction. In addition to diols, periodates can also cleave 1,2-hydroxy ketones, 1,2-diketones, α-keto acids, α-hydroxy acids, amino acids, 1,2-amino alcohols, 1,2-diamines, and epoxides to give aldehydes and carboxylic acids.
Alkenes can also be oxidised and cleaved in the Lemieux–Johnson oxidation, which uses a catalytic loading of osmium tetroxide which is regenerated in-situ by the periodate. The overall process is equivalent to that of ozonolysis.
Reactions proceed via a cyclic intermediate called a periodate ester. The formation of this may be effected by pH and temperature but is most strongly effected by the geometry of the substrate, with cis-diols reacting significantly faster than trans-diols. The oxidative cleavage reduces of the iodine; thus the reaction also converts periodate salts to iodates and periodic acid to iodic acid. 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 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.
Periodate cleavage is often utilized in molecular biochemistry for the purposes of modifying saccharide rings, as many five- and six-membered sugars often have vicinal diols. Historically it was also used to determine the structure of monosaccharides.
Periodate are powerful oxidising agents. They can oxidise catechol to 1,2-benzoquinone and hydroquinone to 1,4-benzoquinone. Sulfides can be effectively oxidised to sulfoxides. Periodates are sufficiently powerful to generate other strong inorganic oxidisers such as permanganate, osmium tetroxide and ruthenium tetroxide.
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.
|Iodine oxidation state||−1||+1||+3||+5||+7|
|Formula||I−||IO−||IO2−||IO3−||IO4− or IO65−|
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