:Periodic acid

Modern industrial scale production involves the oxidation of a solution of sodium iodate under alkaline conditions, either electrochemically on a Lead dioxide anode, or by treatment with chlorine:{{cite book |last1=Greenwood |first1=N. N. |last2=Earnshaw |first2=A |title=Chemistry of the elements |publisher=Butterworth-Heinemann |isbn=978-0-7506-3365-9 |page=872 |edition=2nd |doi=10.1016/C2009-0-30414-6 |year=1997}}

:{{chem2|IO3− + 6 HO− \− 2 e− → IO6(5−) + 3 H2O}} {{spaces|5}} (counter ions omitted for clarity) {{spaces|5}} E° = −1.6 V{{cite book|last=Parsons|first=Roger|title=Handbook of electrochemical constants|url=https://archive.org/details/ost-chemistry-parsons-handbookofelectrochemicalconstants|year=1959|publisher=Butterworths Scientific Publications Ltd|page=[https://archive.org/details/ost-chemistry-parsons-handbookofelectrochemicalconstants/page/n41 71]}}

:{{chem2|IO3− + 6 HO− + Cl2 → IO6(5−) + 2 Cl− + 3 H2O}}

A standard laboratory preparation involves treating a mixture of tribarium dihydrogen orthoperiodate with nitric acid. Upon concentrating the mixture, the barium nitrate, which is less soluble, is separated from periodic acid:{{cite book|author=M. Schmeisser|chapter=Periodic acid|title=Handbook of Preparative Inorganic Chemistry, 2nd Ed. |editor=G. Brauer|publisher=Academic Press|year=1963|place=NY,NY|volume=2pages=322}}

:{{chem2|Ba3(H2IO6)2 + 6 HNO3 → 3 Ba(NO3)2 + 2 H5IO6}}

Orthoperiodic acid has a number of acid dissociation constants.{{cite book|last=Aylett|first=founded by A.F. Holleman; continued by Egon Wiberg; translated by Mary Eagleson, William Brewer; revised by Bernhard J.|title=Inorganic chemistry|year=2001|publisher=Academic Press, W. de Gruyter.|location=San Diego, Calif. : Berlin|isbn=0123526515|page=454|edition=1st English ed., [edited] by Nils Wiberg.}}{{cite book|last=Burgot|first=Jean-Louis|title=Ionic equilibria in analytical chemistry|publisher=Springer|location=New York|isbn=978-1441983824|page=358|date=2012-03-30}} The pK_a of metaperiodic acid has not been determined.

:{{chem2|H5IO6 ⇌ H4IO6− + H+}},{{spaces|5}} pK_a1 = 3.29

:{{chem2|H4IO6− ⇌ H3IO6(2−) + H+}},{{spaces|5}} pK_a2 = 8.31

:{{chem2|H3IO6(2−) ⇌ H2IO6(3−) + H+}},{{spaces|5}} pK_a3 = 11.60

There being two forms of periodic acid, it follows that two types of periodate salts are formed. For example, sodium metaperiodate, {{chem2|NaIO4}}, can be synthesised from {{chem2|HIO4}} while sodium orthoperiodate, {{chem2|Na5IO6}} can be synthesised from {{chem2|H5IO6}}.

Orthoperiodic acid forms monoclinic crystals (space group P2₁/_n) consisting of a slightly deformed {{chem2|IO6}} octahedron interlinked via bridging hydrogens. Five I–O bond distances are in the range 1.87–1.91 Å and one I–O bond is 1.78 Å.{{cite journal|last=Feikema|first=Y. D.|title=The crystal structures of two oxy-acids of iodine. I. A study of orthoperiodic acid, H₅IO₆, by neutron diffraction|journal=Acta Crystallographica|date=10 June 1966|volume=20|issue=6|pages=765–769|doi=10.1107/S0365110X66001828}}{{cite journal|doi=10.1107/S0567740882004932|title=Structure of the 1:1 adduct of orthoperiodic acid and urea|journal=Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry|volume=38|issue=3|pages=1048–1050|year=1982|last1=Fábry|first1=J.|last2=Podlahová|first2=J.|last3=Loub|first3=J.|last4=Langer|first4=V.}}

The structure of metaperiodic acid also includes {{chem2|IO6}} octahedra, however these are connected via cis-edge-sharing with bridging oxygens to form one-dimensional infinite chains.{{cite journal|last=Kraft|first=Thorsten|author2=Jansen, Martin|title=Crystal Structure Determination of Metaperiodic Acid, HIO₄, with Combined X-Ray and Neutron Diffraction|journal=Angewandte Chemie International Edition in English|date=1 September 1997|volume=36|issue=16|pages=1753–1754|doi=10.1002/anie.199717531}}

{{further|Periodate#Reactions}}

Orthoperiodic acid can be dehydrated to give metaperiodic acid by heating to 100 °C under reduced pressure.

:{{chem2|H5IO6 ⇌ HIO4 + 2 H2O}}

Further heating to around 150 °C gives iodine pentoxide ({{chem2|I2O5}}) rather than the expected anhydride diiodine heptoxide ({{chem2|I2O7}}). Metaperiodic acid can also be prepared from various orthoperiodates by treatment with dilute nitric acid.{{cite book|last=Riley|editor-first=Georg|editor-last=Brauer|translator=Scripta Technica, Inc.|others=Translation editor Reed F.|title=Handbook of preparative inorganic chemistry. Volume 1|year=1963|publisher=Academic Press|location=New York, N.Y.|isbn=012126601X|pages=323–324|edition=2nd}}

Like all periodates periodic acid can be used to cleave various 1,2-difunctional compounds. Most notably periodic acid will cleave vicinal diols into two aldehyde or ketone fragments (Malaprade reaction).

File:Periodate cleavage.svg

This can be useful in determining the structure of carbohydrates as periodic acid can be used to open saccharide rings. This process is often used in labeling saccharides with fluorescent molecules or other tags such as biotin. Because the process requires vicinal diols, periodate oxidation is often used to selectively label the 3′-termini of RNA (ribose has vicinal diols) instead of DNA as deoxyribose does not have vicinal diols.

Periodic acid is also used as an oxidising agent of moderate strength, as exemplified in the Babler oxidation of secondary allyl alcohols which are oxidised to enones by stoichiometric amounts of orthoperiodic acid with catalyst PCC.{{Cite journal|last=Killoran|first=Patrick M.|last2=Rossington|first2=Steven B.|last3=Wilkinson|first3=James A.|last4=Hadfield|first4=John A.|date=2016|title=Expanding the scope of the Babler–Dauben oxidation: 1,3-oxidative transposition of secondary allylic alcohols|url=https://linkinghub.elsevier.com/retrieve/pii/S0040403916309340|journal=Tetrahedron Letters|language=en|volume=57|issue=35|pages=3954–3957|doi=10.1016/j.tetlet.2016.07.076}}

Periodic acid is part of a series of oxyacids in which iodine can assume oxidation states of −1, +1, +3, +5, or +7. A number of neutral iodine oxides are also known.

Compounds with a similar structure:

• Perchloric acid, perbromic acid, the related perhalogenic acids
• Telluric acid and perxenic acid, the isoelectronic oxoacids of tellurium and xenon

Compounds with similar chemistry:

• Lead tetraacetate (Criegee oxidation)

{{reflist}}

{{Hydrogen compounds}}

{{Iodine compounds}}

{{Periodates}}

Category:Halogen oxoacids

Category:Oxidizing acids

Category:Periodates