Periodic acid

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Periodic acid
Orthoperiodic acid
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Error creating thumbnail:
Metaperiodic acid
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Orthoperiodic acid
Names
Other names
  • Paraperiodic acid
  • Iodic(VII) acid
  • Hydrogen periodate
Identifiers
3D model (JSmol)
ChemSpider
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EC Number
  • 236-585-6
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UNII
UN number UN3085
  • {{#property:P3117}}Lua error in Module:EditAtWikidata at line 29: attempt to index field 'wikibase' (a nil value).
  • (orthoperiodic): InChI=I=1S/H5IO6/c2-1(3,4,5,6)7/h(H5,2,3,4,5,6,7)
    Key: TWLXDPFBEPBAQB-UHFFFAOYSA-N
  • (metaperiodic): InChI=1/HIO4/c2-1(3,4)5/h(H,2,3,4,5)
    Key: KHIWWQKSHDUIBK-UHFFFAOYAH
  • (orthoperiodic): OI(=O)(O)(O)(O)O
  • (metaperiodic): O[I+3]([O-])([O-])[O-]
Properties
HIO4 (metaperiodic)
H5IO6 (orthoperiodic)
Molar mass 190.91 g/mol (HIO4)
227.941 g/mol (H5IO6)
Appearance Colourless crystals
Melting point 128.5 °C (263.3 °F; 401.6 K)[1]
Solubility soluble in water, alcohols
Acidity (pKa) pKa1 = 3.29
pKa2 = 8.31
pKa3 = 11.60
Conjugate base Periodate
Hazards[2]
GHS labelling:
GHS03: OxidizingGHS05: CorrosiveGHS08: Health hazardGHS09: Environmental hazard
Danger
H271, H314, H372, H400
P210, P260, P273, P303+P361+P353, P305+P351+P338
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazard OX: Oxidizer. E.g. potassium perchlorate
3
0
0
OX
Safety data sheet (SDS) External MSDS
Related compounds
Other anions
Other cations
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Periodic acid (/ˌpɜːraɪˈɒdɪk/ per-eye-OD-ik) is an oxoacid of iodine. It can exist in two forms: orthoperiodic acid, with the chemical formula H5IO6, and metaperiodic acid, which has the formula HIO4. Periodic acids are colourless crystals. Periodic acid features iodine in the highest oxidation state of +7.

Periodic acid was discovered by Heinrich Gustav Magnus and C. F. Ammermüller in 1833.[3]

Synthesis

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Modern industrial scale production involves the oxidation of a solution of sodium iodate under alkaline conditions, either electrochemically on a PbO2 anode, or by treatment with chlorine:[4]

IO−3 + 6 HO− - 2 e− → IO5−6 + 3 H2O       (counter ions omitted for clarity)       E° = −1.6 V[5]
IO−3 + 6 HO− + Cl2 → IO5−6 + 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:[6]

Ba3(H2IO6)2 + 6 HNO3 → 3 Ba(NO3)2 + 2 H5IO6

Properties

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Orthoperiodic acid has a number of acid dissociation constants.[7][8] The pKa of metaperiodic acid has not been determined.

H5IO6 ⇌ H4IO−6 + H+,      pKa1 = 3.29
H4IO−6 ⇌ H3IO2−6 + H+,      pKa2 = 8.31
H3IO2−6 ⇌ H2IO3−6 + H+,      pKa3 = 11.60

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

Structure

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Orthoperiodic acid forms monoclinic crystals (space group P21/n) consisting of a slightly deformed 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.[9][10] The structure of metaperiodic acid also includes IO6 octahedra, however these are connected via cis-edge-sharing with bridging oxygens to form one-dimensional infinite chains.[11]

Reactions

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Further information: Periodate § Reactions

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

H5IO6 ⇌ HIO4 + 2 H2O

Further heating to around 150 °C gives iodine pentoxide (I2O5) rather than the expected anhydride diiodine heptoxide (I2O7). Metaperiodic acid can also be prepared from various orthoperiodates by treatment with dilute nitric acid.[12]

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.[13]

Other oxyacids

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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.

Iodine oxidation state −1 +1 +3 +5 +7
Name Hydrogen iodide Hypoiodous acid Iodous acid Iodic acid Periodic acid
Formula HI HIO HIO2 HIO3 HIO4 or H5IO6

See also

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Compounds with a similar structure:

Compounds with similar chemistry:

References

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