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F. Holleman; continued by Egon Wiberg; translated by Mary Eagleson, William Brewer; revised by Bernhard J. Periodic acid ( / ˌ p ɜːr aɪ ˈ ɒ d ɪ k/ per-eye- OD-ik) is the highest oxoacid of iodine, in which the iodine exists in oxidation state +7. Expanding the scope of the Babler–Dauben oxidation: 1,3-oxidative transposition of secondary allylic alcohols".
H 3 IO 6 2 − ↽ − − ⇀ H 2 IO 6 3 − + H + {\displaystyle {\ce {H3IO6 Orthoperiodic acid forms monoclinic crystals ( space group P2 1/ n) consisting of a slightly deformed IO 6 octahedron interlinked via bridging hydrogens. It can exist in two forms: orthoperiodic acid, with the chemical formula H 5IO 6, and metaperiodic acid, which has the formula HIO 4. This can be useful in determining the structure of carbohydrates as periodic acid can be used to open saccharide rings.
Periodate is part of a series of oxyacids in which iodine can assume oxidation states of −1, +1, +3, +5, or +7. It can combine with a number of counter ions to form periodates, which may also be regarded as the salts of periodic acid. Metaperiodic acid can also be prepared from various orthoperiodates by treatment with dilute nitric acid. Orthoperiodic acid can be dehydrated to give metaperiodic acid by heating to 100°C under reduced pressure.
H 5 IO 6 + H + + 2 e − ⟶ IO 3 − + 3 H 2 O {\displaystyle {\ce {H5IO6 + H+ + 2e- -> IO3- + 3 H2O}}} E° = 1.This process is often used in labeling saccharides with fluorescent molecules or other tags such as biotin. Most notably periodic acid will cleave vicinal diols into two aldehyde or ketone fragments ( Malaprade reaction). It is one of a number of oxyanions of iodine and is the highest in the series, with iodine existing in oxidation state +7.
The structure of metaperiodic acid also includes IO 6 octahedra, however these are connected via cis-edge-sharing with bridging oxygens to form one-dimensional infinite chains. 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.
This is commercially available, but can also be produced by the oxidation of iodates with chlorine and sodium hydroxide. Further heating to around 150°C gives iodine pentoxide ( I 2O 5) rather than the expected anhydride diiodine heptoxide ( I 2O 7). For example, sodium metaperiodate, NaIO 4, can be synthesised from HIO 4 while sodium orthoperiodate, Na 5IO 6 can be synthesised from H 5IO 6. 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. Crystal Structure Determination of Metaperiodic Acid, HIO 4, with Combined X-Ray and Neutron Diffraction".
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