What Is Periodic Acid?
Periodic acid, a type of iodine oxoacid, predominantly refers to orthoperiodic acid with the chemical formula H5IO6. It also encompasses metaperiodic acid (HIO4), both sharing an oxidation number of +7 for iodine. These acids form periodates with various metals in solution, ionizing into periodate and hydrogen ions.
Uses of Periodic Acid
Primarily, periodic acid is utilized in the PAS reaction for polysaccharide detection, distinguishing between aldehydes and ketones using the Schiff reagent. This reaction decomposes polysaccharides into aldehydes, aiding in the detection of polysaccharides in organs. Additionally, periodic acid performs oxidative cleavage of diols into carbonyl compounds.
Properties of Periodic Acid
Metaperiodate sublimates at 100°C and decomposes at 138°C, forming iodic acid (HIO3) and diiodine pentoxide (I2O5). Orthoperiodate, more stable and soluble in water, melts at 122°C and decomposes around 130-140°C. It is a weak acid, with significantly lower acidity than perchloric acid.
Structure of Periodic Acid
The structure of periodic acids varies between their forms. Metaperiodic acid features a one-dimensional chain-like structure, whereas orthoperiodate forms colorless monoclinic crystals with an octahedral structure centered on the iodine atom. In solution, a variety of periodate species exist, shifting with changes in temperature and pH.
Other Information on Periodic Acid
1. Synthesis of Periodic Acid
Orthoperiodic acid is synthesized through the reaction of barium hydrogen orthoperiodate with concentrated nitric acid, which upon heating, yields metaperiodic acid.
2. Reaction of Periodic Acid
Useful for the cleavage of 1,2-bifunctional compounds, periodic acid facilitates the Malaprade reaction—cleaving vicinal diols into aldehydes or ketones, crucial for carbohydrate structural analysis. This reaction is also applicable for selective RNA labeling and as a moderate oxidant in organic synthesis, exemplified by the Babler oxidation of secondary allylic alcohols.