Paraperiodic acid

Add 1 drop (0 05 ml.) of concentrated nitric acid to 2 0 ml. of a 0 5 per cent, aqueous solution of paraperiodic acid (HjIO,) contained in a small test-tube and shake well. Then introduce 1 op or a small crystal of the compound. Shake the mixture for 15 seconds and add 1-2 drops of 5 per cent, aqueous silver nitrate. The immediate production of a white precipitate (silver iodate) constitutes a positive test and indicates that the organic compound has been oxidised by the periodic acid. The test is based upon the fact that silver iodate is sparingly soluble in dilute nitric acid whereas silver periodate is very soluble if too much nitric acid is present, the silver iodate will not precipitate. 

An alternative procedure for the above test is as follows. Mix 2-3 ml. of 2 per cent, aqueous paraperiodic acid solution with 1 drop of dilute sulphuric acid (ca. 2 5N) and add 20-30 mg. of the compound. Shake the mixture for 5 minutes, and then pass sulphur dioxide through the solution until it acquires a pale yellow colour (to remove the excess of periodic acid and also iodic acid formed in the reaction). Add 1-2 ml. of Schiff s reagent (Section 111,70) the production of a violet colour constitutes a positive test. 

The periodic acid reagent is prepared by dissolving 1 - 0 g. of paraperiodic acid HglOg in 200 ml. of distilled water. 

A. Basarow and L. Meyer objected to J. Thomsen s hypothesis. A. Basarow13 regards iodine as septivalent and assumes paraperiodic acid to contain five hydroxyl groups, (HO)2 IO (HO)3, two of which behave as acidic hydroxyls, and three as alcoholic hydroxyls. 

Only four of the acids listed in Table 19.8 have been isolated in pure form perchloric acid (HC104), iodic acid (HIO3), and the two periodic acids, metaperiodic acid (HI04) and paraperiodic acid (H5IC>6). The others are stable only in aqueous solution or in the form of their salts. Chlorous acid (HCIO2) is the only known halous acid. 

Iodine differs from the other halogens because it forms more than one perhalic acid. Paraperiodic acid (HsIOg) is obtained as white crystals (mp 128°C) when periodic acid solutions are evaporated. When heated to 100°C at reduced pressure, these crystals lose water and are converted to metaperiodic acid (HIO4) ... 

Metaperiodic acid is a strong monoprotic acid, whereas paraperiodic acid is a weak polyprotic acid (Kal = 5.1 X 10-4 ka2 = 4.9 X 10-9). It has an octahedral structure in which a central iodine atom is bonded to one O atom and five OH groups ... 

Crystalline paraperiodic acid, H5I06, which is hygroscopic and readily soluble in water, is commercially available. Most of the salts of periodic acid are characterized by their slight solubility in water. For oxidation experiments sodium metaperiodate, NaIC>4, is the most suitable salt because of its solubility in water (9.3% at 20° and 12.6% at 25°).99 Sodium metaperiodate is commercially available and also can be obtained readily from the slightly soluble trisodium paraperiodate, Na3H2I06, by crystallization from nitric acid in the ratio of 150 cc. of water and 45 cc. of concentrated nitric add to 100 g. of salt.9 Trisodium paraperiodate is formed in 90% yield by the reaction of bromine and sodium iodide in aqueous sodium hydroxide solution at 80°.100 It is also produced in 80% yield by the oxidation of sodium iodate with chlorine in aqueous sodium hydroxide solution.99 In connection with this preparation of trisodium paraperiodate from sodium iodate, it should be noted that in the usual periodate oxidation reactions the periodate is converted quantitatively into iodate. Paraperiodic acid has been prepared in about 93% yield from trisodium paraperiodate " 1 it has been prepared also by the electrolytic oxidation 191 >192 of iodic add. 

Oxidation of N-Acetyl-D-glucosylamine.98 Six grams (0.027 mole) of N-acetyl-D-glucosylamine (XLI) in 100 cc. of water is added to 18.5 g. (0.081 mole) of paraperiodic acid in 100 cc. of water, and the reaction mixture is maintained at 25° for four and one-half hours, which is known from analytical data obtained in small-scale experiments to be sufficient time for completion of the reaction with the consumption of two moles of the oxidant. The solution is exactly neutralized with barium hydroxide, the precipitate removed, and the filtrate evaporated to dryness in vacuum. The solid is extracted with 100 cc. of absolute ethanol, the ethanol extract evaporated to dryness, the residue taken up in ethanol, and the solution is centrifuged and again evaporated to dryness. The product, the dialdehyde XLII, can be converted into the corresponding barium salt (see reference 93). 

Hodine also forms H4FO9 (mesodiperiodic acid) and H5IO6 (paraperiodic acid). tHOF oxidation state is best represented as -1. 

The equilibria existing in solutions of periodic acid and its salts have been investigated, with the conclusion that, of the possible hydrates of I(VII) oxide, only paraperiodic acid, HjIOs, exists as a solid in equilibrium with its aqueous solutions. Acidic periodate solutions contain chiefly the two species 104" and HjIOe, the proportion of HsIOg increasing with decreasing pH. 

Reagent Dissolve 0.25 g of paraperiodic acid (HjIOg) in 50 mL of water. 

A solution of 5.4 g (0.015 mol) of glucose phenylosazone in 2 L of warm 66% ethanol is cooled to room temperature and treated with a solution of 10 g (0.044 mol) of paraperiodic acid in 70 mL of water. The yellow-orange precipitate appears at once. After dilution of the mixture with 500 mL of water, the precipitate is filtered with suction, and the crystals are washed with 66% ethanol and dried in vacuo. Two recrystallizations of the residue (3.4 g, 85%) from 66% ethanol give yellow-orange needles of mesoxaldehyde osazone, mp 198 °C (dec). 

When chlorine is passed into a suspension of this salt, avoiding an excess of the gas, a solution of periodic acid is obtained from which, after filtration, paraperiodic acid, HglOg, can be crystallised ... 

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