Periodic acid test

The polyhydric alcohols of Solubility Group II are liquids of relatively high boiling point and may be detected inter alia by the reactions already described for Alcohols (see 6). Compounds containing two hydroxyl groups attached to adjacent carbon atoms (1 2-glyeols), a-hydroxy aldehydes and ketones, and 1 2-diketones may be identified by the periodic acid test, given in reaction 9. 

Periodic acid test. (For 1,2-glycols and a-hydroxyaldehydes and ketones, Section 4.2.55, p. 454). 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 (H5IOe) contained in a small test tube and shake well. Then introduce 1 drop 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. 

Periodic Acid Test for vic-Glycols. Vicinal glycols (hydroxyl groups on adjacent carbon atoms) can be detected by reaction with periodic acid. In addition to 1,2-glycols, a positive test is given by a-hydroxy aldehydes, a-hydroxy ketones, a-hydroxy acids, and a-amino alcohols, as well as 1,2-diketones. 

Periodic acid test (for polyhydric alcohols). This test is based on Malaprade s reaction, in which an alcohol having two hydroxyl groups situated on adjacent carbon atoms is oxidized by periodic add, the latter being reduced to iodic acid which is detected in the test as its silver salt. 

CAUTION. Ethers that have been stored for long periods, particularly in partly-filled bottles, frequently contain small quantities of highly explosive peroxides. The presence of peroxides may be detected either by the per-chromic acid test of qualitative inorganic analysis (addition of an acidified solution of potassium dichromate) or by the liberation of iodine from acidified potassium iodide solution (compare Section 11,47,7). The peroxides are nonvolatile and may accumulate in the flask during the distillation of the ether the residue is explosive and may detonate, when distilled, with sufficient violence to shatter the apparatus and cause serious personal injury. If peroxides are found, they must first be removed by treatment with acidified ferrous sulphate solution (Section 11,47,7) or with sodium sulphite solution or with stannous chloride solution (Section VI, 12). The common extraction solvents diethyl ether and di-tso-propyl ether are particularly prone to the formation of peroxides. 

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. 

Place 2 ml. of the periodic acid reagent in a small test tube, add one drop (no more—otherwise the silver iodate, if formed, will fail to precipitate) of concentrated nitric acid, and shake well. Add one drop or a small crystal of the compound to be tested, shake the mixture for 15-20 seconds, and then add 1-2 drops of 3 per cent, silver nitrate solution. The instantaneous formation of a white precipitate of silver iodate is a positive test. Failure to form a precipitate, or the appearance of a brown precipitate which redissolves on shaking, constitutes a negative test. 

Nitric acid test t 65 wt.% HNOj Five 48 h exposures to boiling solution refreshed after period Average mass loss per unit area of five testing periods -1- 0-99 to -1- 1-20 1. Chromium-depleted areas 2. <7-phase 3. Chromium carbide... 

It is, however, more likely that the discrepancies observed in the periodate oxidation of malonaldehyde concern mainly the hydroxylation step. In the mechanism proposed (5) for this reaction, it is the enol form of malonaldehyde which is hydroxylated. However, titrations of a solution of malonaldehyde, prepared by hydrolysis of an aqueous solution (33) of carefully distilled 1, 3, 3-tri-ethoxypropene (46, 47), both with strong base and with iodine, indicate that only about 80% of the enol form is present in the equilibrium solution. On the other hand, the thio-barbituric acid test (58, 59) gave consistently higher values for the malonaldehyde content of the solution. The fact that only about 80% of the enol form is present in the equilibrium solution is all the more important as it can be shown (56) by titration with strong base that the enolization is slow, and moreover does not seem to go to completion. 

The periodate-thiobarbituric acid test developed by Waravdekar and Saslaw269 is used in the modification of Weissbach and Hur-witz229 for identification of the products of /3-eliminative cleavage. 

Exposure to periodate yields 3-formylpyruvic acid, which reacts with thiobarbituric acid, giving rise to red condensation products having an absorption maximum at 545-550 nm. Certain workers have used the thiobarbituric acid test without previous oxidation of the unsaturated oligo-D-galactosiduronates with periodic acid.3-29,239,260 Even here, the products of reaction with thiobarbituric acid have an absorption maximum at 550 nm. 

Sandri (15) tested three titration methods with good results. One was titration with potassium bromate in the presence of potassium bromide and hydrochloric acid, using a starch-iodine end-point. Another was addition of excess periodic acid-potassium iodide with sodium... 

Tollen s test may result in the simple oxidation to a carboxylic acid, or it may cause fragmentation of the carbon backbone similar to the oxidation reaction seen with periodic acid (see the later related section). Figure 16-8 shows the general reaction that occurs when you conduct Tollen s test. 

The periodic acid-thiobarbituric acid test has some disadvantages that may cause severe errors in the determination of sialic acid. 

A further method for estimation of sialic acid using periodic acid has been reported by Massamiri and coworkers.121 Fonnaldehyde produced by mild, oxidative cleavage of the D-en/f/iro-glycerol-l-yl side-chain of Neu can be detected by 3-methyl-2-benzothiazolinone hydrazone, which yields a green-blue color (absorbance maximum 625 nm). This test, which is slightly more sensitive than the thiobarbi-... 

The test for ethylene glycol was performed by oxidation of the sample with periodic acid and detection of the resulting formaldehyde with fuchsin-sulfurous acid reagent (9). 

The second of the three types of qualitative teste mentioned above involves acid-catalyzcd cleavage of an epoxide with periodic acid.38 The IOf ion then oxidizes the resulting 1,2-diol in the usual manner, while itself undergoing reduction to iodato. In the presence of AgH ion, the gradual formation of iodate will be marked by precipitation of silver iodate (Eq. 983). An obvious drawback to this procedure is that any other functional groups capable of reducing IOf iou will interfere with epoxide detection. 

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