Hypoiodite test

The iodoform test is sometimes called the hypoiodite test because sodium hypoiodite is formed according to Equation 25.21. In more general terms, the reactions just described can be called the haloform test or the sodium hypohalite test... 

For years the iodoform test was a laboratory method for the identification of a methyl ketone (a ketone where one of the R groups is a methyl group). A positive test produced the compound iodoform. Iodoform, CHI3, is a yellow precipitate with a characteristic odor. The oxidation utilizes sodium hypoiodite, which is generated in situ by the reaction of iodine with sodium hydroxide. Figure 10-35 shows an example of the iodoform test. 

Hypoiodites are used for qualitative tests for methyl ketones (Lieben test). For this purpose, a compound to be tested is stirred with an aqueous solution of sodium hydroxide (80 mol/mol of methyl ketone). Iodine (4.5 mol of 12) is added portionwise with stirring, and the mixture is set aside for 20 min at 25 °C before acidification. In the presence of a methyl keto group, a yellow heavy precipitate of iodoform settles at the bottom of the test tube. Iodoform can be identified easily not only by its characteristic smell but also by its melting point (120-123 °C) [1173], This test applies not only to methyl ketones but to any compound that can be converted in the reaction medium into a species containing the COCH3 group, for example, isopropyl or ethyl alcohol. 

Whether or not an alcohol contains one particular structural unit is shown by the iodoform test. The alcohol is treated with iodine and sodium hydroxide (sodium hypoiodite, NaOI) an alcohol of the structure... 

This sensitive test permits use of the readily available sodium hypochlorite in place of sodium hypoiodite for recognition of methyl carbonyl or methylcarbinol structures. It is useful also when detection of a haloform by infrared spectroscopy or gas-liquid chromatography is unsatisfactory, as may happen in certain reactions in which haloform-type cleavage occurs to a slight extent. ... 

When iodine is used the reaction serves for recognition of a group RCOCH3 (R = H, alkyl, or aryl) or of a group such as ethanol that is oxidizable to RCOCH3 by hypoiodite (Lieben s iodoform test) ... 

According to H. Moissan, chromium trioxide does not react with chlorine free from hydrogen chloride and K. H. Butler and D. McIntosh observed that the trioxide is insoluble in liquid chlorine, and has no effect on the b.p. of the liquid. According to A. Michael and A. Murphy, a soln. of chlorine in carbon tetrachloride in a sealed tube at 175° forms chromyl and carbonyl chlorides. H. Moissan observed that bromine has no action on the trioxide. I. Walz found that a cone, soln. of chromium trioxide, when poured on iodine, rapidly turns black and assumes a syrupy consistency, and the liquid thus formed does not respond to the tests for free iodic or hydriodic acid chromium hypoiodite may be formed. A mixture of sulphuric acid and chromium trioxide oxidizes iodine to iodic acid. 0. Ruff and H. Krug found that the trioxide is vigorously attacked by chlorine trifluoride. 

OBr hypoiodite with methyl ketones forms the basis of the iodoform (CHI3) test for CH3C=0 groups in organic compounds ... 

The hypoiodite reaction is quite selective for magnesium. Considerable quantities of reducing agents, ammonium and aluminum salts, and such ions which form colored hydroxides or higher oxides interfere with the test. Phosphate and oxalate ions interfere by yielding crystalline magnesium phosphate and oxalate. In contrast to the surface-rich hydroxide, these products are not capable of adsorbing free iodine. 

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