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Reductive Character of Hydroiodic Acid
Hideo Togo
Emeritus Professor of Chiba University, Research Advisor in GODO SHIGEN Co. LTD
Electronegativity of fluorine atoms, chlorine atoms, bromine atoms, and iodine atoms decreases as follows: 4.0, 3.0, 2.8, and 2.6, respectively. However, polarizability of fluorine atoms, chlorine atoms, bromine atoms, and iodine atoms increases as follows: 10.4, 36.6, 47.7, and 71.0, respectively. Moreover, pKa of hydrofluoric acid, hydrochloric acid, hydrobromic acid, and hydroiodic acid is as follows: 3.2, -7, -9, and -10, respectively. Thus, among those four halogen atoms, electronegativity of iodine atom is the smallest, polarizability of iodine atom is the biggest, and hydroiodic acid is the strongest acid. Hydroiodic acid has mild reducing ability, and therefore, colorless liquid hydroiodic acid slowly changes to a yellow solution and then a brown solution, together with generation of molecular iodine, under atmospheric conditions. Hydrogen iodide (HI) is a colorless gas at room temperature (mp: -50.8 °C, bp: -35.1 °C, and density: 2.85 g/cm3 (-47 °C)). 57% Hydroiodic acid (aq. 57% HI) is a colorless liquid and is prepared by distillation (bp is 127 °C). It is commercially available.
Warming treatment of alcohols with aq. 57% hydroiodic acid generates the corresponding alkyl iodides in good yields efficiently, as shown in Eq. 1.1.1
Warming treatment of α-aryl-α-(hydroxy)propionic acids with excess amounts of aq. 57% hydroiodic acid generates α-(aryl)propionic acids, as shown in Eq. 1.2a.2 This reaction proceeds through formation of α-aryl-α-(iodo)propionic acid from α-aryl-α-(hydroxy)propionic acid with aq. 57% hydroiodic acid, and then reduction of α-aryl-α-(iodo)propionic acid by aq. 57% hydroiodic acid, together with generation of molecular iodine. Similarly, warming treatment of α-aryl-α-(hydroxy)propionitriles with aq. 57% hydroiodic acid generates α-(aryl)propionitriles, as shown in Eq. 1.2b.2
Warming treatment of benzyl alcohols bearing a 2-pyridyl group at the α-position with aq. 57% hydroiodic acid in acetic acid generates aryl(2-pyridyl)methanes, as shown in Eq. 1.3.3 Moreover, warming treatment of fructose with aq. 57% hydroiodic acid in a mixture of toluene and water generates 5-methyl-2-furaldehyde, as shown in Eq. 1.4.4
Microwave irradiation (MW) or warming treatment of nitroarenes bearing a cyano group or ketone group with aq. 57% hydroiodic acid generates anilines bearing a cyano group or acetyl group, as shown in Eq. 1.5.5,6 Here, nitrile and ketone groups are not reduced at all under the conditions.
Sulfoxides can be exothermically and rapidly reduced to the corresponding sulfides at room temperature by aq. 57% hydroiodic acid. Sulfinic acids (RSO2H) and sulfonyl chlorides (RSO2Cl) can be also reduced to the corresponding disulfides under warming conditions by aq. 57% hydroiodic acid. Treatment of sodium arenesulfinates with aq. 57% hydroiodic acid at room temperature generates the corresponding diaryl disulfides smoothly, via formation of arenesulfinic acid (pKa ~2), as shown in Eq. 1.6.7
Sulfonic acids and sulfonate salts are not reduced at all by LiAlH4 or iBu2AlH under warming conditions. However, warming treatment of sulfonic acids or sodium sulfonates with KI and PPA (polyphosphoric acid) in the presence of Bu4NI (cat.) in sulfolane generates the corresponding disulfides, as shown in Eq. 1.7.8 Here, the ketone group is not reduced at all. Treatment of (triaryl)phosphine oxides, oxalyl chloride, and NaI in the presence of TMEDA in acetonitrile generates the corresponding (triaryl)phosphines efficiently, as shown in Eq. 1.8.9
The above-mentioned reductive reactions by hydroiodic acid or iodide salts are reflected in the reducing character of hydroiodic acid or iodide anions. This character comes from the high polarizability and high nucleophilicity of iodine atoms. Therefore, the above-mentioned reductive reactions cannot be carried out at all with hydrofluoric acid, hydrochloric acid, or hydrobromic acid.
References
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Introduction of the author
Hideo Togo
Hideo Togo was born in 1956 in Ibaraki, Japan. He completed his doctoral thesis in 1983 at University of Tsukuba. Then, he became a post-doctoral fellow at University of Lausanne in Switzerland (1983–1984) and at CNRS (Professor Sir Derek H. R. Barton) in France (1984–1985). Then, he became a research associate at University of Tsukuba in 1987 and then moved to Chiba University in 1989 as a research associate. He became an associate professor in 1994, and a full professor in 2005. He retired from Chiba University in 2021, and became an emeritus professor at Chiba University and a research advisor at the research center of technology of GODO SHIGEN Co. LTD.