Hydrogen iodide

HI is a colorless gas that reacts with oxygen to give water and iodine. With moist air, HI gives a mist (or fumes) of hydroiodic acid. It is exceptionally soluble in water, giving hydroiodic acid. One liter of water will dissolve 425 liters of HI gas, the most concentrated solution having only four water molecules per molecule of HI. Hydroiodic acid Hydroiodic acid is an aqueous solution of hydrogen iodide. Commercial "concentrated" hydroiodic acid usually contains 48–57% HI by mass. The solution forms an azeotrope boiling at 127 °C with 57% HI, 43% water. The high acidity is caused by the dispersal of the ionic charge over the anion. The iodide ion radius is much larger than the other common halides, which results in the negative charge being dispersed over a large volume. This weaker H+···I− interaction in HI facilitates dissociation of the proton from the anion and is the reason HI is the strongest acid of the hydrohalides. :{{chem2|HI_{(g)} + H2O_{(l)} → H3O+_{(aq)} + I−_{(aq)}|}} Ka ≈ 1010 :{{chem2|HBr_{(g)} + H2O_{(l)} → H3O+_{(aq)} + Br−_{(aq)}|}} Ka ≈ 109 :{{chem2|HCl_{(g)} + H2O_{(l)} → H3O+_{(aq)} + Cl−_{(aq)}|}} Ka ≈ 106 ==Synthesis==

The industrial preparation of HI involves the reaction of I2 with hydrazine, which also yields nitrogen gas: : When the synthesis is performed in water, the HI can be purified by distillation. Anhydrous HI can be prepared by reaction of iodine with tetrahydronaphthalene: : HI can also be distilled from a solution of NaI or other alkali iodide that is treated with the dehydration reagent phosphorus pentoxide (which gives phosphoric acid). Concentrated sulfuric acid is unsuited for acidifying iodides, as it oxidizes the iodide to elemental iodine. An historical route to HI involves oxidation of hydrogen sulfide with aqueous iodine: : Additionally, HI can be prepared by simply combining H2 and I2: : In the laboratory, yet another method involves hydrolysis of PI3, the iodine analog of PBr3. In this method, I2 reacts with phosphorus to create phosphorus triiodide, which then reacts with water to form HI and phosphorous acid: : ==Reactions==

Solutions of hydrogen iodide are easily oxidized by air: : : is brown in color, which makes aged solutions of HI often appear dark. Like HBr and HCl, HI adds to alkenes, In the following idealized equation diethyl ether is split two equivalents of ethyl iodide: : The reaction is regioselective, as iodide tends to attack the less sterically hindered ether carbon. HI was commonly employed as a reducing agent early on in the history of organic chemistry. Chemists in the 19th century attempted to prepare cyclohexane by HI reduction of benzene at high temperatures, but instead isolated the rearranged product, methylcyclopentane (see the article on cyclohexane). As first reported by Kiliani, hydroiodic acid reduction of sugars and other polyols results in the reductive cleavage of several or even all hydroxy groups, although often with poor yield and/or reproducibility. In the case of benzyl alcohols and alcohols with α-carbonyl groups, reduction by HI can provide synthetically useful yields of the corresponding hydrocarbon product (). This process can be made catalytic in HI using red phosphorus to reduce the formed I2. ==Applications==

Commercial processes for obtaining iodine all focus on iodide-rich brines. The purification begins by converting iodide to hydroiodic acid, which is then oxidized to iodine. The iodine is then separated by evaporation or adsorption. ==References==