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.^[6]

The industrial preparation of HI involves the reaction of I₂ with hydrazine, which also yields nitrogen gas:^[7]

2 I₂ + N₂H₄ → 4 HI + N₂

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:^[8]

C₁₀H₁₂ + 2 I₂ → C₁₀H₈ + 4 HI

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).^[9] 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:^[10]

H₂S + I₂ → 2 HI + S

Additionally, HI can be prepared by simply combining H₂ and I₂:^[9]

H₂ + I₂ → 2 HI

This method is usually employed to generate high-purity samples. For many years, this reaction was considered to involve a simple bimolecular reaction between molecules of H₂ and I₂. However, when a mixture of the gases is irradiated with the wavelength of light equal to the dissociation energy of I₂, about 578 nm, the rate increases significantly. This supports a mechanism whereby I₂ first dissociates into 2 iodine atoms, which each attach themselves to a side of an H₂ molecule and break the H−H bond:^[11]

H₂ + I₂ + (₅₇₈ nm radiation) → H₂ + 2I → I···H···H···I → 2HI

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:

3 I₂ + 2 P + 6 H₂O → 6 HI + 2 H₃PO₃

Solutions of hydrogen iodide are easily oxidized by air:

4 HI + O₂ → 2 H₂O + 2 I₂

HI + I₂ ⇌ HI₃^[12]

HI₃ is brown in color, which makes aged solutions of HI often appear dark.

Like HBr and HCl, HI adds to alkenes,^[13] in a reaction that is subject to the same Markovnikov and anti-Markovnikov guidelines as HCl and HBr.

HI + RCH=CH₂ → RCH(I)−CH₃

HI is also used in organic chemistry to convert primary alcohols into alkyl iodides.^[14] This reaction is an S_N2 substitution, in which the iodide ion replaces the "activated" hydroxyl group (water):

HI + RCH₂OH → RCH₂I + H₂O

HI is sometimes preferred over other hydrogen halides.

HI (or HBr) can also be used to cleave ethers. Commonly, it is applied to the cleavage of aryl-alkyl ethers to give phenols and the alkyl iodide.^[14] In the following idealized equation diethyl ether is split two equivalents of ethyl iodide:

2 HI + (CH₃CH₂)₂O → 2CH₃CH₂I + H₂O

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,^[15] 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.^[16] 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 (ROH + 2HI → RH + H₂O + I₂).^[13] This process can be made catalytic in HI using red phosphorus to reduce the formed I₂.^[17]

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.^[18]

• Hydroiodic acid