is filtered): H2S + I2 → 2 HI + S Additionally, HI can be prepared by simply combining H2 and I2: 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 : {H2} + {I2} ->[\text{578 nm radiation}] {H2} + 2I -> I{\cdots}H{\cdots}H{\cdots}I -> 2HI In the laboratory, another method involves hydrolysis of PI3, the iodine equivalent 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 → 2 PI3 + 6 → 6 HI + 2 H3PO3 Key reactions and applications Solutions of hydrogen iodide are easily oxidized by air: 4 HI + O2 → 2 + 2 I2 HI + I2 ⇌ HI3 is dark brown in color, which makes aged solutions of HI often appear dark brown. Like HBr and HCl, HI adds to alkenes: HI + H2C=CH2 → HI is also used in organic chemistry to convert primary alcohols into alkyl iodides. This reaction is an SN2 substitution, in which the iodide ion replaces the "activated" hydroxyl group (water): HI is preferred over other hydrogen halides because the iodide ion is a much better nucleophile than bromide or chloride, so the reaction can take place at a reasonable rate without much heating. This reaction also occurs for secondary and tertiary alcohols, but substitution occurs via the SN1 pathway. HI (or HBr) can also be used to cleave ethers into alkyl iodides and alcohols, in a reaction similar to the substitution of alcohols. This type of cleavage is significant because it can be used to convert a chemically stable and inert ether into more reactive species. In this example diethyl ether is split into ethanol and iodoethane: The reaction is regioselective, as iodide tends to attack the less sterically hindered ether carbon. If an excess of HI is used, the alcohol formed in this reaction will be converted