Hydrogen Iodide - Key Reactions

Key Reactions

HI will undergo oxidation if left open to air according to the following pathway:

4 HI + O₂ → 2H₂O + 2 I₂

HI + I₂ → HI₃

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

• Like HBr and HCl, HI add to alkenes

HI + H₂C=CH₂ → H₃CCH₂I

HI is also used in organic chemistry to convert primary alcohols into alkyl halides. This reaction is an S_N2 substitution, in which the iodide ion replaces the "activated" hydroxyl group (water). HI is preferred over other hydrogen halides in polar protic solvents 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. The large iodide anion is less solvated and more reactive in polar protic solvents and thus causes the reaction to proceed faster because of stronger partial bonds in the transition state. This reaction also occurs for secondary and tertiary alcohols, but substitution occurs via the S_N1 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 cleaved into ethanol and iodoethane. The reaction is regioselective, as iodide tends to attack the less sterically hindered ether carbon.

Hydroiodic acid is subject to the same Markovnikov and anti-Markovnikov guidelines as HCl and HBr.

• HI reduces certain α-substituted ketones and alcohols replacing the α substituent with a hydrogen atom.