Radical cations from alkyl iodides in aqueous solution
Abstract
Radical cations RI+˙ and (RI∴IR)+ are generated upon ˙OH radical-induced oxidation of alkyl iodides in acidic (pH < 4.5) aqueous solution. Their formation requires the reaction of protons with an intermediate ˙OH-adduct. Although alkyl iodide radical cations have been reported to exist in a solid, low-temperature matrix, they have so far not been detected in the liquid, particularly the aqueous phase. Both radical cations establish an equilibrium: RI+˙+ RI ⇌(RI∴IR)+. For R = Me the stability constant of the 2σ–1σ* three-electron-bonded species has been estimated to K 5 × 104 mol–1 dm3. (RI∴IR)+ exhibits a strong optical absorption band in the visible; λmax. is red-shifted with increasing electron-releasing power of R and ranges from 415 nm (R = Me) to 470 nm (R = s-C4H9). The molecular radical cation absorbs in a comparatively much narrower range from 310–320 nm. It is suggested that it exists as (RI∴OH2)+, i.e. as an associate with one water molecule. The alkyl iodide radical cations are strong oxidants. (MeI∴IMe)+, for example, oxidizes I– and Me2S with rate constant values of 7.7 × 109 mol–1 dm3 s–1 and 3.6 × 109 mol–1 dm3 s–1, respectively.