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; λ_max. 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.