Demethoxylation of Methoxylated Phenols and Hydroxybenzoic Acids
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Oxidative Demethoxylation of Methoxylated Phenols and Hydroxybenzoic Acids by the OH Radical. An in Situ Electron Spin Resonance, Conductometric Pulse Radiolysis, and Product Analysis Study Steen Steenken" and Peter O'Nelll Institut iur Strahienchemie im Max-Planck-Institut fur Kohienforschung, Stifistrasse 34-36, 0 4 3 3 0 Mulheim, West Germany (Received October 4, 1976) Publication costs assisted by Institut iur Strahlenchemie im Max- Planck-Institut {ur Kohlenforschung
OH radicals react with methoxylated phenols and methoxylated hydroxybenzoic acids by attachment to ring positions occupied by methoxyl groups (ipso attack) in addition to attachment to other positions. The OH radical adds preferentially to those ring positions which are activated by the electron-donatingOH or OCH3 groups. The radicals formed by addition of OH to ring carbons carrying methoxyl groups eliminate methanol to yield semiquinones ( k = 103-104 s-I).
Introduction With benzene derivatives, oxidative replacement of substituents X by the OH radical as shown in reaction 1
has been observed for X = F,' N02,2and NH2.3 The formation of chloride4 on irradiation of chlorobenzene in aqueous solution probably proceeds by this mechanism as well. Reaction 1 has also been discussed for X = OH5 or OCH3,6although it could not be unambiguously demonstrated to occur, Step 1B is of the 1,l-elimination type which has been observed also with radicals derived from h a l ~ - and ~ - ~nitrouracils,1° nitro- and bromofurans," and chloroethylenes.12 From pulse radiolysis studies on radical cation13 and zwitterion14formation by reaction of OH in acid solution with methoxylated benzenes and benzoic acids, respectively, evidence was obtained14,15for the involvement of reaction 1with X = OCH3 The present investigation was performed in order to study this type of reaction in detail using methoxylated phenols and hydroxybenzoic acids. Experimental Section N20 saturated aqueous solutions containing 5-10 mM substrate were irradiated with 6oCoy-rays using doses from 4.8 X 10l8to 2.4 X 10lgeV mL-l at a dose rate of 8.1 X 1017 eV mL-' mi&. Samples (10 pL) of the irradiated solutions were analyzed for methanol by gas chromatography using the double column technique with back-flushing16(precolumn: length 1.5 m, packed with 10% Marlophen on Teflon; main column: length 3 m, packed with P 4000; both columns were operated at 85 "C). The detection limit corresponded to -0.05 mM methanol. The 3-MeV van de Graaff accelerator and the optical and conductivity detection systems have been de~ c r i b e d . l ~Solutions ~'~ were irradiated at 20 f 2 "C with electron pulses of 1ps duration. The pII of the solutions was adjusted using HC104 or NaOH. Dosimetry was performed using either 10 mM FeS04 in 0.8 N H2S04or C(N02!4.19 The in situ radiolysis ESR experiments were carried out using the method described by Eiben and Fessenden.20
The coupling constants and g factors were determined as d e ~ c r i b e d l ~and > ~ are ' estimated to be accurate to 30 mG respectively. and 5 X The substrates were obtained from Fluka or Aldrich. They were of the highest purity available and were used as received with the exception of !2,6-dimethoxyphenol, which was zone refined prior to use. Results and Discussion Production of Methanol. On 6oCoy-irradiation of NzO saturated 5-10 mM solutions of 2-, 3-, or 4-methoxyphenol (MP), 2,3-, 2,6-, or 3,5-dimethoxyphenol (DMP), 2hydroxy-5-methoxybenzoic acid (bmethoxysalicylic acid), 3-methoxy-4-hydroxybenzoicacid (vanillic acid), or 3,5dimethoxy-4-hydroxybenzoicacid (syringa acid) at pH 3-7, methanol is formed as a reaction product. The concentration of methanol in irradiated substrate solutions is linearly dependent on dose in the range 4.8 X 1018-(1.2-2.4)1019 eV mL-l. At the highest dose the removal of substrate corresponds to
