The reactivity of biogenic monoterpenes towards OH• and SO4-• radicals in de-oxygenated acidic solution

The reactivity of some selected biogenic monoterpene compounds towards important aqueous phase free-radical oxidants, namely OH . and SO4-., have been investigated using the complementary experimental techniques of pulse radiolysis and laser flash photolysis (lambda = 248 nm). Rate constants for the reactions of the OH . radical with cis-verbenol and methacrolein have been determined to be (6.8 +/- 0.5) x 10(9) dm(3) mol(-1) s(-1) and (8.0 +/- 0.7) x 10(9) dm(3) mol(-1) s(-1), respectively (T = 20 degrees C, pH 4.0, Ionic strength similar to 0 mol dm(-)3). Rate constants and activation energies for the reactions of the SO4-. radical have been measured for the following compounds (T = 20 degrees C, pH 4.0, Ionic strength = 0.03 mol dm(-3)): alpha-pinene (k = (3.1 +/- 0.1) x 10(9) dm(3) mol(-1) s(-1); E-act. = (8.9 +/- 1.3) kJ mol(-1)), alpha-terpineol (k = (4.1 +/- 0.1) x 10(9) dm(3) mol(-1) s(-1); E-act. = (13.4 +/- 0.6) kJ mol(-1)), cis-verbenol (k = (3.2 +/- 0.2) x 10(9) dm(3) mol(-1) s(-1); E-act. = (10.0 +/- 0.7) kJ mol(-1)), verbenone (k = (1.6 +/- 0.1) x 10(9) dm(3) mol(-1) s(-1); E-act. = (6.1 +/- 0.7) kJ mol(-1)), myrtenal (k = (1.85 +/- 0.1) x 10(9) dm(3) mol(-1) s(-1); E-act. = (7.5 +/- 0.7) kJ mol(-1)) and methacrolein (k = (1.18 +/- 0.1) x 10(9) dm(3) mol(-1) s(-1)). In most instances the absorption spectra of the intermediate products formed by these reactions have been measured which, in conjunction with strategic conductiometric studies, have been used to suggest plausible mechanisms for the oxidation in acidic de-oxygenated solution.