Effects of Metal Ions on Aqueous-Phase Decomposition of α-Hydroxyalkyl-Hydroperoxides Derived from Terpene Alcohols.

We report the results of a mass spectrometric study of the effects of atmospherically relevant metal ions on the decomposition of α-hydroxyalkyl-hydroperoxides (α-HHs) derived from ozonolysis of α-terpineol in aqueous solutions. By direct mass spectrometric detection of chloride adducts of α-HHs, we assessed the temporal profiles of α-HHs and other products in the presence of metal ions. In addition, reactions between α-HHs and FeCl ₂ in the presence of excess DMSO showed that the amount of hydroxyl radicals formed in a mixture of α-terpineol, O ₃ , and FeCl ₂ was 5.7 ± 0.8% of the amount formed in a mixture of H ₂ O ₂ and FeCl ₂ . The first-order rate constants for the decay of α-HHs produced by ozonolysis of α-terpineol in the presence of 5 mM acetate buffer at a pH of 5.1 ± 0.1 were determined to be (4.5 ± 0.1) × 10 ^-4 s ^-1 (no metal ions), (4.7 ± 0.2) × 10 ^-4 s ^-1 (with 0.05 mM Fe ²⁺ ), (4.7 ± 0.1) × 10 ^-4 s ^-1 (with 0.05 mM Zn ²⁺ ), and (4.8 ± 0.2) × 10 ^-4 s ^-1 (with 0.05 mM Cu ²⁺ ). We propose that in acidic aqueous media, the reaction of α-HHs with Fe ²⁺ is outcompeted by H ⁺ -catalyzed decomposition of α-HHs, which produces the corresponding aldehydes and H ₂ O ₂ , which can in turn react with Fe ²⁺ to form hydroxyl radicals.