Reactivity of Electronically Excited SO₂ with Alkanes

We studied the reaction of electronically excited sulfur dioxide in the triplet state (³SO₂) with a variety of alkane species, including propane, n-butane, isobutane, n-pentane, n-hexane, cyclohexane, n-octane, and n-nonane. Reaction rate constants for the photoinitiated reaction of SO₂ with all of these species were determined and found to be in the range from 3.7 × 10^-13 to 5.1 × 10^-12 cm³molecule^-1s^-1. We found that reaction proceeds via a hydrogen abstraction to form HOSO• and organic radical (R•) species and that reactivity is correlated with the energy required to break a C-H bond and the length of the alkane chain. Abstraction rates were found to be fastest for reaction with hydrogen on a tertiary carbon. Similarly, abstraction from secondary carbons is found to be faster than from primary carbons. The reactivity of ³SO₂ with alkanes increases with chain length as additional secondary carbons are added.