Temperature-Dependent Structure–Activity Relationship of OH + Haloalkene Rate Coefficients under Atmospheric Conditions and Supporting Measurements

Hydrofluoroolefins
(HFOs) and related haloalkenes are a family
of compounds, primarily man-made, with many industrial applications.
Gas-phase electrophilic addition of OH to the olefinic bond represents
the primary sink for these chemicals in the atmosphere. The degree
and type of halogenation strongly affect their chemical reactivity,
leading to differing reactivities with the OH radical that have presented
a challenge for structure–activity relationships (SARs). Here,
we investigate and extend the SARs to estimate temperature-dependent
OH reaction rate coefficients, k(T), at tropospheric temperatures. We considered two techniques: the
group-additivity approach of Atkinson and co-workers and the recent
method of Tokuhashi and co-workers; we found the latter to make superior
predictions for halogenated olefins. We extended Tokuhashi et al.’s
SAR to include more olefins and to predict temperature dependencies.
We compared SAR predictions against new absolute k(T) for two HFOs (3,3,3-trifluoropropene and 1,1,3,3-tetrafluoropropene)
measured using the pulsed-laser photolysis–laser-induced fluorescence
technique from 212 to 373 K. The Arrhenius expressions were determined
as kOH+CF3CHCH2(T) = (8.86 ± 0.82) × 10–13 exp[(159 ± 26)/T] and kOH+CF2HCHCF2(T) = (7.46 ± 0.34) × 10–13 exp[(365 ±
12)/T]. The measured k(T) was predicted accurately between 200 and 400 K using the modified
Tokuhashi approach. Our new measurements of the OH reaction rate coefficient
with 3,3,3-trifluoropropene were in excellent agreement with literature
determinations. We represent the first reported k(T) for 1,1,3,3-tetrafluoropropene. Given that new
HFOs enter the market frequently, such estimation techniques may be
a helpful screening tool for assessing their environmental impact
before they are examined further and reach the mass-production phase.