Rate constants for reactions of OH radicals with (Z)-CF₃CCl=CHCl, CHF₂CF=CF₂, (E)-CF₃CH=CHF, (Z)-CF₃CH=CHF, CH₃CF=CH₂, and CH₂FCH=CH₂

Highlights

• Rate constants for reactions of OH with various halogenated propenes were measured.

• Infrared absorption spectra of halogenated propenes were measured.

• Atmospheric lifetimes and global warming potentials of the propenes were estimated.

• Ozone depletion potential of (E)-CF₃CH=CHCl was estimated.

• The correlation between structure and reactivity was discussed.

Abstract

The rate constants for the reaction of OH radicals with six halogenated propenes—(Z)-CF₃CCl = CHCl (k₁), CHF₂CF=CF₂ (k₂), (E)-CF₃CH=CHF (k₃), (Z)-CF₃CH=CHF (k₄), CH₃CF=CH₂ (k₅), and CH₂FCH=CH₂ (k₆)—were measured over 250–430 K using the absolute rate method. The Arrhenius expressions for the rate constants were determined as k₁ = (9.30 ± 0.54) × 10⁻¹³ exp [−(180 ± 20)/T], k₂ = (1.43 ± 0.04) × 10⁻¹² exp [(490 ± 10)/T], k₃ = (4.86 ± 0.13) × 10⁻¹³ (T/298)^0.727 exp [(110 ± 10)/T], k₄ = (4.55 ± 0.12) × 10⁻¹³ (T/298)^0.367 exp [(350 ± 10)/T], k₅ = (3.22 ± 0.20) × 10⁻¹² exp [(490 ± 20)/T], and k₆ = (2.87 ± 0.19) × 10⁻¹² exp [(440 ± 20)/T] cm³ molecule⁻¹ s⁻¹. Infrared absorption spectra of the halogenated propenes were measured at room temperature. The atmospheric lifetimes, global warming potentials of the halogenated propenes, and ozone depletion potential of (Z)-CF₃CCl = CHCl were estimated. The correlation between the reactivity and structure of 44 halogenated alkenes, including the six halogenated propenes, was investigated.

Introduction

Because unsaturated compounds have higher reactivity with OH radicals than saturated compounds, they are expected to have shorter atmospheric lifetimes and lower global warming potentials (GWPs) and ozone depletion potentials (ODPs). Thus, halogenated alkenes such as CF₃CF=CH₂ and (E)-CF₃CH=CHCl have been developed as potential chlorofluorocarbons (CFCs) replacements. However, owing to the presence of the C–F bond, these compounds show absorption in the infrared (IR) region of the atmospheric window; thus, they can contribute to the radiative forcing of climate when released into the atmosphere. In addition, chlorinated compounds may decompose and release chlorine atoms in the stratosphere, which can contribute to the depletion of the stratospheric ozone layer.

The GWP depends on the atmospheric lifetime and radiative efficiency. The ODP also depends on the atmospheric lifetime. Because halogenated alkenes are decomposed mainly through OH radical addition to a C=C bond, the atmospheric lifetime can be estimated from the reaction rate with OH radicals. Therefore, it is indispensable to assess the environmental impact of halogenated alkenes by measuring their reaction rates with OH radicals and their IR spectra.

In this study, we report the results of kinetic measurements of the rates of reactions between OH radicals and six halogenated propenes ((Z)-CF₃CCl = CHCl, CHF₂CF=CF₂, (E)-CF₃CH=CHF, (Z)-CF₃CH=CHF, CH₃CF=CH₂, and CH₂FCH=CH₂) over the temperature range of 250–430 K. The IR absorption spectra of these compounds were measured at room temperature. Subsequently, their atmospheric lifetimes with respect to their reactions with OH radicals, as well as their GWPs and ODP were estimated. For the reaction of OH radicals with (E)-CF₃CH= CHF, Søndergaard et al. (2007) reported the room temperature reaction rate using the relative rate (RR) method. Orkin et al. (2010) and Antiñ;olo et al. (2017)reported the temperature dependence of the reaction rate using an absolute rate method. Zhang et al. (2015) reported the temperature dependence of the reaction rate using RR method. For (Z)-CF₃CH= CHF, Nilsson et al. (2009) and Zhang et al. (2015) reported the room temperature reaction rate and the temperature dependence, respectively, using the RR method.Antiñolo et al. (2017)reported the temperature dependence of the reaction rate using an absolute rate method. For CH₃CF=CH₂, Tovar et al. (2014) reported the room temperature reaction rate using RR method. For CH₂FCH=CH₂, Albaladejo et al. (2003) reported the temperature dependence of the reaction rate using the absolute rate method. However, there are no previously reported values for the reaction rate of OH radicals with (Z)-CF₃CCl = CHCl or CHF₂CF=CF₂. Søndergaard et al. (2007), Orkin et al. (2010), and Zhang et al. (2015) have reported the IR cross-section for (E)-CF₃CH=CHF. Nilsson et al. (2009) and Zhang et al. (2015) reported IR cross-section for (Z)-CF₃CH=CHF. However, there are no previously reported IR absorption cross-section for (Z)-CF₃CCl = CHCl, CHF₂CF=CF₂, CH₃CF=CH₂, or CH₂FCH=CH₂. As mentioned above, some unsaturated compounds are already being used as replacements. In particular, chlorine containing unsaturated compounds are attracting attention as replacements. The present experimental study and the compilation of reaction rates and infrared spectra can provide guidance for the development of new replacement compounds.

In our previous studies (Tokuhashi et al., 2018a, 2018b), we examined the correlation between the reactivity and structure of halogenated alkenes and calculated the reaction rates of OH radicals with 26 halogenated alkenes by considering the atoms or atomic groups attached to the carbons on both sides of the double bond. In this report, we have further studied the correlation between the reactivities and structures of 44 halogenated alkenes including the original 26 compounds.