Gas-phase degradation of 2-butanethiol initiated by OH radicals and Cl atoms: kinetics, product yields and mechanism at 298 K and atmospheric pressure

Abstract

Relative rate coefficients and product distribution of the reaction of 2-butanethiol (2butSH) with OH radicals and Cl atoms were obtained at atmospheric pressure and 298 K. The experiments were performed in a 480 L borosilicate glass photoreactor in synthetic air coupled to a long path "in situ" FTIR spectrometer. The rate coefficients obtained by averaging the values from different experiments were: k _OH = (2.58 ± 0.21) × 10^-11 cm³ per molecule per s and k _Cl = (2.49 ± 0.19) × 10^-10 cm³ per molecule per s. The kinetic values were compared with related alkyl thiols and homologous alkyl alcohols, where it was found that thiols react faster with both oxidants, OH radicals and Cl atoms. SO₂ and 2-butanone were the major products identified for the reactions of 2-butanethiol with OH radicals and Cl atoms. The product yield of the reaction of 2-butanethiol and OH radicals were (81 ± 2)%, and (42 ± 1)% for SO₂ and 2-butanone, respectively. For the reactions of 2-butanethiol with Cl atom, yields of SO₂ and 2-butanone were (59 ± 2)% and (39 ± 2)%, respectively. A degradation mechanism was proposed for the pathways that leads to formation of identified products. The product distribution observed indicated that the H-atom of the S-H group abstraction channel is the main pathway for the reaction of OH radicals and Cl atoms with 2-butanethiol.

Fig. 1. Plots of kinetic data of reaction of 2butSH with OH radicals at 298 K and atmospheric pressure of synthetic air using 2-methyl-propene and E-2-butene as reference compounds.

Fig. 2. Plots of kinetic data of reaction of 2butSH with Cl atoms at 298 K and atmospheric pressure of synthetic air using 2-methyl-propene and propene as reference compounds.

Fig. 3. Trace A shows the IR spectrum for a mixture of 2butSH and methyl nitrate (CH₃ONO) in air acquired before irradiation and the trace B shows the residual spectrum after irradiation and subtraction of the 2butSH, CH₃ONO, CO, CO₂ and H₂O abortions. Traces C show the reference spectrum for SO₂, traces D show the reference spectrum of 2-butanone, traces E show the reference spectrum of (HCHO), traces F show the reference spectrum of formic acid (HC(O)OH), traces G show the reference spectrum of peroxyacetyl nitrate (PAN) and H traces show the residual spectrum after to subtract the spectra of the identified compounds. For comparison proposes all spectra were normalized.

Fig. 4. Plot of the products concentrations of SO₂ and 2-butanone as function of reacted 2butSH with OH radicals at atmospheric pressure of synthetic air and 298 K.

Fig. 5. Trace A shows the IR spectrum for a mixture of 2butSH and Cl₂ in air acquired before irradiation and the trace B shows the residual spectrum after irradiation and subtraction of the 2butSH, CO and CO₂ absorptions. Traces C show the reference spectrum for SO₂, traces D show the reference spectrum of 2-butanone, traces E show the reference spectrum of HCl and F traces show the residual spectrum after to subtract the spectrum of the identified compounds. For comparison proposes all spectra were normalized.

Fig. 6. Plot of the products concentrations of SO₂, 2-butanone as function of reacted 2butSH with Cl atoms at atmospheric pressure of synthetic air and 298 K.

Fig. 7. Proposed mechanism for the OH radical and Cl atoms oxidation of 2butSH at 298 K and atmospheric pressure of synthetic air. We show the more important possible H-abstraction pathways to form the identified products. X = OH or Cl. The molecules enclosed in the dashed black line rectangle were identified in the reaction with OH radicals and the enclosed in black line rectangles are the identified in the reaction with Cl atoms.