Relative Rate Constants for the Reactions of Chlorine Atoms with Unsaturated Ethers.

PABLO R. DALMASSO, RAÚL A. TACCONE, JORGE D. NIETO, MARIANO A. TERUEL, SILVIA I. LANE.

Orléans. Francia

Congreso; 19th Internacional Symposium on Gas Kinetics (GK 2006).; 2006

Unsaturated ethers are widely used in different industries. Allyl ethers (ROCH₂CH=CH₂) and vinyl ethers (ROCH=CH₂), where R is an alkyl group, are mainly used as raw material for epoxy silane coupling agents and unsaturated polyesters containing allyl groups, in cosmetics, for manufacturing of coatings and in the preparation of fragances¹. Even though an evaluation of the amount of the world production of these compounds is not available at present, their uses certainly results in their release into the atmosphere.

While the OH radical is considered to be the major daytime oxidant, chlorine atoms present at an average global concentration of ~1x10⁴ atoms cm^{-3 2}, can play an important role in atmospheric chemistry since atomic Cl usually reacts with volatile organic compounds (VOCs) with a much greater rate constant than OH radicals³. Moreover, in the coastal urban areas the concentrations of Cl atoms, for a brief period at dawn in the marine boundary layer, are estimated to be significantly large, as high as 10⁵ atoms cm^-3 or more^4,5. Thus, reactions with Cl atoms in coastal areas may represent a significant loss process for VOCs.

The aim of this study was to extend the existing scant data base of rate constants for the reactions of chlorine atoms with allyl and vinyl ethers, as part of ongoing work in our laboratory regarding the atmospheric impact of these oxygenated compounds. The relative rate method was used to study the following reactions:

 Cl  +  CH₂=CHCH₂OCH₂CH₃ ---> Products         (1)

Cl  +  CH₂=CHCH₂OCH₂CH=CH₂ ---> Products  (2)

Cl  +  CH₂=CHOCH₂CH₂Cl ---> Products            (3)

Relative rate constants were measured  by comparing the Cl atom reaction rate with the ethers to that with n-pentane used as the reference compound. All the experiments were carried out in an 80 L collapsible Tedlar bag, at room temperature and atmospheric pressure using N₂ as bath gas. Using a value of (2.71 ± 0.68) x 10^-10 cm³ molecule^-1 s^{-1 6} for the reaction rate constant of Cl with n-pentane, the following values of k₁, k₂, and k₃ were derived for the rate constants of Cl atoms with the ethers studied: (5.5 ± 1.5) x 10^-10 , (3.5 ± 1.2) x 10^-10  and <1 x 10^-11, respectively, in units of cm³ molecule^-1 s^-1. To the best of our knowledge this is the first experimental determination of the rate constants for these reactions. It must also be pointed out that there are no previous determinations for these ethers with the other tropospheric oxidants.

Based on the rate constants reported above, the tropospheric lifetimes of  the three ethers were calculated as: 2 days for CH₂=CHCH₂OCH₂CH₃ , 3.3 days for CH₂=CHCH₂OCH₂CH=CH₂ , and > 115 days for CH₂=CHOCH₂CH₂Cl, taking an average global concentration of 1x10⁴ atoms cm^-3 for Cl atoms. These relatively short lifetimes indicate that these compounds will be oxidised rapidly near their anthropogenic sources, thus making a negligible  contribution to global warming but probably increasing local atmospheric pollution.

References

1. Perstorp web site: www.perstorp.com  and BASF web site. www.basf.de.

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4. C.W. Spicer,  E.G.  Chapman, B.J.  Finlayson-Pitts,  R.A.  Plastidge, J.M.  Hubbe, J.D. Fast,  C.M. Berkowitz, Nature 394 (1998) 353-356.

5. M.J.  Ezell, W.  Wang,  A.A. Ezell, G.  Soskin, B.J.  Finlayson-Pitts,  Phys. Chem. Chem. Phys. 4  (2002) 5813-5820.

6. NIST-Chemical Kinetics Database on the Web, 2002. Standard Reference Database 17, Version 7.0 (Web Version), Release 1.2.