Rate constants for the gas-phase reactions of hydroxyl radicals and chlorine atoms with aliphatic alcohols and ethers have been determined at 298 ± 2 K and at a total pressure of 1 atmosphere. The OH radical rate data were obtained using both the absolute technique of pulse radiolysis combined with kinetic UV spectroscopy and a conventional photolytic relative rate method. The Cl atom rate constants were measured using only the relative rate method. Values of the rate constants in units of 10−12 cm3 molecule−1 s−1 are: 






The above relative rate constants are based on the values of (OH + c-C6H12) = 7.49 × 10−12 cm3 molecule−1 s−1 and (Cl + c-C6H12) = 311 × 10−12 cm3 molecule−1 s−1. Attempts to corre late the trends in the rate constant data in terms of the bond dissociation energies and inductive effects are discussed.

Absolute and relative rate constants for the reactions of hydroxyl radicals and chlorine atoms with a series of aliphatic alcohols and ethers at 298 K

Laser flash photolysis of chloroacetone was used to measure the rate constants and activation energies for
 the reactions of the Cl• atom with a number of oxygen-containing compounds and inorganic anions in aqueous
 solution. For the organic compounds there is a strong correlation at 25°C between k(Cl•+RH)
 and k(•OH+RH), where RH is CH3OH, CH3CH2OH, CH3CH(OH)CH3, (CH3)3COH, HCHO, CH3CHO, CH3CO2H,
 HCO2H and HCO2−, respectively. For CH3CO2−, k(Cl•+RH)k(•OH+RH), and for CH3COCH3 and CH3COCH2Cl, k(Cl•+RH)k(•OH+RH).
 Possible reasons for these differences are discussed in terms of preferential attack by Cl• at O–H groups
 in the neutral molecules, rather than H-abstraction from C–H as with •OH, and electron transfer for the reactions
 of Cl• with the anions. For the inorganic anions X=OCN−, NO3−, SO42−, ClO3−, SCN−, HCO3−, N3−,
 NO2−, HSO3, k(Cl•+X) ranges from 1.0×108 (NO3−) to 5.3×109 dm3 mol−1 s−1 (SCN−) but there is no strong correlation
 between k and the reduction potential of X. Comparison of the reactivity of Cl•
 with reported rate constants for the
 reactions of Cl2•− indicates that, in many cases, these rate constants are largely accounted for by the fraction of
 Cl• present in equilibrium with Cl2•−. The implications of these results for atmospheric chemistry
 are discussed.

The reactivity of chlorine atoms in aqueous solution. Part III. The reactions of Cl• with solutes

The relative rate technique has been used to determine the rate constants for the reaction of chlorine atoms with a series of oxygenated organic species. Experiments were performed at 295 ± 2 K and atmospheric pressure of synthetic air or nitrogen. The decay rates of the organic species were measured relative to that of ethane or n-butane. Using rate constants of 5.7 × 10−11 cm3 molecule−1 s−1, and 2.25 × 10−10 cm3 molecule−1 s−1 for the reaction of Cl with ethane and n-butane respectively the following rate constants were derived, in units of 10−11 cm3 molecule−1 s−1: propane, (16.0 ± 0.4);i-butane, (15.1 ± 0.9) n-pentane, (31.0 ± 1.6); n-hexane, (34.5 ± 2.3); cyclohexane, (36.1 ± 1.5); methanol, (4.57 ± 0.40); ethanol, (8.45 ± 0.91); n-propanol, (14.4 ± 1.2); t-butylalcohol, (3.26 ± 0.19); acetaldehyde, (8.45 ± 0.79); propionaldehyde, (11.3 ± 0.9); dimethylether, (20.5 ± 0.8); diethylether, (35.6 ± 2.8); and methyl-t-butylether, (16.6 ± 1.2). Quoted errors represent 2σ, and do not include any errors due to uncertainties in the rate constants used to place our relative measurements on an absolute basis. The results are discussed with respect to the mechanisms of these reactions and to previous literature data.

Gas phase reaction of Cl atoms with a series of oxygenated organic species at 295 K

The kinetics and products of the homogeneous gas-phase reactions of the OH radical with the chloroethenes were investigated at 298 ± 2 K and atmospheric pressure. Using a relative rate technique and ethane as a scavenger for the chlorine atoms produced in these OH radical reactions, rate constants (in units of 10−12 cm3 molecule−1s−1) of 8.11 ± 0.24, 2.38 ± 0.14, and 1.80 ± 0.03 were obtained for 1,1-dichloroethene, cis-1, 2-dichloroethene and trans-1,2-dichloroethene, respectively. Under these conditions, the major products observed by long pathlength FT-IR absorption spectroscopy were HCHO and HC(O)Cl from vinyl chloride; HC(O)Cl from cis- and trans-1,2-dichloroethene; HCHO and COCl2 from 1,1-dichloroethene; HC(O)Cl and COCl2 from trichloroethene; and COCl2 from tetrachloroethene. In the absence of a Cl atom scavenger, significant yields of the chloroacetyl chlorides, CHxCl3−xC(O)Cl, were observed from 1,1-dichloro-, trichloro- and tetrachloroethene, indicating that these products resulted from reactions involving chlorine atoms. The yields of all of these products are reported and the mechanisms of these gas-phase reactions discussed. In addition, OH radical reaction rate constants were redetermined, in the presence of a Cl atom scavenger, for cis- and trans-1,3-dichloropropene and 3-chloro-2-chloromethyl-1-propene, being (in units of 10−12 cm3 molecule−1 s−1) 8.45 ± 0.41, 14.4 ± 0.8, and 33.5 ± 3.0, respectively.

Atmospheric reactions of chloroethenes with the OH radical

Chlorine atoms react with a variety of organic molecules by abstraction of an H atom, making HCl and a radical co-product, and investigations of these reactions provide a large and valuable body of data for improved fundamental understanding of the mechanisms of reactions involving polyatomic molecules. The kinetics and dynamics of reactions of Cl atoms with alkanes have been extensively studied both by experimental and computational methods, and the key outcomes and conclusions are reviewed. These reactions serve as benchmarks for the interpretation of recent experimental data on the dynamics of reactions of Cl atoms with heteroatom functionalized organic molecules such as alcohols, ethers, amines, alkyl halides and thiols. Although bearing many similarities to the dynamics of the alkane reactions, significant differences are found: in particular, the extent of HCl rotational excitation from reactions of Cl atoms with the functionalized molecules is much greater than the very cold rotational distribution...

The dynamics of chlorine-atom reactions with polyatomic organic molecules

Absolute rate constants for the reaction of atomic chlorine with dimethyl ether and methanol, Cl+CH3OCH3→HCl+CH2OCH3 (1),Cl+CH3OH→HCl+CH2OH (2), have been measured over the temperature range 200–500 K, using the flash photolysis–resonance fluorescence technique. In both systems, the results were independent of substantial variations in reactant concentration, total pressure (Ar) and flash intensity (i.e., initial [Cl]). The rate constants were also shown to be invariant with temperature. The best representation for this temperature range was found to be k1= (1.76±0.15) ×10−10 cm3 molecule−1 s−1 and k2= (6.33±0.70) ×10−11 cm3 molecule−1 s−1, respectively, where the error is one standard deviation. These are the first determinations of the rate constants for Reactions (1) and (2). These reactions are theoretically discussed and compared to the related reactions of Cl with CH4, C2H6, and H2CO. Molecules of the type CH3OX (where X=H, OH, etc.) are likely to be formed in the stratosphere as products of the oxidation of methane. The molecules CH3OH and CH3OCH3 are considered as prototypes of molecules containing the methoxy group. The potential importance of the fast reactions Cl+CH3OX in stratospheric chemistry is discussed.

J. V. Michaela

Papers

Rate constants for the reaction of atomic chlorine with methanol and dimethyl ether from 200 to 500 K