Absolute rate constants for elementary reactions in the chlorination of CH4, CD4, CH3Cl, CH2Cl2, CHCl3, CDCl3 and CBrCl3

Abstract

The rates of reactions of ClzP atoms with methane, and with a series of substituted methanes, have been measured using mass spectrometric analysis of molecular reactant consumption in the presence of excess Cl atoms, with pseudo first order kinetic analysis ; this approach minimizes secondary reactions and the problem of reaction stoichionietries. The following rate constants were obtained for CI + RH reactions : l~g~~( k c~~/ c m~ molecule-' s-I) = (-10.294+0.043)-(778k 16) KIT, 300-686 K ; l o g l o ( k~~s~i / c m 3 molecule-' s-l) = (-9.673 k 0.087) -(776 & 30) K / T , 300-604 K ; l~g~~( k c~~~i~/ c m~ molecule-' s-I) = (-10.078 k0.074)-(629& 26) K / T , 298-621 K ; l~g~~( k c~~~~/ c m~ molecule-' s-I) = (-10.480+0.051)-(599k 19) KIT, 297-652 K ; l~g~~( k c~~c i~/ c m~ moIecuIe-' s-I) = -12.64+ 0.08, 652 K. The correlations of these results with data from competitive chlorination studies are discussed in relation to the uncertainty surrounding the Arrhenius parameters for the primary reference reaction, Values for the kinetic isotope (H/D) rate constant ratios, (~C H J~C D~) and ( k c~c i~ /~C D C I~) , were C1+ HZ+HCl+ H. measured over wide ranges of temperature : loglo(kCHs/kCD4) = (-0.155f0.024)+(384+ 12) K / T , 298-1018 K ; 298-1006 K. lOglo(kCHCl~/kCDC13) = (-0.014+0.020)+(167If:9) K / T , These results are in agreement with the predictions of the semi-empirical BEBO (bond-energy-bond order) method.Chlorination reactions with molecules RH, involving abstraction of an H atom, and leading to formation of HCl and a radical R, constitute one of the largest series of elementary reactions for which relative (although not absolute) rate constants have been measured. These kinetic data have provided some of the most extensive results for the testing of theories of absolute reaction rates, including kinetic isotope effects. Examples include the reactions : Cl+H,, D2, HT l * ; Cl+ CH4, CD4, CzH6, CzD6.2 However, surprisingly few direct measurements of absolute values for rate constants are a~ailable.~ All the reactions in the series C1+CH4, CH,Cl, CH2C12, CHC13, have activation energies less than 20 kJ f110l-l,~ whilst the values for -AU,",, in these exothermic reactions gradually increase in this series from about 4 kJ mol-l for Cl+ CH4, to about 42 kJ mol-1 for C1+ CHC13.4 Measurements of the ratios of rate constants (ICR*H/ICR"H) for reactions of this type have been carried out using competitive methods, with C1 atoms generated either thermally or photolytically from C12 : kWH Cl + R'H+R' + HCI, C1+ R"H+R" + HCl. 1547 7kRttn9

Survey of Rate Coefficients in the C-H-Cl-O System

“…·CI repulsion, however, the activation energy changes little from CH3CI to CH2Cl2 to CHCI3. The experimental data for this reaction, with the exception of that of Clyne and Walker (1973), believed to contain errors (Manning and Kury 10 1977), agree with one another and with SAR predictions (Senkan and Quam 1992) and ambient temperature recommendations , as seen in Figure 4.27. We chose to represent the experimental data using the three-parameter rate expression given in Table 4.7.…”

“…The experimental studies of this reaction, with the exception of that of Clyne and Walker (1973), which is believed to be in error (Manning and Kurylo 1977), generally agree with one another and with SAR predictions (Senkan and Quam 1992) and ambient temperature recommendations , as shown in Figure 4.26. We elected to use the three-parameter rate coefficient expression with m = 2 listed in Table 4.7.…”

“…CI + CHCl 3 ~ CCl 3 + HCI The experimental data available for this reaction are more limited than for the other chlorinated methanes. Discounting the results of Clyne and Walker (1973) owing to the apparent presence of systematic error, we are left with the relative rate measurements of Fettis and Knox (1964), who studied this reaction in a static reactor and determined product distributions using gas chromatography against CH 4 as an internal standard, and those of Chiltz et al (1963). The original rate coefficient determinations of Fettis and Knox (1964) were based on a rate coefficient for CI + CH4 that is now known to be incorrect.…”

Competitive photochlorination and kinetic isotope effects for hydrogen/deuterium abstraction from the methyl group in C2H6, C2D6, CH3CHCl2, CD3CHCl2, CH3CCl3, and CD3CCl3

“…This weak effect is unity within the limits of experimental error. However, the pre-exponential factors and activation energies for reactions [3] and [5] are different. The difference is barely outside the quoted error limits but the trends confirm our former observations concerning P-secondary kinetic isotope effects in the photochlorination of ethyl chlorides (1 1, 12): both the A-factor and the activation energy decrease as a result of deuterium substitution in the adjacent group, i.e., in going from CH3CHC12 to CD3CHC12, the activation energy difference accounting for the inverse temperature dependence.…”

Primary kinetic isotope effect in the gas phase photochlorination of ethyl chloride-1-d1

“…Another check for self-consistency involves the use of eq. [8]. (k, + k2)/k3, k6/k7, k6/kl, and kz/k, at 300 K. Another independent value, k7/k3, may be included for comparison; the value for In [(k, + k2 + k6)/(k3 + k,)] becomes 1.16 which is still satisfactory.…”