Cl atom reaction with

Chlorine atoms react with aromatic hydrocarbons, but only at a significant rate with those having saturated side chains from which the chlorine atom can abstract a hydrogen or unsaturated side chains to which it can add. For example, the rate constant for the Cl atom reaction with benzene is 1.3 X 10"15 enr3 molecule-1 s-1 (Shi and Bernhard, 1997). On the other hand, the rate constants for the reactions with toluene and p-xylene are 0.59 X 10-10 and 1.5 X 10-l() enr3 molecule"1 s"1, respectively (Shi and Bernhard, 1997), and that for reaction with p-cymene is 2.1 X 10"10 cm3 molecule"1 s-1 (Finlayson-Pitts et al., 1999). Hence... 

Shi, J. C., and M. J. Bernhard, Kinetic Studies of Cl-Atom Reactions with Selected Aromatic Compounds Using the Photochemical Reaction-FTIR Spectroscopy Technique, hit, J. Chem. Kinet, 29, 349-358 (1997). 

TABLE 2. Absolute rate constants (in 10 13 cm3molecule, s 1) for Cl atom reactions with bromomethanes at the temperatures 273,333, 303, and 363 K. 

Two important areas of collision dynamics can be addressed via recoil methods and should receive increased attention in the future. The first is collisional dissociation of translationally hot molecules. Indirect evidence for the importance of this process has been obtained in hot atom reaction studies of the recoil Cl atom reaction with CH4 and H2. A consistent kinetic theory interpretation for the production of the replacement reaction product, CH3CI, in the first case indicated that a... 

Taatjes, C. A. (1999). Time-resolved infrared absorption measurements of product formation in Cl atom reactions with alkenes and alkynes. Int. Rev. Phys. Chem. 18,419. 

The OH, NO3, and Cl-atom reactions with DME all proceed via abstraction, and thus the subsequent chemistry will be indentical for all three cases, e.g.,... 

In 1 atm. air at 295 K, the reaction of R0CH(0 )CH3 with O2 to generate the acetate is about twice as rapid as the decomposition to produce the formate. About 20% of the Cl-atom reaction with HFE-7500 appears to occur at the CH3 group, which leads to additional formate production, presumably via formation and subsequent oxidation of the aldehydic species, R0CH2CH=0. 

A similar mechanism, leading to CH2CI and CO, has been proposed for the Cl-atom reaction with ketene (Wallington et al., 1996a). 

Now that we have a model, we must check its consistency with various experiments. Sometimes such inconsistencies result in the complete rejection of a model. More often, they indicate that we need to refine the model. In the present case, the results of careful experiments show that the collision model of reactions is not complete, because the experimental rate constant is normally smaller than predicted by collision theory. We can improve the model by realizing that the relative direction in which the molecules are moving when they collide also might matter. That is, they need to be oriented a certain way relative to each other. For example, the results of experiments of the kind described in Box 13.2 have shown that, in the gas-phase reaction of chlorine atoms with HI molecules, HI + Cl — HC1 I, the Cl atom reacts with the HI molecule only if it approaches from a favorable direction (Fig. 13.28). A dependence on direction is called the steric requirement of the reaction. It is normally taken into account by introducing an empirical factor, P, called the steric factor, and changing Eq. 17 to... 

Because many of the alternates and replacements for CFCs have an abstractable hydrogen atom, reaction with OH in the troposphere dominates their loss. Table 13.4 gives some rate constants for the reaction of OH with these compounds the kinetics summary of De-More et al. (1997) should be consulted for other compounds. It is seen that the rate constants at 298 K are typically in the range of 10-l3-10-ls cm3 molecule-1 s-1, depending on the degree of halogen substitution and the nature of the halogen, e.g., F, Cl, or Br. Typical A factors are of the order of 1 X 10 12 cm3 molecule-1 s-1 per H atom (DeMore, 1996). 

A different experimental approach to the study of chlorine atom reactions with olefins will be mentioned briefly. Wijnen(l06) has studied the photolysis of phosgene as a source of chlorine atoms in the presence of ethylene, and Guercione and Wijnen (49) have carried out similar experiments with propylene. The features of these processes are quite different from those encountered in photochlorination in the presence of molecular chlorine, since the chain propagating reaction (3) cannot occur. Although in the photolysis of phosgene Cl and COC1 are initially formed, it appears that all COC1 radicals further decompose into CO and Cl. 

Compared to toluene, benzene is much less reactive towards Cl-atoms. There are only six studies of this reaction, and these are inconsistent with each other [16,17, 19,24,27,42]. In all studies except one, a relative rate technique was used to determine reaction rate constants at room temperature. Atkinson and Aschmann [16] irradiated Cl2-benzene-n-butane-air mixtures and from the relative loss rates of benzene and n-butane obtained the value of (1.5 0.9) x 10 cm /(molecules) for the Cl-atom reaction rate of benzene. Wallington et al. [17] and Noziere et al. 

We performed extensive studies to determine the Cl-atom rate constant of benzene. n-Butane and ethane were among first molecules that were used as references to measure the relative rate constant of Cl-atom reaction of benzene. By the end of these experiments, the concentration of reference molecules reduced close to the detection limits whereas no decay of benzene outside the uncertainty limits was observed. Although the relative rate technique is particularly powerful in the measurement of reactions with comparable rate constants, for reactions with very different rate constants, this method is less accurate. Since it has become clear that the Cl-atom rate constant of benzene is slow, chloromethane, dichloromethane, and trichloromethane (chloroform) were used as references. These sets of experiments consisted of 20 individual experiments in which the concentration of hydrocarbons ranged from 10 to 15 mTorr and the concentration of CI2 varied from 10 to 100 mTorr. The relative rate constants for chloromethane and dichloromethane have been previously measured by Niki et al. [65]. Specifically, they were combined as a check on the experimental procedures. No difference in the values of the rate... 

Therefore, in analogy with methane system, in the course of experiments as Cl-atom reacts with chloro or dichloro-methane, there is a potential for formation of HO-radicals. Hence in the initial stages of the reaction at lower rates of methane conversion we expect that HO will primarily attack benzene. As the reaction proceeds, oxidation products of CH4 and benzene increase in concentration and compete for the HO radicals. Thus, we expect to observe a curvature in relative rate plot of benzene at longer irradiation times, as shown in Figure 13.6. In FTIR studies, for the case of experiments in nitrogen diluent, curvature was observed but substantially lower concentrations of benzene were consumed. It is... 

We used two reference molecules, ethane and n-butane. Equation (3) can be used to determine the relative rate of Cl -I- toluene with respect to the reference molecules ethane and n-butane. In this case, "HC" is toluene and RH is either ethane or n-butane. Figures 13.8 and 13.9 are plots of these experiments. All these plots are linear with > 0.99 and with zero intercepts within 2a. Depicted in Table 13.4 are the results of these experiments compared with the literature values. This work is in excellent agreement with that of Wallington et al [17] Since our results in air and N2 are identical within experimental uncertainties, these values are combined together. The ratio of (0.977 0.026) and (0.267 0.01) are obtained using ethane and n-butane, respectively. Using the absolute values of 5.7 X 10 and 1.94 x 10 cm /(molecules), we obtain the values of (5.57 0.15) x 10 and (5.18 0.19) x 10 cm /(molecules) for the Cl-atom reactions of toluene using ethane and n-butane, respectively. It is to be noted that... 

Clyne and Clark carried out some of the earliest work on reactions that produce electronically excited NCI. They observed emission from NC1(6) when Cl atoms reacted with CIN3. N3 radicals were detected and they proposed that production of NCI (6) was linked to the reaction of Cl with N3. Overall, their results were consistent with the sequence... 

Figure 7.30 The experimental (shaded areas) and calculated decay rates via tunneling for normal and deuterated ethyl chloride. The reaction coordinate for HCl or DCl loss is primarily the transfer of the H (D)-atom from the terminal carbon to the Cl atom. Taken with permission from Booze et al. (1991).

The reactions of endothermic compounds with atoms are useful sources of energy rich species and free radicals. Ozone and chlorine dioxide are the compounds which have been most fully studied, but atom reactions with the difficult compounds nitrogen trichloride and the azides of chlorine and bromine are also interesting. The reactions of H( S), 0( P), N( S), F, Cl, Br and IfPs/a) have all received study by the discharge-flow method, and these cases are chosen to exemplify the features of this class of reactions. 

A very satisfactory approach for studies of Cl atom reactions uang resonance absorptimi is based on a O-atom resonance lamp onffloying argon Ar P2.0 metastables. This utilizes the reactive collision of Ar P2,o atoms with Q2, which occurs with a hi cross ection (k = 7.1 x 10, and gives ArQ excimers (see Chapter 4 of this volume) vidiicli predissodate to Cl (4r) excited atoms - ... 

Vibrationally exciting hydrogen cyanide accelerates its reactions with H, O, or Cl substantially, qualitatively the same result as with water. The energetics for the hydrogen atom abstraction reaction are very similar to each other and to those for water, as Fig. 4 shows. We use two diagnostics to test the spectator picture in reactions of HCN with the different atoms. One is the reactivity of different states as seen in the action spectrum compared to the photoacoustic spectrum. The other is the product state distributions. We find that the extent to which the nonreacting bond is a spectator depends on the identity of the attacking atom. Reactions with Cl atoms are much further from the simple spectator picture than those with H or O atoms. 

The distribution of the CN product among its rovibrational states also points to the unique behaviour of Cl atoms in reactions with vibrationally excited HCN. Figure 6 shows the populations of the vibrational states from reaction of HCN((X)4) with O, H, and Cl atoms and HCN(302) with Cl atoms [9]. Clearly, the reaction with Cl atoms creates CN with significantly more vibrational energy than those with H or O atoms. The reaction with Cl atoms forms more than 40% of the CN radicals in vibrationally excited states, but those with H or O produce less than 25% in vibrationally excited states, differences that are well outside the uncertainties in the measurement. Similarly, the 560 cm of rotational energy in the CN product of the Cl-atom reactions exceeds the 140 to 315 cm" from the O- or H-atom reactions. These results point to the qualitative differences between the reactions with Cl atoms and those with H or O atoms. 

FIGURE 20.17 A simple example of how steric factors influence the probability of reactions occurring, (a) A sodirnn atom approaches an HCl molecule, but the orientations are not conducive for a reaction to occur, so after colliding they simply go on their way. (b) Here the orientation of the HCl is more conducive to reaction, so upon collision the Cl atom bonds with the Na atom and new product species are formed. 

Also, in Si-methylated 1,3,5-trisilacyclohexanes containing a carbosilane side chain on a skeletal Si atom or on a skeletal C atom, reaction with ICl forming Si—Cl bonds occurred ... 

Note that Wallington et al. (2004) conducted similar Cl-atom kinetic and mechanistic studies on the related species, CF3CHFCF2OCF3. Although most of the reaction appears to occur at the CHF group in both molecules, the Cl-atom reaction is considerably ( 50%) faster with CF3CHFCF2OCF2H than with CF3CHFCF2OCF3. 

The enhancement in reactivity of a given aliphatic ether with OH, Cl, and NO3 compared to its parent alkane is readily apparent from the data presented in table III-H-1. Enhancements are on the order of a factor of 4 for the OH reactions, 10-70 for the NO3 reactions (note that only limited data are available for these reactions), and 1.5 for the Cl-atom reactions. Whether the enhancement in reactivity is restricted to the C—H bonds located near the ether linkage, or whether the enhancement extends further down the molecule, is difficult to ascertain given the data currently available and is open to interpretation. This point will be discussed further in section III-H-3. 

OH. This plot, an update to the one provided in Calvert et al. (2008), gives logio[fc(Cl + alkane)] = 0.521 x logjoWOH + alkane)] —3.67, with = 0.85. Thus, while the reactivities of the ethers/alcohols with Cl-atoms and with OH are clearly correlated, the correlation is significantly different to the one observed for reaction of the alkanes with the same two oxidants. As examples, nonane, 2-methyl- 1-propanol and di-isopropyl ether (DIPE) react with OH at nearly identical rates [A (OH - - nonane) = 9.8 x 10 ... 

A number of studies have been conducted on the reaction of alkyl nitrates with OH radicals and Cl-atoms. No data are available on the reaction with NO3 radicals and ozone, but these reactions are expected to be slow and of negligible atmospheric significance. No mechanistic studies of the OH- or Cl-atom reactions are available. 

The reaction is initiated when some CI2 molecules absorb sufficient energy to dissociate into Cl atoms (represented above as Cl ). Cl atoms collide with CH4 molecules to produce methylradicals (H3C ), which combine with CI2 molecules to form CH3CI molecules. When any or all of the last three reactions proceed to the extent of consuming the free radicals present, the reaction stops. The initiation step occurs much less frequently than the propagation steps. For example, the dissociation of a single CI2 molecule probably produces thousands of chlorination reactions. 

Rettner C T 1994 Reaction of an H-atom beam with Cl/Au(111)—dynamics of concurrent Eley-Rideal and Langmuir-Hinshelwood mechanisms J. Chem. Phys. 101 1529... 

Figure B2.3.17. REMPI spectra of the HCl and DCl products from the reaction of Cl atoms with (CH3)3CD [63], The mass 36 and 2 ion signals are plotted as a fiinction of the 2-photon wavenumber. Assigmnents of the...

The reaction of an atom with a diatomic molecule is the prototype of a chemical reaction. As the dynamics of a number of atom-diatom reactions are being understood in detail, attention is now being turned to the study of the dynamics of reactions involving larger molecules. The reaction of Cl atoms with small aliphatic hydrocarbons is an example of the type of polyatomic reactions which are now being studied [M, 72, 73]. 

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