Chlorine rate constants

For chlorine enhanced conversion and initial rate vs. the second order chlorine rate constant (presence of TCE) (Figure 3b), the trends are clear conversion and initial rate increase with increasing kci values. For values of kci above 2e-ll cm molecules i, the enhanced conversion for all corresponding compoimds is greater than 80% per pass. 

Figure 3b Maximum enhanced conversion in a single pass and the enhanced initial rate (TCE added) vs. literature second order chlorine rate constants. (Same symbols as (2a)).

The relative basicities of aromatic hydrocarbons, as represented by the equilibrium constants for their protonation in mixtures of hydrogen fluoride and boron trifluoride, have been measured. The effects of substituents upon these basicities resemble their effects upon the rates of electrophilic substitutions a linear relationship exists between the logarithms of the relative basicities and the logarithms of the relative rate constants for various substitutions, such as chlorination and... 

Ha.logen Compounds. Fluorine is unreactive toward ozone at ordinary temperatures. Chlorine is oxidized to Cl20 and Cl20y, bromine to Br Og, and iodine to I2O2 and I4O2. Oxidation of haUde ions by ozone increases with the atomic number of haUde. Fluoride is unreactive chloride reacts slowly, ultimately forming chlorate and bromide is readily oxidized to hypobromite (38). Oxidation of iodide is extremely rapid, initially yielding hypoiodite the estimated rate constant is 2 x 10 (39). HypohaUte ions are oxidized to haUtes hypobromite reacts faster than hypochlorite (40). 

Dichlorine monoxide, which exists in very low equiUbrium concentrations in dilute HOCl solutions, is nevertheless a kineticaHy significant reactant. Eor example, although tetracyanonickelate(II) can be oxidized by chlorine in aqueous solution to /n j -Ni(III) (CN)4(H20)2 , the second-order rate constant at 25°C for oxidation with CI2O is 40 times greater than for CI2 and 2.6 x 10 greater than for HOCl (31). 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

The growth of long chains ( > 10 ) in the perfectly mixed 1 1 crystals of ethylene with chlorine and bromine at 20-70 K was studied in detail by Wight et al. [1993]. Active radicals were generated by pulse photolysis of CI2 or Br2. The rate constant was found to be /Cc = 8-12s below Tc = 45 K. The chain grows according to the well known radical mechanism including the reactions... 

Rate constants have been measured for the reactions of boron compounds with a series of bromomethanes and bromofluoromethanes. Previously it was shown that the reactivity of the chlorine in chlorofluoromethane is substantially reduced by increasing fluorine substitution. The corresponding decrease in the reactivity of bromolluoromethane u as not observed [ifS]. 

A single chlorine atom can bring about the decomposition of tens of thousands of ozone molecules. Bromine atoms can substitute for chlorine indeed the rate constant for the Br-catalyzed reaction is larger than that tor the reaction just cited. 

It has been proposed that aromatic solvents, carbon disulfide, and sulfur dioxide form a complex with atomic chlorine and that this substantially modifies both its overall reactivity and the specificity of its reactions.126 For example, in reactions of Cl with aliphatic hydrocarbons, there is a dramatic increase in Ihe specificity for abstraction of tertiary or secondary over primary hydrogens in benzene as opposed to aliphatic solvents. At the same time, the overall rate constant for abstraction is reduced by up to two orders of magnitude in the aromatic solvent.1"6 The exact nature of the complex responsible for this effect, whether a ji-coinplex (24) or a chlorocyclohexadienyl radical (25), is not yet resolved.126- 22... 

The ratio of rate constants for attack of a chlorine atom on the primary (kp) and tertiary (k,) hydrogen atoms is 1 4.2. Calculate the expected ratio of I II. 

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... 

Table 3. Relative rate constants (Kfgi) and half lifes (tj/2> for a mixture of aromatic bromine compounds calculated from the concentrations of educts after reaction with 3.5 g fly ash containing chlorine. 

For a review that lists many rate constants for abstraction of hydrogen at various positions of many molecules, see Hendry, D.G. Mill, T. Piszkiewicz, L. Howard, J.A. Eigenmann, H.K. J. Phys. Chem. Ref. Data, 1974, 3, 937 Roberts, B.P. Steel, A.J. Tetrahedron Lett., 1993, 34, 5167. See Tanko, J.M. Blackett, J.F. J. Chem. Soc., Perkin Trans. 2,1996, 1775 for the absolute rate constants for abstraction of chlorine by alkyl radicals. 

Jeffers PM, Ward LM, Woytowitch LM, et al. 1989. Homogenous hydrolysis rate constants for selected chlorinated methanes, ethanes, ethenes and propanes. Environmental Science and Technology 23 965-969. 

The results of chain transfer studies with different polymer radicals are compared in Table XIV. Chain transfer constants with hydrocarbon solvents are consistently a little greater for methyl methacrylate radicals than for styrene radicals. The methyl methacrylate chain radical is far less effective in the removal of chlorine from chlorinated solvents, however. Vinyl acetate chains are much more susceptible to chain transfer than are either of the other two polymer radicals. As will appear later, the propagation constants kp for styrene, methyl methacrylate, and vinyl acetate are in the approximate ratio 1 2 20. It follows from the transfer constants with toluene, that the rate constants ktr,s for the removal of benzylic hydrogen by the respective chain radicals are in the ratio 1 3.5 6000. Chain transfer studies offer a convenient means for comparing radical reactivities, provided the absolute propagation constants also are known. 

For polychlorinated biphenyls (PCBs), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 d for 3-chlorobiphenyl to 34 d for 236-25 pentachlorobiphenyl (Anderson and Hites 1996). It was estimated that loss by hydroxy-lation in the atmosphere was a primary process for the removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a hydroxyl radical at the 1-position followed by transannular dioxygenation at the 2- and 5-positions followed by ring fission (Brubaker and Hites 1998). Reactions of hydroxyl radicals with polychlorinated dibenzo[l,4]dioxins and dibenzofurans also play an important role for their removal from the atmosphere (Brubaker and Hites 1997). The gas phase and the particulate phase are in equilibrium, and the results show that gas-phase reactions with hydroxyl radicals are important for the... 

No systematic studies of a number of compoimds have yet appeared to discover correlations suggestive of mechanism. This paper presents the fractional conversions and reaction rates measured under reference conditions (50 mg contaminants/m ) in air at 7% relative humidity (1000 mg/m H2O), for 18 compounds including representatives of the important contaminant classes of alcohols, ethers, alkanes, chloroethenes, chloroalkanes, and aromatics. Plots of these conversions and rates vs. hydroxyl radical and chlorine radical rate constants, vs. the reactant coverage (dark conditions), and vs. the product of rate constant times coverage are constructed to discern which of the proposed mechanistic suggestions appear dominant. 

The conversion and initial rate in the presence and absence of TCE versus the product of the second order rate constant and the dark adsorption appear in Figure 4a,b. Figure 4a shows considerable scatter in the data, reveahng only broad, general trends between conversion or rate and hydroxyl second order rate constant. However, the plots of enhanced conversion and initial rate vs. the corresponding chlorine second order rate constant multiplied by the dark adsorption data are smoother (Figure 4b). 

Reaction rate constants are postulated as shown in Table II for degradation in water (biolysis and photolysis), in bottom sediments (probably biolysis), and for permanent burial of sediment. The values were selected from a perusal of the literature and must be regarded as speculative. A factor of 20 reduction in reaction rate constant is assumed for addition of each chlorine. 

Edney, E.O., Kleindienst, T.E., Corse, E.W. (1986) Room temperature rate constants for the reaction of OH with selected chlorinated and oxygenated hydrocarbons. Int J. Chem. Kinet. 18, 1355-1371. 

These reactions of diclorine with olefins are endothermic, AH varies from 63 kJ mol-1 (Ph2C=CH2) to 191 kJ mol-1 (MeC=CMe). The rate constants of chlorine reactions with olefins calculated by the IPM method are presented in Table 3.16. 

The ability of MPO to catalyze the nitration of tyrosine and tyrosyl residues in proteins has been shown in several studies [241-243]. However, nitrite is a relatively poor nitrating agent, as evident from kinetic studies. Burner et al. [244] measured the rate constants for Reactions (24) and (25) (Table 22.2) and found out that although the oxidation of nitrite by Compound I (Reaction (24)) is a relatively rapid process at physiological pH, the oxidation by Compound II is too slow. Nitrite is a poor substrate for MPO, at the same time, is an efficient inhibitor of its chlorination activity by reducing MPO to inactive Complex II [245]. However, the efficiency of MPO-catalyzing nitration sharply increases in the presence of free tyrosine. It has been suggested [245] that in this case the relatively slow Reaction (26) (k26 = 3.2 x 105 1 mol-1 s 1 [246]) is replaced by rapid reactions of Compounds I and II with tyrosine, which accompanied by the rapid recombination of tyrosyl and N02 radicals with a k2i equal to 3 x 1091 mol-1 s-1 [246]. 

As for values in solvents with intermediate D, (ca. 10-15) only a very few of the many claims to have obtained kp+ in such solvents can be taken at face value and their critical evaluation is beyond the scope of this article. Suffice it to say that the most credible values appear to lie in the range 104 to 106 1 mol"1 s"1 for chlorinated solvents of D ca. 10 and ca. 300 K. It is therefore much too early yet to discuss whether these kp values conform to any of the theoretical relations between the rate-constant of an ion-molecule reaction and some function of D such as that of Laidler and Landskroener [12] or Hiromi [13] (see also [14]). 

The chlorine atom adds in the gas phase to propadiene (la) with a rate constant that is close to the gas-kinetic limit. According to the data from laser flash photolysis experiments, this step furnishes exclusively the 2-chloroallyl radical (2a) [16, 36], A computational analysis of this reaction indicates that the chlorine atom encounters no detectable energy barrier as it adds either to Ca or to Cp in diene la to furnish chlorinated radical 2a or 3a. A comparison between experimental and computed heats of formation points to a significant thermochemical preference for 2-chloroal-lyl radical formation in this reaction (Scheme 11.2). Due to the exothermicity of both addition steps, intermediates 2a and 3a are formed with considerable excess energy, thus allowing isomerizations of the primary adducts to follow. 

Many of the quantitative rate data on radical reactions which are to be found in the scientific literature have been obtained by comparison of reaction rates rather than by direct measurement of absolute rate constants (Ingold, 1973). For example, it is a straightforward matter to compare the rate of chlorine abstraction from CC14 by phenyl radicals with the rate of hydrogen abstraction from cyclohexane by the same species, simply by comparing the PhCl/PhH product ratio from a suitable competition experiment (Bridger and Russell, 1963). In contrast, direct measurements of the absolute rates of these reactions have yet to be carried out (although indirect estimates are available). 

There are extensive data for the acid-catalyzed protiodesilylation of XCgELrSiMes in methanol-aqueous perchloric acid or acetic acid-aqueous sulphuric acid at 50°C225. Correlation analysis of the partial rate factors (relative rate constants) by means of the Yukawa-Tsuno equation (Section n.B) finds p = —5.3 and r+ = 0.65. These values are consistent with a relatively low demand for stabilization of the transition state by electron delocalization, i.e. the transition state is early along the reaction coordinate, p-NO2 is highly deactivating with / = 14 x 10 but 0-NO2 is even more deactivating, with / = 6.8 x 10-5. This contrasts with the deactivation order discussed above for nitration and chlorination (Table 6), and may be explained in terms of the early transition state, well removed from the Wheland intermediate. 

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