Alkanes rate constants with

Where rate constants have been measured for the identical type of reaction of an atom or radical with a range of compounds it may be possible to correlate the values of the rate constants with structural features in the compounds concerned. The structure-reactivity relationship so developed may then be applied to predict values of the rate constants for the reactions with other compounds where experimental data are lacking. The reactions of H, O and OH with alkanes have been treated in this way. 

Explicit mechanisms attempt to include all nonmethane hydrocarbons believed present in the system with an explicit representation of their known chemical reactions. Atmospheric simulation experiments with controlled NMHC concentrations can be used to develop explicit mechanisms. Examples of these are Leone and Seinfeld (164), Hough (165) and Atkinson et al (169). Rate constants for homogeneous (gas-phase) reactions and photolytic processes are fairly well established for many NMHC. Most of the lower alkanes and alkenes have been extensively studied, and the reactions of the higher family members, although little studied, should be comparable to the lower members of the family. Terpenes and aromatic hydrocarbons, on the other hand, are still inadequately understood, in spite of considerable experimental effort. Parameterization of NMHC chemistry results when NMHC s known to be present in the atmosphere are not explicitly incorporated into the mechanism, but rather are assigned to augment the concentration of NMHC s of similar chemical nature which the... 

Figure 1 kci vs. koH- Second order gas phase rate constants for the reaction of Cl atoms vs. the corresponding OH radicals rate constants for the reactions with a. n-alkanes [11] b. n-alcohols [12] c. n-ethers [12] d. chloroethenes [13] and e. 1-chloroalkanes [14],... 

DeMore, W.B., Bayes, K.D.(1999) Rate constants for the reactions of hydroxyl radical with several alkanes, cycloalkanes, and dimethyl ether. J. Phys. Chem. A, 103, 2649-2654. 

Lloyd, A.C., Darnall, K.R., Winer, A.M., Pitts, Jr., J.N. (1976) Relative rate constants for reaction of the hydroxyl radical with a series of alkanes, alkenes, and aromatic hydrocarbons. J. Phys. Chem. 80, 189-794. 

The results of the calculation of the activation energies and the rate constants of peroxyl, alkoxyl, and alkyl radicals with alkanes and cycloalkanes are presented in Table 6.3-Table 6.5. 

Fig. 4 Photoreaction of benzophenone with the n-alkanes. Plot of the second-order rate constants in CC14 at 25°C against the number of CH2 groups. (Data from Winnik and Maharaj, 1979)...

FIGURE 1.24. Potential-dependent forward and backward rate constants of the ferrocene-ferrocenium couple attached to a gold electrode hy a long-chain alkane thiol assembled together with unsubstituted alkane thiols of similar length. Solid line use of Equations (1.37) to (1.39) with X, = 0.85 eV, ks — 1.25 s 1. Adapted from Figure 4A in reference 65, with permission from the American Association for the Advancement of Science. 

The value of = 1 X 10 s for the first-order rate constant for collapse of an ion pair between Me-4 and pentaflourobenzoate ion is larger than the second-order rate constant rcoo = 5x10 M s reported for the bimolecular addition of alkane carboxylates to Me-4. This second-order rate constant is limited by the rate constant for formation of an ion pair between Me-4 and a carboxylate ion. The larger barrier to encounter-limited reactions of carboxylate ions compared with the diffusion-limited reactions of anions such as azide ion, = 5 X 10 represents the barrier to desolvation of nucleophile that must precede formation of an ion pair between Me-4 and a carboxylate ion (Scheme 13). ... 

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

The values of the reaction rate, for polyhalogenated alkanes in Fe(II)/goe-thite suspensions are noted in Table 16.3 together with their pseudo-rate constants and half-lives. The reaction rates are affected by contact time, sorption density, and solution pH. Pecher et al. (2002) note that a contact time of 20 hours is necessary... 

Table 16.3 Names, abbreviations, pseudo-first-order rate constants, and half-lives of polyhalo-genated alkanes in Fe(II)/goethite suspension. Experimental conditions 25 m L" goethite, pH 7.2, tgq>24 h. Fe(II) = 1 mM. b Standard deviation, c number of replicates, d t =5 h. Reprinted with permission from Pecher K, Haderline SB, Schwarzenbach RP (2002) Reduction of polyhalo-genated methanes by surface-bound Fe(II) in aqueous suspensions of iron oxides. Environ Sci Technol 36 1734-1741. Copyright 2002 American Chemical Society...

TABLE 2-2 Rate Constants for Polyesterification (26.9° C) of Sebacoyl Chloride with a,co-Alkane Diols in Dioxane" b... 

Determination of OH relative rate constants for compounds that photolyze significantly in actinic radiation requires a nonphotolytic source of OH. Three such OH sources are H202-N02-C0 mixtures (Campbell et al., 1975, 1979 Audley et al., 1982), the thermal decomposition of H02N02 in the presence of NO (Barnes et al., 1982), and 03-hydrazine reactions (Tuazon et al., 1983) or 03 alkane reactions in the dark (Finlayson-Pitts et al., 1993). However, in these cases, the reactant must not react with 03, HOz, or H202, and care must be taken in interpreting the data since these systems have the potential of being rather complex. Indeed, the rate constants derived have not always agreed well with literature values. Until the... 

Beichert, P L. Wingen, J. Lee, R. Vogt, M. J. Ezell, M. Ragains, R. Neavyn, and B. J. Finlayson-Pitts, Rate Constants for the Reactions of Chlorine Atoms with Some Simple Alkanes at 298 K Measurement of a Self-Consistent Set Using both Absolute and Relative Rate Methods, J. Phys. Chem., 99, 13156-13162 (1995). 

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