Alkanes rate constants, 240, Table

A corresponding correlation is obtained for the rate constants of a,a -phenyl substituted alkanes 26 (R1 = C6H5, R2 = H, R3 = alkyl) (see Fig. 1 )41). It has, however, a different slope and a different axis intercept. When both correlations are extrapolated to ESp = 0, a difference of about 16 kcal/mol in AG is found. This value is not unexpected because in the decomposition of a,a -phenyl substituted ethanes (Table 5, no. 22—27) resonance stabilized secondary benzyl radicals are formed. From Fig. 1 therefore a resonance energy of about 8 kcal/mol for a secondary benzyl radical is deduced. This is of the expected order of magnitude54. ... 

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. 

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... 

Table 6.2 summarizes rate constants for some OH-al-kane reactions for recent recommendations for other alkanes, see Atkinson (1994, 1997a) and Atkinson et al. (1997a). 

TABLE 6.2 Rate Constants and Temperature Dependence" 11 for Reaction of OH Radicals with Some Alkanes... 

The first thing that stands out in Table 6.2 is that the OH-CH4 rate constant, 6.2 X 10 15 cm3 molecule 1 s-1, is much smaller than those for the higher alkanes, a factor of 40 below that for ethane. This relatively slow reaction between OH and CH4 is the reason that the focus is on non-methane hydrocarbons (NMHC) in terms of ozone control in urban areas. Thus, even at a typical peak OH concentration of 5 X 106 molecules cm 3, the calculated lifetime of CH4 at 298 K is 373 days, far too long to play a significant role on urban and even regional scales. Clearly, however, this reaction is important in the global troposphere (see Chapter 14.B.2b). 

TABLE 6.3 Rate Constants at Room Temperature for Reaction of NO, Radicals with Alkanes"... 

Table 6.4 summarizes the rate constants for the reactions of chlorine atoms with alkanes. Structure-reactivity relationships have again been developed for... 

TABLE 6.4 Rate Constants for the Reactions of Cl Atoms with Alkanes"... 

Reaction with OH is, however, reasonably rapid as might be expected and is of the same order of magnitude as the OH-alkane reactions. Table 6.22, for example, shows the room temperature rate constants for the reactions of OH at 298 K with some alkyl nitrates. With 2-butyl nitrate as an example, the lifetime with... 

As mentioned in Section 9.3, Jackson (141) has obtained estimates of the chain-transfer coefficient of the growing radical with polymer in the free-radical polymerization of ethylene, C,p, by choosing the value so as to fit the MWD. As the polymerization conditions for the polymers mentioned in Table 10.1 are not disclosed, it is necessary to choose typical conditions 220° C and 2000 atm will be selected. Under these conditions Ctp, the ratio of the rate constant for attack on polymer (per monomer unit) to that for propagation, in a homogeneous phase, was found to be about 4.0 x 10 3. This is in good agreement with the known transfer coefficients for the lower alkanes (160), when allowance is made for the differences in pressure and temperature (100). The relation between Ctp and k is ... 

From the activation energies and the preexponential factors, the rate constants at 873 K can be calculated. They are listed in Table II. They show that for the gas-phase homogeneous reactions, the reactions of O atoms and OH radicals with ethene are very rapid and somewhat faster than their reactions with ethane. This fact would limit the maximum yield of ethene. It is well known that if the reactions of an alkane and an alkene are both first order in the hydrocarbon, then the maximum yield for the alkene of about 35% would be obtained when the rate constants, kA and kn, for the two reactions have equal values ... 

The reactivity of a number of alkane complexes has been examined and this field has been reviewed through 1996 by Hall and Perutz. Flash photolysis of Cr(CO)6 in cyclohexane showed that solvation occurs within the first picosecond after photolysis, a fact that appears to rule out spin crossing as an important component in the dissociation of CO from Cr(CO)6. The stability of CpRe(CO)2(alkane) is particularly striking. Comparison of the rate constants for heptane solvated metal complexes with CO, Table 1, reveals that the rate constant for CpRe(CO)2(heptane) is five orders of magnitude slower than that of CpV(CO)3 (heptane). In fact, the stability of the CpRe(CO)2(alkane) complexes is so high that it has been possible to carry out low-temperature NMR on the cyclopentane complex generated by continuous photolysis of... 

Effect of Other Hydrocarbons. The complex role of C3H6 in this system has been analyzed in terms of its reactions with O, O3, and OH, and a similar analysis is applied to determining the role of other hydrocarbons in the smog chemistry. Relative rate constants for the reactions of a number of hydrocarbons with O, O3, and OH hydrocarbons are presented in Table I, together with the relative reactivities of these hydrocarbons based on NO-NO2 conversion rates observed in smog chambers. The relative OH rate constants correlate remarkably well with the reactivities for all the types of hydrocarbons listed in the table. By contrast, the O-atom and O3 rates correlate with the reactivities only for the olefinic compounds. For the aromatics, aldehydes, and alkanes in the table, the relative O-atom and O3 rate constants are negligibly small compared with the relative reactivities. The relative OH rate... 

Triplet sensitization did not lead to intramolecular adducts for higher alkanes but resulted in polymer formation (vide infra) a reaction scheme including excited complex formation in a primary step was proposed. The rate constant for excited complex formation, k, could be calculated as a function of the number of methylene units and diminished by lengthening of the chain (Table 8). The quantum yield of intersystem crossing was much lower (Table 8) than in the isolated chromophore, N-butylamalelmlde ( isc = 0.25), and substantially lower than the quantum yield of reaction, (Table 8). 

The stoichiometries and kinetics for the reaction of 02 - with halogenated hydrocarbons (alkanes, alkenes, and aromatics) are summarized in Table 7-1.18-24 [The normalized first-order rate constants, fcj / [S], were determined by the rotated ring-disk electrode method under pseudo-first-order conditions ([substrate] > [O2-)]. ... 

The difference in the reactivities of Cl - C4 alkanes is mainly caused by the difference in the Hex values. Estimations based on the data of Table 1 show that the difference in rate constants at 700 - 1000 K between methane and butanes over the same catalyst can exceed 10. The H-atom affinity in the case of efficient catalysts for methane activation should be the highest. As a result, if the 0-H bond strength is high enough to compensate the energy expenditure in the reaction (1), the process of active sites regeneration (reoxidation) becomes more impeded and the difference in the optimal reaction temperatures for different alkanes can reach 100 K or more. 

Acetic acid and other organic acids are thermodynamically unstable at sedimentary conditions and will eventually decarboxylate to CO2 and alkanes (24). Experimental studies of acetic acid decarboxylation show that the rate is extremely sensitive to temperature and the types of catalytic surfaces available (Table II 25-261. Extrapolated rate constants for acetic acid decarboxylation at 100 C differ by more than 14 orders of magnitude between experiments conducted in stainless steel and catalytically less active titanium (Table II 26). Inherent (uncatalyzed) decarboxylation rates are similar for acetic acid and acetate (26). However, in catalytic environments their rates of decarboxylation differ markedly (25-261. and therefore a pronounced pH effect on total decarboxylation rate is observed. 

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