Alkane activation bond strengths

The hydroxyl radical will be the predominant entity which attacks the alkane to regenerate an alkyl radical (Reaction 10) under conditions where isomerization and decomposition are the usual fate of alkylperoxy radicals. The activation energy for attack on an alkane molecule by OH, although difficult to determine accurately (30), is low (I, 3) (1-2 kcal. per mole). This has an important consequence. The reaction will be unselective, being insensitive to C—H bond strength. Each and every alkyl radical derived from the alkane skeleton will therefore be formed. To describe the chain-propagation steps under conditions where isomerization is a frequent fate of alkylperoxy radicals it is necessary, then, to consider each and every alkylperoxy radical derived from the alkane and not just the tertiary radicals. 

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. 

Generally C-C bonds in alkanes are much weaker than the corre onding C-H bonds, as own in Table 1. Clearly then active sites do not distinguish between these bonds on a bond strength basis only and in view of the structure of these alkanes it is reasonable to suggest that steric factors must play a role here, since C-C bonds are generally more difficult to accommodate within an active site than C-H bonds. 

Hydrocarbons (HCs) from C2 to CIO are commonly used to test activity. The efficiency of the HC in the SCR of NOx in competition with the combustion reaction with oxygen increases with increasing molecular weight. This is probably due to the parallel increase in the heat of adsorption and decrease in the C-H bond strength.132 Alkanes, alkenes and oxygenated HCs have been studied in this reaction and the activity in the NOx elimination frequently follows this order, with the lower light-off temperatures being displayed for the latter type of compounds. Independent of the nature of... 

A second type of polar effect on activation energies is revealed in comparisons of the reactions of CF3 and CH3 radicals with non-polar molecules. In general, it is found that Eqhs Eqf3 2—3 kcal mole" for attack on non-polar alkanes or silanes. In terms of the Polanyi relations for CH3 and CF3, the difference in bond strengths D(CF3—H) — D(CH3-H) = 2 kcal mole" would account for about 1 kcal mole" difference in the activation energies. The additional 1—2 kcal mole" lowering in as compared with Fchs for tbe same substrate is... 

Activities of metals for alkane exchange increase with metal-metal bond strength (i.e. with sublimation energy), presumably because of stronger bonding of the radicals formed when the alkane dissociates, and their consequent higher coverages. ... 

These early discoveries form the basis for the growth of this field over the last 15 years. Advances have been made in understanding the fundamental basis for C-H activation, including both intermediates involved and the bond strengths of the metal-carbon bonds that are formed. A detailed understanding of both oxidative addition and electrophilic activation of C-H bonds is now in hand. New discoveries of alkane and arene functionalization allow the formation of useful derivatives in synthetically useful quantities. Many new examples of the use of transition metal C-H activation in organic transformations are being reported. 

Ibers, DiCosimo and Whitesides showed that the bis-neopentyl platinum complex of equation 16 gave facile cyclometalation. They were surprised that the reverse reaction, which would of course be an alkane activation with neopentane as the substrate, did not take place because, thermodynamically, the bonds formed should compensate for the bonds lost. Presumably, it is the unfavorable entropic term which is the major factor in preventing the reverse reaction, but the substantial steric bulk of the neopentyl group may tend to reduce the Pt—neopentyl bond strength in a bis-neopentyl complex. 

This is in line with electron withdrawal from the silanol O by EFai, leading to a decreased O-H bond strength and enhanced Br0nsted acidity [36]. An overview of potential EFai species as probed by different techniques is available [37]. However, superacidic sites in zeolites, viz. USY, compared to real superacids at low temperature are unable to activate a-bonds in alkanes. They have strengths only comparable to that of sulfuric acid [38, 39]. 

While all of the substrates discussed above are not shown in Fig. 2, the same analysis can be performed with all of them (alkynes, substituted methanes). One caveat that we encountered was that many of these substituted derivatives proved to be very stable. Loss of alkane from the n-pentyl hydride complex has a half-hfe of about an hour at 25°C. Methane loss from 3 has a half-life of about 5 h. Loss of benzene from 2, however, is extremely slow (months), and therefore, the rate of benzene reductive elimination at 25°C was determined by extrapolation from the rate at higher temperatures. The Eyring plot of hi( /T) vs. 1/T gave activation parameters for reductive elimination of benzene A// = 37.8 (1.1) kcal/mol and = 23 (3) e.u., which can be used to calculate the rate at other temperatures. As mentioned above, the substituted derivatives are much more stable. Reductive elimination of the alkynyl hydrides was examined at lOO C, as was the elimination of many of the substituted methyl derivatives. In these cases, the rate of benzene elimination was calculated from the Eyring parameters at the same temperature as that where the rate of reductive elimination was measured, so that the barriers could be directly compared as in Fig. 2. The determinatimi of AG° for all substrates allows Eq. 7 to be used to determine relative metal-carbon bond strengths for these compounds. Table 1 summarizes these data, giving A AG, AG°, and Drei(Rh-C) for all substrates. 

The kinetics of ligand substitution on Cr(CO)5(heptane) was studied by Yang et al. and the rate constants vary by 20 for different entering groups. As noted above, the AH for CO and H2O substitution on Cr(CO)5(CgH,2) is smaller than the Cr—(CjH,2> bond strength. These observations seem most consistent with associative activation. On the other hand, van Eldik and co-workers have done several studies in mixed alkane/amine solvents and interpret the observed values of AV in terms of dissociative activation. 

The selectivity toward product molecules with the same number of carbon atoms as in the reactant varies significantly for the different transition metals. The stronger the metal-carbon surface bond strength, the lower the activation energy for dissociation and the lower the selectivity becomes. For this reason platinum, which represents the transition metal with the lowest reactivity, is the preferred metal for the catalytic isomerization or aromatization of alkanes. The Group I-B metals, Cu, Ag, and Au, form weaker metal-carbon bonds, but have a reactivity that is too low. In the case of Ag and Au the interaction energies become so weak that neither H2 nor C-H bonds dissociate. On Cu such bond dissociation is possible, but with a considerable activation energy. 

A similar conclusion applies to a Mg-V-O catalyst in which Mg3(V04)2 is the active component. The relative rates of reaction for different alkanes on this catalyst follow the order ethane < propane < butane 2-methylpropane < cyclohexane (Table I) [12-14]. This order parallels the order of the strength of C-H bonds present in the molecule, which is primary C-H > secondary C-H > tertiary C-H. Ethane, which contains only primary C-H bonds, reacts the slowest, whereas propane, butane, and cyclohexane react faster with rates related to the number of secondary carbon atoms in the molecule, and 2-methylpropane, with only one tertiary carbon and the rest primary carbons, reacts faster than propane which contains only one secondary carbon. Similar to a Mg-V-O catalyst, the relative rates of oxidation of light alkanes on a Mg2V207 catalyst follow the same order (Table I). 

---