Hydrogen termination activation energy

Equations 4.2.31 and 4.2.32 represent radical recombination reactions that might be considered as alternative termination reactions. Since these reactions and reaction 4.2.26 are radical recombination reactions, they will all have very low activation energies. The relative rates of these processes will then be governed by the collision frequencies and thus the concentrations of the reacting species. The relative concentrations of hydrogen and bromine atoms can be determined from the steady-state form of equation 4.2.17. 

The traditional chain oxidation with chain propagation via the reaction RO/ + RH occurs at a sufficiently elevated temperature when chain propagation is more rapid than chain termination (see earlier discussion). The main molecular product of this reaction is hydroperoxide. When tertiary peroxyl radicals react more rapidly in the reaction R02 + R02 with formation of alkoxyl radicals than in the reaction R02 + RH, the mechanism of oxidation changes. Alkoxyl radicals are very reactive. They react with parent hydrocarbon and alcohols formed as primary products of hydrocarbon chain oxidation. As we see, alkoxyl radicals decompose with production of carbonyl compounds. The activation energy of their decomposition is higher than the reaction with hydrocarbons (see earlier discussion). As a result, heating of the system leads to conditions when the alkoxyl radical decomposition occurs more rapidly than the abstraction of the hydrogen atom from the hydrocarbon. The new chain mechanism of the hydrocarbon oxidation occurs under such conditions, with chain... 

This reaction is certainly exothermic and by analogy with similar hydrogen abstraction reactions might be expected to have an activation energy of anywhere from zero to 8 kcal. If it should be proved that this is the terminating interaction or a possible terminating interaction of primary and secondary alkylperoxy radicals, then we must begin to think anew about the entire mechanism for the over-all chain decomposition and oxidation of hydrocarbons. I believe it would also require us to review seriously our interpretation of the spin resonance work on alkylperoxy radicals at low temperatures. 

An alternative termination reaction to hydrogenation of the growing alkyl chain is p-CH cleavage. Computations by Cheng et al. (31) indicated low activation energy barriers for the equilibration between adsorbed alkyl species and adsorbed alkene reported barriers are approximately 50 kj/mol. On the Co(OOOl) surface, the alkene is slightly more stable than the adsorbed alkyl (by about 10 kJ/mol). If desorption of the alkene into the gas phase is included, an overall barrier of 70 kJ/mol results for... 

The activation energy of the concerted mechanism is only 3-7 kcalmoD lower than that for the first step of the stepwise mechanism. However, the geometries of the two transition states are dissimilar, one bond being formed in the stepwise structure while two bonds are formed in the concerted case, and this leads to different KIEs. The secondary KIEs calculated for concerted TS (terminal hydrogens) are always inverse (and vary from 0.93 to 0.99, depending on the position and the level of theory), in agreement with expectations for sp to sp hybridization changes. The most reliable values are 3% and... 

Tracks et provided a kinetic explanation of hydrogen termination based on the calculation of activation energies involved in breaking up the various silicon bonds in the following reactions ... 

Reaction (2.13a) has the smallest activation energy, meaning that the Si-OH bond is highly unstable in HF solutions and if present will be quickly replaced by other bonds. Reaction (2.13b), which results in hydrogen termination, is kinetically more favorable than reactions (2.13c) and (2.13d). This means that fluoride termination tends to be replaced by hydrogen termination, which, whether in the form of mono-, di-, or trihydride, is kinetically more stable. Similar to HF, the attack by F and HFz leading to hydrogen termination has also been proposed. ... 

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