Oxygen atoms, concentration determination reaction mechanisms

The most direct way to test the validity of a mechanism is to determine what intermediates are present during the reaction. If oxygen atoms were detected, we would know that Mechanism I is a reasonable description of NO2 decomposition. Likewise, the observation of NO3 molecules would suggest that Mechanism II is reasonable. In practice, the detection of intermediates is quite difficult because they are usually reactive enough to be consumed as rapidly as they are produced. As a result, the concentration of an intermediate in a reaction mixture is very low. Highly sensitive measuring techniques are required for the direct detection of chemical intermediates. 

The procedure adopted by Avramenko and Kolesnikova in an attempt to establish the primary reaction mechanisms consisted of the experimental determination for the various products formed of the shapes of the rate curves as a function of the initial concentration of the reactant olefins. These curves were then compared with the theoretical curves based on an assumed sequence of formation of products. Different shapes of rate curves were predicted for products formed with participation of a single oxygen atom (primary or secondary products), of two oxygen atoms ( quadratic products ), etc. 

The evidence for the proposed mechanism comes from kinetic, spectroscopic (multinuclear NMR), X-ray structure, and theoretical calculations. The kinetic rate law under optimum catalytic conditions is very complex. Under pseudo-first-order conditions, where the concentrations of both 9.35 and the hydroperoxide are much greater than that of allyl alcohol, the rate expression 9.5 is obeyed. In this expression the inhibitor alcohol is an inert alcohol such as isopropanol or f-butanol that is deliberately added to slow down the reaction for convenient rate measurements. The inert alcohol acts as an inhibitor, since it competes with both hydroperoxide and allyl alcohol for coordination to the Ti center. Note that expression 9.5 is consistent with the formation of an intermediate like 9.36, before the rate-determining oxygen atom transfer step. 

It is scarcely surprising that some deviations from the theory have been observed. One of the most favourable flames, hydrogen/oxygen, was examined by Padley and Sugden. They compared their experimentally-determined hydrogen atom concentrations in the reaction zone with the observed burning velocities. The excellent fit obtained emphasizes the dominance of this mechanism in these flames, rather than the general applicability of the theory. 

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