Self-decelerating reaction

The experimental curves in Fig. 2.9 are compared with the curves calculated according to the second-order kinetic equation for two different initial temperatures. The divergence between the pairs of curves at the final stages of the process reaches 15 %, while the experimental is less than 3 %. This proves that the equation is inadequate. On the other hand, a kinetic equation that takes into account the effect of self-deceleration fits the experimental data along the whole curve. Therefore, we can predict that the reaction in the system under discussion will be incomplete. 

Hydrogen uptake/time curves were of the expected form. Racemic reaction in the absence of alkaloid modifier was a slow self-poisoning reaction for which the initial rate was the maximum rate. Alkaloid-modified reactions were rapid and gave S-shaped uptake/time curves, i.e. they showed an initial acceleration, a period of reaction at the maximum rate, and deceleration as reactant was exhausted. Over Pt/graphite (Table 1) 0.17 mmol of... 

Since the rate law for self-decelerating processes is unknown, extrapolation of conversion with time is impossible. In principle, methods like IR spectroscopy would be suitable but they often suffer from lack of accuracy, notably when small changes have to be detected near the end of the reaction. 

As the initial temperature increases, the initial self-heat rate increases following the zero-order line. One very interesting phenomenon occurs when the maximum self-heat coincides with the initial self-heat rate. Above this temperature, the reaction decelerates, even though the temperature itself continues to increase. This temperature is designated as T and can be evaluated when Tm = T0 = T and 7 = ATab -f T ... 

Given the difference between the selectivity outcome for cyclocondensation and that for production of terminal vs. internal alkenes, we can conclude that different dictates will have different reactivities. This is self-evident from our prior kinetic studies in that each alkyl substituent has a slight decelerating effect one expects a preference for terminal deoxygenation over internal unless the internal diol has an electron-withdrawing substituent. Less important will be the difference in reactivity between syn and anti diolates, although if one or the other is particularly less reactive, it risks developing a thermodynamic sink that effectively removes active catalyst from the system. Further work is needed to quantitatively establish rates of reaction under stoichiometric conditions in order to derive a predictive model for selectivity. 

The term of polymerization in bulk is utilized whenever the polymer formed is monomer-soluble the latter plays the role of solvent for a reaction medium that witnesses a very strong viscosity buildup as the yield increases the kinetics can then take an explosive mode, accelerated by the gel effect (or Trommsdotff effect). This phenomenon is actually a self-acceleration of the rate of polymerization (possibly until becoming explosive) followed by a strong deceleration of the process (see Figure 8.7). 

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