EVALUACIÓN RADIOLÓGICA Y TOMOGRÁFICA

Figure 5.18: The methoxycarbonylation of phenyl acetylene.

Phenyl acetylene was of particular interest as studies 52 determined that the additional steric bulk from the phenyl ring influenced the product distribution The rate of the methoxycarbonylation were then compared to that of styrene.

Figure 5.19: The methoxycarbonylation of phenyl acetylene in 10 ml of methanol under constant CO.

The reaction was run in a higher methanol concentration to establish the order with respect to the substrate concentration. The gas uptake curve is shown in Figure 5.19 and the graphical rate expression in Figure 5.20. The rate as a function of phenyl acetylene graph showed that at lower substrate concentrations there was a linear fit running through the

Figure 5.20: The rate as a function of the substrate concentration for the methoxycarbonylation of phenyl acetylene in 10 ml of methanol.

The natural log of the rate was then plotted against the natural log of the phenyl acetylene concentration to determine the order with respect to phenyl acetylene. The linear fit confirms that the reaction is first order at lower substrate concentrations and tends to higher order at higher concentrations.

Figure 5.21: The natural log of the rate as a function of the natural log of the phenyl acetylene concentration for the methoxycarbonylation of phenyl acetylene in 10 ml of

methanol.

The reaction was also run at lower methanol concentrations (2ml) to establish the order with respect to the methanol concentration. Once again, the rate was plotted against the substrate concentration and then the rate divided by the substrate concentration was plotted against the methanol concentration to determine the order with respect to methanol. The plots are shown in figure 5.22 and 5.23 respectively. As the order with respect to substrate is complex, the determination with order with respect to methanol concentration can only be determined for lower substrate concentrations when the reaction is known to be first order in substrate.

Figure 5.22: The rate as a function of the substrate concentration for the methoxycarbonylation of phenyl acetylene in 2 ml of methanol.

Figure 5.23: The rate divided by the substrate concentration as a function of the methanol concentration for the methoxycarbonylation of phenyl acetylene in 2 ml of methanol.

When the rate was divided by the substrate concentration and plotted against the methanol concentration, it was found that, at low substrate concentration, the rate was almost independent of methanol concentration. This suggests that the reaction is zero order in methanol concentration at lower substrate concentrations.

The system was then run under constant volume conditions to record the CO consumption as a function of time. The gas uptake curve for this reaction is shown in figure 5.24

Figure 5.24: The methoxycarbonylation of phenyl acetylene in 10 ml of methanol in a constant volume system.

This reaction was carried out in the presence of 10 ml of methanol, which would result in pseudo zero order kinetics in methanol. The rate as a function of substrate concentration was then plotted to show the order in substrate concentration. In this instance, the reaction was found first order in substrate concentration throughout the reaction, which is shown in figure 5.25.

Figure 5.25: The rate as a function of the substrate concentration for the methoxycarbonylation of phenyl acetylene in 10 ml of methanol in a constant volume

system.

As the reaction was run under constant volume conditions, the CO partial pressure was changing as a function of time. The apparent overall first order dependence combined with the already determined first order dependence of the rate on phenyl acetylene concentration suggests that the reaction may be zero order in pCO, at least at low substrate concentrations. This is confirmed by a plot of the rate divided by the phenyl acetylene concentration as a function of the partial pressure of CO, which approximates to a straight line parallel to the x axis (figure 5.26)

Figure 5.26: The rate, divided by the substrate concentration, as a function of the partial pressure of CO, for the methoxycarbonylation of phenyl acetylene in 10 ml of methanol in a

constant volume reactor.

At lower concentrations of phenyl acetylene, the reaction is first order in phenyl acetylene, zero order in methanol and zero order in pCO. This suggests that the coordination of phenyl acetylene to the palladium is rate determining. All of the palladium remains in the catalytic cycle all of the time. In other words the equilibrium shown in figure 5.27 lies completely to the right; otherwise a negative dependence on CO should be observed.

Figure 5.27: The hydrido carbonyl complex in equilibrium with hydrido alkyl complex for the methoxycarbonylation of styrene.

However we note that in the reaction run at constant volume, the initial pressure was 30 bar, and 100% conversion of substrate would result in only a 17% decrease in CO concentration. So a possible alternative explanation for the observed zero order in pCO is that the measurements are not sufficiently sensitive to pick up changes as a result of this small pressure drop and that the reaction is effectively run under pseudo zero order conditions in CO.

The selectivity towards the linear product, methyl cinnamate was found to be in excess of 99% throughout these reactions. The reaction was, therefore, proposed to follow the hydride mechanism. The mechanism and origin of selectivity is explored in more detail in section