Isomerization of cyclopropanes

Calculate the energy change En tor isomerization of cyclopropane to propene in the gaseous state at 0 K... 

An example of a first-order elementary reaction is the isomerization of cyclopropane to propene, for which the reverse reaction hardly proceeds, and we may write the rate as... 

For the gas-phase unimolecular isomerization of cyclopropane (A) to propylene (P), values of the observed first-order rate constant, kuni, at various initial pressures, P0, at 470 C in a batch reactor are as follows ... 

A second example of an inverse statistical weight isotope effect is that of the secondary H/D KIE on C-C bond rupture during the gas phase unimolecular isomerization of cyclopropane to propene. Theory and experiment are compared in Fig. 14.2 for reactions 14.37 and 14.38. 

Fig. 14.2 Pressure dependence of the 2°-D isotope effect, kn/ko, on the unimolecular isomerization of cyclopropane at 755 K c-C3H6 -> C3H6 and C-C3D6 — C3D6 (The data are from Blades, A. T., Can.

The thermal isomerization of cyclopropane to propylene is perhaps the most important single example of a unimolecular reaction. This system has been studied by numerous workers. Following the work of Trautz and Winkler (1922), who showed that the reaction was first order and had an energy of activation of about 63,900 cal mole measured in the temperature range 550-650° C, Chambers and Kistiakowsky (1934) studied the reaction in greater detail and with higher precision from 469-519° C. They confirmed that it was first order and, for the reaction at its high-pressure limit, obtained the Arrhenius equation... 

The cis-trans isomerization of cyclopropanes is not restricted to the deuterium-substituted molecules, cis- and traws-l,2-Dimethylcyclo-propane have been shown to imdergo reversible geometrical isomerization as well as slower structural isomerization. All the processes are homogeneous and kinetically first order, and almost certainly unimolecular. The reaction scheme is shown below. 

If stored as a liquid, even at —78°, cyclopropane undergoes a fairly rapid polymerization reaction. However, in the gas phase, at temperatures above 325° (in a stream of helium), it isomerizes smoothly to yield methylacetylene. This is clearly analogous to the isomerization of cyclopropane to propylene. 

This is because there is a potential energy barrier for isomerization or dissociation that can only be overcome by adding energy to-the appropriate bonds. Let us consider as prototypes the isomerization of cyclopropane to propylene... 

The isomerization of cyclopropane follows the Lindemann mechanism and is found to be unimolecular. The rate constant at high pressure is 1.5 x 10- s- and that at low pressure is 6 X 10- torr- s-K The pressure of cyclopropane at which the reaction changes its order, found out ... 

Catalysis. The isomerization of cyclopropane to propylene and the rearrangement of protoadamantane to adamantane were studied on HY zeolite and samples of materials A and B. 

Catalytic and Electron Transfer Properties. The isomerization of cyclopropane on HY zeolites activated at temperatures less than 600° C is attributed to catalysis by Bronsted acid sites (12, 13), and the activation temperature for maximum activity was in the range 300°-400°C (13). On the other hand, rearrangement of protoadamantane to adamantane proceeds by hydride ion abstraction at Lewis acid sites (lfy. Materials B, therefore, appear to have good Bronsted activity (Figure 5) and in view... 

The ability of water molecules to promote a reaction depends on many factors. In most cases, zeolites with monovalent cations have low activity. However, the addition of water molecules to X and Y zeolites with monovalent ions increased the isomerization of cyclopropane (63). De-cationized zeolites can be promoted readily with water, and the process is reversible (2, 60, 64). It was shown (2) that the promoting ability of water molecules in faujasites is less when the Si02/Al203 increases. Dealu-minated faujasites are even more difficult to promote. For erionite and mordenite the maximum effect of water was observed only after treatment with liquid water and subsequent heating (2). The effect of water on zeolites saturated with polyvalent cations is less pronounced (65, 66, 67). However, the presence of multivalent cations stabilizes the catalytic activity. Water and alcohols were reported to promote ion exchanged zeolites for n-pentane isomerization (68) and n-hexadecane hydrocracking (69). 

The activity enhancement of MeFSM-16 by sulfiding with hydrogen sulfide in the isomerization of cyclopropane at 150°C... 

Figure 2. The effect of sulfiding temperature of MeFSM-16 on the isomerization of cyclopropane.

Early kinetic studies on the structural isomerization of cyclopropane to propene provided estimates of activation parameters73 75 and prompted speculation that the reaction might well involve a trimethylene diradical intermediate. This possibility seemed reinforced when the thermal interconversion of the els and trans isomers of l,2-d2-cyclo-propane at 414 to 474 °C (equation 1) was reported in 195876. This structurally degenerate isomerization was found to be substantially faster than conversion to deuterium-labeled propenes—about 24 times faster at the high pressure limit76 77. 

Another, quite independent theoretical assessment, based on ab initio calculations and the generalized valence bond method, found that the barrier height for cis,trans isomerization of cyclopropane is essentially the same (calculated value, 60.5 kcal mol-1) whether one or both of the thermal CH2 groups are rotated after opening of the CC bond 243. Thus, in 1972, there seemed to be general agreement among theoreticians that the stereomutations of cyclopropane should take place with k, about equal to kl2. 

Avery (43) and Ward (71) have shown that at room temperature closely similar spectra are obtained by the adsorption of cyclopropane or propene on Pd/Si02 and Ni/Si02, respectively. Isomerization of cyclopropane to propene most probably occurs as an intermediate situation. 

Isomerization of cyclopropane was studied by Hall and co-workers (84, 103), who reported catalyst activity to increase with vacancy concentration. They postulated that the active site consisted of a vacancy and neighboring Al—OH group. Hydrogenation of ethylene at room temperature was found to increase with increase in catalyst reduction and irreversible hydrogen was apparently not involved in the reaction (104). 

Furue H, Pacey PD. Performance of a cylindrical flow reactor in a kinetic study of the isomerization of cyclopropane. J Phys Chem 1980 84 3139-3143. 

Horsley et a/.164 have determined a reaction pathway for the geometrical isomerization of cyclopropane by a calculation and search of the full 21-dimensional hypersurface The calculations were performed using a minimal basis set of STO s in a modified SCF calculation using the restricted Hartree-Fock formalism suggested by Nesbet.165 In the region of the trimethylene diradical, a 3 x 3 configuration interaction calculation was included. 

Table 4. Isomerization of Cyclopropanes 155 to Silyl Enol Ethers 156 According to Eq. 68...

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