Cyclopropane to propylene

A question that intrigued several kineticists around 1920 was the following. For bi-molecular reactions of the type A -1- B = Products collision theory gave at least a plausible conceptual picture If the collision between A and B is sufficiently vigorous, the energy barrier separating reactants and products can be crossed. How, though can one explain the case of monomolecular elementary reactions, e.g. an isomerization, such as cyclopropane to propylene, or the decomposition of a mol-... 

The gas-phase isomerisation reaction of cyclopropane to propylene satisfies the following rate expression ... 

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... 

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. 

We first describe our representation of chemical reactions. Consider the isomerization reaction of cyclopropane to propylene,... 

It is found by experiment that rates almost always have power-law dependences on the densities (such as concentration, density on a surface, or partial pressure) of chemical species. For example, our first example of the homogeneous reaction of cyclopropane to propylene exhibits a rate of decomposition that can be written as... 

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... 

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. 

Preliminary studies (6) have shown that the ratio of ethylene to carbon monoxide is a function of the temperature as well as the wavelength. This ratio is a measure of the relative importance of reaction (24) as compared to (25) and (26). The ratio of cyclopropane to propylene which measures the relative rates of (25) to (26) is independent of temperature and pressure at 3130 A. and has a value of 15.5. In photolysis at 2537 A., the same ratio is 2 and besides is found to be sensitive to the total pressure. The geometry of the system lias also been found to be a factor. 

The models described above assume that the reaction occurs only in the liquid phase. In some cases, such as isomerization of cyclopropane to propylene on a silica-alumina catalyst,43 reduction of crotonaldehyde over a palladium catalyst,45 and hydration of olefins to alcohols over tungsten oxide,58 the reactions could occur in the gas as well as in the liquid phases. 

Among the numerous examples of homogeneous first-order reactions are the rearrangement of cyclopropane to propylene/ certain cis-trans-isomerizations, and the inversion of sucrose. 

The catalytic data of Figure 4 show that small cobalt particles in NaX zeolite selectivity convert cyclopropane to propylene by ring opening reactions. We believe that these reactions are catalyzed at cobalt sites and not at Bronsted sites since we have tried to poison any Bronsted sites in additional experiments with sodium vapor. 

Bassett and Habgood [70] and Nakagaki and Nishino [71], who studied the transformation of cyclopropane to propylene, demonstrated the possibility of determining the chromatographic characteristics from chromatograms taken downstream of the reactor column. 

Only two experimental investigations have been carried out to study isotope effects in unimolecular reactions at low pressures. Weston has studied the tritium isotope effect in the isomerization of cyclopropane to propylene, while Gray and Pritchard have studied the individual rates of decomposition of octadeutero-cyclobutane and unlabeled cyclobutane. Few details of the work by Gray and Pritchard are available. The isotope effect does not appear to change much with pressure. Strangly enough these authors find that the reaction exhibits an inverse isotope effect with A (QHs)/ (QD8) = I... 

Methylmaleic acid to methyl fumaric acid Dimethylmaleic ester to dimethylfumaric ester Vinyl allyl ether to allylacetaldehyde Cyclopropane to propylene... 

Both products appear to be a cyclopropane to propylene type of thermal isomerization. The lower activation energy for formation of cyclohexene is probably the result of strain in the five-membered ring which is released upon bridgehead bond homolysis. 

Two possible pathways were envisioned for the reaction (a) cyclopropane to propylene-like rearrangement followed by 1,5-hydrogen shifts, that can equilibrate C4 and C6 as well as C3 and Cl with a slower 1,5-deuterium shift (due to the primary isotope effect) and (b) the second pathway would involve a retro-electrocyclization to a cycloheptatriene destroying the aromatic it system in the process, and this undergoes a 1,5-deuterium shift to the 1,2-benzocycloheptatriene which subsequently undergoes a 1,5-hydrogen shift to equilibrate C4 and C6 but also must equilibrate C3 with Cl. Further, a 1,5-deuterium shift in the 7-deuterio material gives the isomer from path (a) (Scheme 12.8). 

The energy level of the transition state connecting the trimethylene with propylene was very similar to the one going directly from cyclopropane to propylene [30,31,33,34]. We have calculated a rate constant using Eq. (6.1) and performed RRKM calculations to adapt the calculated high-pressure limit first-order rate constant to the pressure where the experiment was done. The comparison between our calculated rate constants and the experimental values of Waage and Rabinovitch [35], for example, was very good [32,36]. 

In 1960, Bassett and Habgood [31] investigated the isomerization of cyclopropane to propylene catalyzed by the Linde molecular sieve 13x exchanged with Ni ions on a packed column. Cyclopropane was injected as a pulse onto the column, and the formation of the product was observed as an extremely broadened peak. Gil-Av and Herzberg-Minzly [32] investigated Diels-Alder... 

The conversion of cyclopropane to propylene is first order with a half-life of 18 minutes at 500 C. (a) What fraction of a given quantity of cyclopropane remains after 54 minutes (b) Starting with 2.00 g, how many grams remain after (i) 72 minutes, (ii) 45 minutes (c) How many minutes will be... 

Examples are the dissociation of molecular bromine, Br2, the decomposition of sulfuryl, SO2CI2, into SO2 and CI2, in the isomerization of cyclopropane to propylene. All these reactions are elementary, and thus truly unimolecular. 

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