Isomerization, cyclopropane Unimolecular reactions

P3.03.23. EFFECT OF PRESSURE ON UNIMOLECULAR REACTIONS. CYCLOPROPANE ISOMERIZATION. 

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... 

Much of the recent work on the cyclopropane-propylene isomerization has had one of two objectives, either to try and determine which of the two reaction paths suggested by the early workers is involved, or to test the various theories of unimolecular reactions. Comer and Pease (1945), using catalytic hydrogenation to analyse their reaction product, but otherwise working under similar conditions to Chambers and Kistiakow-sky, suggested that all the results obtained could be represented just as well by the reaction scheme... 

Pritchard, H. O., R. G. Sowden and A. F. Trotman-Dickenson, Studies in energy transfer II. The isomerization of cyclopropane—a quasi-unimolecular reaction, Proc. Roy. Soc. A, 217, 563-571 (1953). 

During the past few years a very considerable amount of work has been done on the decompositions and isomerizations of cyclic compounds. Perhaps the most important reason for the recent interest in these reactions of cyclic hydrocarbons is that a number of them occur in a reasonably simple manner, so that they are particularly suitable for the evaluation of theories of unimolecular reactions. The simplest of these processes is the conversion of cyclopropane into propene... 

The thermal isomerization of cyclopropane into propene has been very extensively investigated from the experimental point of view, and a considerable amount of work has been done to apply the various theories of unimolecular reactions. The reaction rate is not influenced by the surface or by inhibitors such as nitric oxide or propene the process therefore appears to be a homogeneous unimolecular reaction. The main experimental results are summarized in Table 2. In this and... 

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... 

The activation entropies for the four unimolecular reactions are positive and can be interpreted if we assume that the molecule, in its activated state, is less ordered than in the ground state. Disordering occurs because a bond must be weakened or broken for reaction to be possible. The cyclopropane isomerization is a particularly good example. As a C-C bond breaks the molecule passes through an intermediate, possibly a diradical. 

Besides undergoing reversible, thermal, geometrical isomerization, cyclopropane also undergoes a slower, irreversible structural isomerization. The kinetics of both reactions of trans-C Hj] cyclopropane at 480 °C in the 10 Torr pressure range have been reported, as well as the kinetics of the structural isomerization of cyclopropane between 454 and 538 °C in the pressure range 0.4—137 atm. First-order kinetics for the gas-phase isomerization of 1,1,2-trimethylcyclopropane between 427 and 481°C have been established. The kinetic data and the formation of the complex mixture of products were interpreted in terms of a unimolecular, biradical-intermediate mechanism, in line with precedent. 

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. 

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. 

The structural isomerization of a number of alkyl substituted cyclopropanes has been investigated. In all cases the reaction is probably unimolecular. In general several olefins are formed. The results obtained are shown in Table 2. It is to be expected (especially if a diradical is an... 

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 ... 

Biradicals are frequently postulated to arise as intermediates in a number of chemical reactions and unimolecular isomerizations. Sometimes there are reasonable alternative concerted mechanisms in which the intermediate (or transition-state complex) is not a biradical. Such a case of much interest37,61 involves the reactions of singlet [5] and triplet [7] methylenes with olefins. We note that the permutational symmetry does not determine whether or not a reaction is concerted rather it is determined by the shapes of the intermolecular potential surfaces.78 The lowest 1Ai methylene is expected to react by a concerted mechanism, since it correlates with the ground state of the product cyclopropane higher excited singlets need not react via a concerted mechanism. 

The pyrolysis of substituted cyclopropane leads to three types of unimolecular isomerizations (see Fig. 3). The first kinetic study of the conversion of cyclopropane into propylene (reaction a) was undertaken by Trautz and Winkler in 1922 ... 

Nucleophilic capture of the spirooctadienyl cation opens the 3-member ring. This behavior characterizes many reactions of many other cyclopropane-containing carbocations, as well, y-radiolysis of perdeuterated propane forms CsD ions, most of which either transfer D or form isopropyl adducts. As the propane pressure is raised from 1000 mbar to 2000 mbar, however, the isopropyl/ -propyl adduct ratio falls from 30 1 to about 5.5 1. This implies the formation of corner-protonated cyclopropane, which reacts with nucleophiles as though it were an -propyl cation. With increased pressure, vibrationally excited protonated cyclopropane experiences more frequent nonreactive collisions, which deactivate it and slow down its rate of unimolecular isomerization to isopropyl cation. 

With reactions in solution the question arises as to whether the solvent should be included in the molecularity. It is usually considered that it counts in the molecularity if it enters into the overall reaction but not if it exerts only an environmental effect. Thus, a process in aqueous solution in which a compound was being hydrolyzed, with the reaction of a water molecule with each solute molecule, would be regarded as bimolecular. The isomerization of cyclopropane in solution, however, is unimolecular. 

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. 

Most unimolecular processes are apparently observed in the first-order (high-pressure or high-concentration) region, and we will continue to assume first-order kinetics for unimolecular steps in multistep mechanisms. Most proposed mechanisms do not include unimolecular steps, and there are not very many reactions known to have a one-step unimolecular mechanism. The first such reaction discovered was the isomerization of cyclopropane to propene. Others are the dissociation of molecular bromine and the decomposition of sulfuryl chloride. ... 

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