Unimolecular thermal isomerization reactions

A number of allylic compounds, including allyl vinyl ethers, allyJic azides, allylic thiocyanates, and allylic sulfinates, undergo unimolecular thermal isomerization reactions. Most of these reactions are concerted, four-center processes and in most of them the allylic substituent, as well as the allylic carbon skeleton, undergoes alteration. 

A number of interesting and useful organic reactions involve isomerizations of substances having one or more carbon-carbon double bonds. This chapter deals with the kinetics of reactions of alkenes, cycloalkenes and substituted alkenes which involve migration of carbon-carbon double bonds, with or without structural alteration of the carbon skeleton of the starting materials. These reactions include prototropic and anionotropic rearrangements, several concerted unimolecular isomerizations such as the Cope and Claisen rearrangements, and a number of non-concerted thermal isomerization reactions. 

Unimolecular reactions that take place by way of cyclic transition states typically have negative entropies of activation because of the loss of rotational degrees of freedom associated with the highly ordered transition state. For example, thermal isomerization of allyl vinyl ether to 4-pentenal has AS = —8eu. ... 

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... 

In the case of 16a and 16b, it has been demonstrated that the rate constants are almost independent of concentration and solvent polarity, which indicates that the racemization is a unimolecular reaction and does not involve ionic species in the rate-controlling step. The value of k2 is typically about 4.2 x 10 s at 25 °C. Activation parameters have been calculated from the rate constants measured in a temperature range from 20.4 to 39.8°C, revealing the values for A// = 24.3 kcalmol , and AA = —2.0calK . The same authors also reported thermal isomerization in solution between diastereomers of 20 accompanied by decomposition. Gradual isomerization in solution of m-9 into trans- obeying the first-order kinetics (k = 4.0 x 10 s , = 0.966) has also been reported <2005T6693>. As in the case of 16... 

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... 

The kinetics and mechanisms of the gas phase pyrolysis of cyclopropene in the temperature range 193-243°C has been examined experimentally and theoretically The major and minor products of reaction are propyne and allene respectively propyne results from a unimolecular isomerization with an activation energy of 147.3 kJ mol Moreover, cyclopropene is most likely the important intermediate in the thermal isomerization of allene to propyne These observations are accommodated by the reactions of equation 65. The activation energy for the conversion of allene into cyclopropene is 269 kJmol" and that for the reverse process is 182 kJmol ... 

Several kinds of thermal unimolecular isomerization reactions of disilenes are now known. 

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although conformational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thermally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, = A. The quantity Hh from transition state theory provides... 

RRKM theory is a microcanonical transition state theory and as such, it gives the connection between statistical unimolecular rate theory and the transition state theory of thermal chemical reaction rates. Isomerization or dissociation of an energized molecule A is assumed in RRKM theory to occur via the mechanism... 

CH3NC — CH3CN has also been carried out, the principal object being the interpretation of the transition state. This particular tautomerism has also been the subject of an investigation in which experiments have been made to assess the importance of radical chain effects in the thermally induced reaction. It is concluded that there is no reason to doubt the fact that the isomerization of methyl isocyanide is an excellent test for the theory of unimolecular reactions. 

In the following section, the UVA IS spectroscopy of azobenzenes, the knowledge of which is a prerequisite for photochemical isomerization reactions, is described. Conditions and quantum yields of the photoisomerization will be dealt with in Section 89.3 for the azobenzene molecule and some of its derivatives. The mechanism of these simple light-induced unimolecular reactions (i.e., the dynamics of molecular changes) has only recently been studied directly by means of experimental laser spectroscopy methods. The results obtained thus far are summarized and compared with conclusions from quantum chemical calculations in Section 89.4. Thermal Z—isomerization reactions influence the concentration ratio of E- and Z-isomers in the photostationary state on irradiation of azobenzenes. They are described separately in Section 89.5. Finally, a selection of interesting prospective applications of the photoisomerism of azobenzenes is presented in Section 89.6. 

Our interest in thermally activated unimolecular reactions is in the change of kuni with pressure from the high to the zero pressure limit, and in the pressure dependence of the isotope effect over that range. One particularly interesting study carried out by Rabinovitch and Schneider (reading list) focused on the isomerization of methyl isocyanide, CH3NC, to methyl cyanide, CH3CN... 

The quasi-equilibrium theory (QET) of mass spectra is a theoretical approach to describe the unimolecular decompositions of ions and hence their mass spectra. [12-14,14] QET has been developed as an adaptation of Rice-Ramsperger-Marcus-Kassel (RRKM) theory to fit the conditions of mass spectrometry and it represents a landmark in the theory of mass spectra. [11] In the mass spectrometer almost all processes occur under high vacuum conditions, i.e., in the highly diluted gas phase, and one has to become aware of the differences to chemical reactions in the condensed phase as they are usually carried out in the laboratory. [15,16] Consequently, bimolecular reactions are rare and the chemistry in a mass spectrometer is rather the chemistry of isolated ions in the gas phase. Isolated ions are not in thermal equilibrium with their surroundings as assumed by RRKM theory. Instead, to be isolated in the gas phase means for an ion that it may only internally redistribute energy and that it may only undergo unimolecular reactions such as isomerization or dissociation. This is why the theory of unimolecular reactions plays an important role in mass spectrometry. 

Pericyclic reactions are unimolecular, concerted, uncatalyzed transformations. They take place in a highly stereoselective manner governed by symmetry proper-ties of interacting orbitals. - Characteristic of all these rearrangements is that they are reversible and may be effected thermally or photochemically. The compounds in equilibrium are usually interconverted through a cyclic transition state,224 although biradical mechanisms may also be operative. A few characteristic examples of pericyclic rearrangements relevant to hydrocarbon isomerizations are presented here. 

The major reaction in the thermal decomposition of 2,3-dihydrofuran (9) is a unimolecular isomerization to cyclopropanecarboxaldehyde (89JPC-1139). In an analogous [1,3] sigmatropic reaction, the isomerization of 2-methyl-4,5-dihydrofuran (10) leads to acetylcyclopropane, which can rearrange to 3-penten-2-one (94JPC2341). The latter product may also be formed directly from 10. 

The stilbenes have played a crucial role in the development of modern photochemistry. Direct or triplet sensitized irradiation of trans-stilbene (t-1) in dilute solution results in isomerization to cis-stilbene (c-1) as the exclusive uni-molecular photochemical reaction (1-3). Direct irradiation of c-1 results in isomerization to both t-1 and trans-4a,4b-di-hydrophenanthrene (2), which revert to c-1 both thermally and photochemically and can be trapped by oxidants such as iodine or oxygen to yield phenanthrene (3) (4-6). Triplet sensitized irradiation of c-1 yields only t-1. These unimolecular isomerization pathways are summarized in eq. 1. 

The thermal reactions of indole have been studied. The authors65 suggested that the indole to benzyl cyanide isomerization involves a series of unimolecular steps which... 

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