Rearrangement mechanisms

In the suggested mechanisms, rearrangement of the bond system in BRC is stipulated by transfers of two protons and one hydride ion, which is accompanied by electron transfer from one BRC end to another. 

Dehydration usually goes by the El mechanism. Rearrangements may occur to form more stable carbocations. 

Apart from these three degradation mechanisms, rearrangements of the fractions formed may take place. A polymer does not undergo only one pyrolysis route always, but multiple routes may be taken simultaneously. The type of reaction is totally governed by the strength of bonds in the molecules. The lowest energy path will be favored. 

The rearrangement of 4,5-disubstituted pyridazine to a 2,5-disubstituted pyrazine was postulated to follow a similar mechanism, rearrangement of a perfluoroalkyl group taking place by an addition-elimination mechanism (Scheme 40).102... 

Thus, the experimental data presented are consistent with the intermediacy of 7T-olefin-cobalt(III) complexes. By invoking a (t-tt rearrangement mechanism, rearrangements catalyzed by the vitamin coenzyme can be generalized. 

Although the formation of the Zaitsev products is typical in El processes, it cannot be taken as evidence for the carbocationic mechanism Rearrangement of the intermediate carbocation and consecutive alkene isomerization (double bond migration) induced by the acidic catalyst may occur. These processes strongly affect product distribution and eventually give rise to the formation of a very complex mixture of isomeric alkenes. 

Friedel-Crafts Alkylation (Section 22.1 C) The electrophile is a carbocation formed as an ion pair by interaction of an alkyl halide with a Lewis acid. Rearrangements from a less stable carbocation to a more stable carbocation are common. The mechanism involves an initial reaction between the alkyl halide and Lewis acid AICI3 to )deld an intermediate that can be thought of as a carbocation, AlCl " ion pair. The carbocation portion of the ion pair reacts as a very strong electrophile with the weakly nucleophilic aromatic 77 cloud to form a resonance-stabilized cation intermediate that loses a proton to give the final product. Because carbocations are involved in the mechanism, rearrangements can be a problem, especially with... 

Whenever a carbocation is an intermediate in a mechanism, rearrangements are possible. Besides the study of carbocation structures and reactivity in stable ion media, the majority of the information chemists have on carbocation rearrangements comes from SnI solvolysis reactions. A hydrogen, alkyl, or aryl group on a carbon adjacent (p) to the cationic carbon can shift to form a different carbocation. This is called a Wagner-Meerwein shift. Eq. 11.30 shows a thermoneutral example. 

On the other hand, the existence of quantum degrees of freedom for solvent causes some part of the medium polarization to change quantum-mechanically and not classically. For example, protons in H2O molecules perform a subbarrier transition to another position, which causes a change in the dipole moment or in its orientation. These quantum transitions necessitate a correction for the probability of tunneling of the medium, i.e. the probability of quantum-mechanical rearrangement of its polarization. This correction somewhat lowers the reaction rate. 

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