Unimolecular rearrangement

FIGURE 7. Relative energies of aniline and its energetically lower-lying isomers. Values given in parentheses in kJ moU1 were obtained from B3LYP/6-311++G(d,p) + ZPE calculations 

A remarkable feature is the elimination of closed-shell molecules. Because the elimination of an H atom has an exit barrier in the triplet state, dissociation from the T state is also significant (formation of excited states will be discussed in a subsequent paragraph). 

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Tables 12.1-12.3 below give some examples of the magnitude of each term for two bimolecular reactions (Diels-Alder and Sn2 reactions, forming either one or two molecules as the product) and a unimolecular rearrangement (Claisen reaction). All values have been calculated at the MP2 level with the 6-31G(d) basis for the Diels-Alder and Claisen reactions, and the 6-31+G(d) basis for the S l reaction. The values are given in kcal/mol at a temperature of 300 K (RT = 0.60 kcal/mol).

Ideally all reactions should result from unimolecular homolysis of the relatively weak 0-0 bond. However, unimolecular rearrangement and various forms of induced and non-radical decomposition complicate the kinetics of radical generation and reduce the initiator efficiency.46 Peroxide decomposition induced by radicals and redox chemistry is covered in Sections 3.3.2.1.4 and 3.3.2.1.5 respectively. 

This possibility can be dismissed22 because it is asymmetric. Read backward, the first step would be not the second-order axial addition of Cl" but a unimolecular rearrangement, and an improbable one at that. Aside from this, the fact that the pathway is not symmetric rules it out. If it is invalid in the special case of an exchange, then it can hardly provide the general pathway for substitution at square-planar metal centers. 

To summarize, the mechanism of a reaction converts starting materials to products through a specific sequence of bimolecular collisions and unimolecular rearrangements. 

Like fragmentations, unimolecular rearrangements are always uphill at the beginning of the process, because a bond breaks. Unlike fragmentations, rearrangements are downhill at the end as a new bond forms. An example of this kind of energy profile for the Isomerization reaction of ds-2-butene appears in Figure 15-14Z). 

Concerted Cycloadditions, Unimolecular Rearrangements, and Thermal Eliminations... 

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