Potential energy unimolecular nucleophilic substitution

Section 4.9 The potential energy diagrams for separate elementary steps can be merged into a diagram for the overall process. The diagram for the reaction of a secondary or tertiary alcohol with a hydrogen halide is characterized by two intermediates and three transition states. The reaction is classified as a unimolecular- nucleophilic substitution, abbreviated as SnI. 

Gas-phase SN2 nucleophilic substitution reactions are particularly interesting because they have attributes of both bimolecular and unimolecular reactions.1 As discovered from experimental studies by Brauman and coworkers2 and electronic structure theory calculations,3 potential energy surfaces for gas-phase SN2 reactions of the type,... 

A process following the rate law shown in Eq. (20.65) is said to be an SN1 (substitution, nucleophilic, unimolecular) process. The term unimolecular refers to the fact that a single species is required to form the transition state. Because the rate of such a reaction depends on the rate of dissociation of the M-X bond, the mechanism is also known as a dissociative pathway. In aqueous solutions, the solvent is also a potential nucleophile, and it solvates the transition state. In fact, the activated complex in such cases would be indistinguishable from the aqua complex [ML H20] in which a molecule of H20 actually completes the coordination sphere of the metal ion after X leaves. This situation is represented by the dotted curve in Figure 20.1 where the aqua complex is an intermediate that has lower energy than [ML,]. The species [ML H20] is called an intermediate because it has a lower energy than that of the activated complex, [MLJ. 

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