Nucleophilic substitution process description

Not shown in the description above, but assumed to be important, is solvation at the rear side of in the ion pairs. According to this scheme, attack by a nucleophile or solvent can occur on the covalent substrate, the intimate ion pair, the solvent-separated ion pair, or on the dissociated carbonium ion. Nucleophilic attack on the covalent substrate or on the intimate ion pair will be akin to a displacement process, and will take place with inversion of configuration. At the solvent-separated ion-pair stage, collapse of the solvent shell can occur from the front to produce retention of configuration or from the back to produce inversion, or the carbonium ion can become symmetrically solvated to produce racemic product. The macroscopic properties of the nucleophilic substitution reaction result from competition among these various rate processes. 

Alternative models for the description and analysis of pericyclic reactions continue to appear. Multicentre pericyclic reactions in ground states have been treated as a sequence of one-centre reactions analogous to 5 2 nucleophilic substitution reactions the procedure is reminiscent of the FMO approach and can, with modification, analyse excited state processes. A second approach is presented in which multicentre reactions that retain at least one common element of symmetry in the reaction process connecting reactants to products are divided into individual steps. If none of these steps is symmetry forbidden, then the total process is allowed. Directions for deciding the individual steps are given. 

Electron transfer processes are also involved in a family of reactions which are designated by the mechanistic description SrnI. This refers to a nucleophilic substitution via a radical intermediate, involving a unimolecular decomposition of a radical anion of the substrate. There are two families of such reactions which have been... 

The frontier molecular orbital approach provides a description of the bonding interactions that occur in the Sn2 process. The orbitals involved are depicted in Fig. 5.3. The frontier orbitals are a filled lone pair orbital on the approaching nucleophile Y and the o antibonding orbital associated with the carbon undergoing substitution... 

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