Dissymmetric solvation

The most common mechanistic course by which organic molecules are racemized is by cleavage of one of the ligands from an asymmetric carbon atom to afford a planar tricoordinate intermediate. In the absence of special effects, such as dissymmetric solvation, such an intermediate is capable of being captured equally well from either side, and produces equal quantities of enantiomers as products. 

Stereochemical analysis can add detail to the mechanistic picture of the Sj l substitution reaction. The ionization mechanism results in foimation of a caibocation intermediate which is planar because of its hybridization. If the caibocation is sufficiently long-lived under the reaction conditions to diffirse away from the leaving group, it becomes symmetrically solvated and gives racemic product. If this condition is not met, the solvation is dissymmetric, and product with net retention or inversion of configuration may be obtained, even though an achiral caibocation is formed. The extent of inversion or retention depends upon the details of the system. Examples of this effect will be discussed in later sections of the chapter. 

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