Intimate ion pair

In chemistry the intimate ion pair concept introduced by Saul Winstein ^[1] describes the interactions between a cation, anion and surrounding solvent molecules. In an ordinary aqueous solutions of inorganic salts an ion is completely solvatated and shielded from the counterion. In less polar solvents two ions can still be connected to some extent. In a tight or intimate or contact ion pair there are no solvents molecules between the two ions. When solvatation increases, ionic bonding decreases and a loose or solvent-separated ion pair results. The ion pair concept explains stereochemistry in solvolysis.

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The concept of intimate ion pairs is used to explain the slight tendency for inversion of stereochemistry during an SN1 reaction. It is proposed that solvent or other ions in solution may assist in the removal of a leaving group to form the carbocation which reacts in an SN1 fashion; similarly, the leaving group may associate loosely with the cationic intermediate. The association of solvent or an ion with the leaving group effectively blocks one side of the incipient carbocation, while allowing the backside to be attacked by a nucleophile. This leads to a slight excess of the product with inverted stereochemistry, whereas a purely SN1 reaction ought to lead to a racemic product.

References

1. ^  Salt Effects and Ion Pairs in Solvolysis and Related Reactions. III.1 Common Ion Rate Depression and Exchange of Anions during Acetolysis S. Winstein, E. Clippinger, A. H. Fainberg, R. Heck, G. C. Robinson; J. Am. Chem. Soc.; 1956; 78(2); 328-335.