Intimate ion pair

In chemistry, the intimate ion pair concept, introduced by Saul Winstein, describes the interactions between a cation, anion and surrounding solvent molecules.

[1] In ordinary aqueous solutions of inorganic salts, an ion is completely solvated and shielded from the counterion.

When solvation increases, ionic bonding decreases and a loose or solvent-shared ion pair results.

The concept of intimate ion pairs is used to explain the slight tendency for inversion of stereochemistry during an SN1 reaction.

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.