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In chemistry, the intimate ion pair concept, introduced by 
The concept of intimate ion pairs is used to explain the slight tendency for inversion of stereochemistry during an S1 reaction. It is proposed that solvent or other ions in solution may assist in the removal of a leaving group to form a carbocation which reacts in an S1 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 S1 reaction should lead to a
Saul Winstein
Saul Winstein (October 8, 1912 – November 23, 1969) was a Jewish Canadian chemist who discovered the '' Winstein reaction.'' He argued a non-classical cation was needed to explain the stability of the norbornyl cation. This fueled a debat ...
, describes the interactions between a cation, anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
and surrounding solvent
A solvent (s) (from the Latin '' solvÅ'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
molecules. In ordinary aqueous solutions of inorganic salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...
s, an ion is completely solvated and shielded from the counterion. In less polar solvents, two ions can still be connected to some extent. In a ''tight'', ''intimate'', or ''contact'' ion pair, there are no solvent molecules between the two ions. When solvation
Solvation (or dissolution) describes the interaction of a solvent with dissolved molecules. Both ionized and uncharged molecules interact strongly with a solvent, and the strength and nature of this interaction influence many properties of the ...
increases, ionic bonding
Ionic bonding is a type of chemical bonding that involves the electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in ionic compounds ...
decreases and a ''loose'' or ''solvent-shared'' ion pair results. The ion pair concept explains stereochemistry in solvolysis
In chemistry, solvolysis is a type of nucleophilic substitution (S1/S2) or elimination reaction, elimination where the nucleophile is a solvent molecule. Characteristic of S1 reactions, solvolysis of a chirality (chemistry), chiral reactant affor ...
.
The concept of intimate ion pairs is used to explain the slight tendency for inversion of stereochemistry during an S1 reaction. It is proposed that solvent or other ions in solution may assist in the removal of a leaving group to form a carbocation which reacts in an S1 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 S1 reaction should lead to a racemic
In chemistry, a racemic mixture, or racemate (), is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as racemates. ...
product. Intimate ion pairs are also invoked in the Si mechanism. Here, part of the leaving group detaches and attacks from the same face, leading to retention.
See also
*Ion association
In chemistry, ion association is a chemical reaction whereby ions of opposite electric charge come together in solution to form a distinct chemical entity. Ion associates are classified, according to the number of ions that associate with each ...
*Asymmetric ion-pairing catalysis Asymmetric ion-pairing catalysis in chemistry is a type of asymmetric catalysis taking place specifically with charged intermediates or charged reagents. In one type of catalysis ion-pairing exists with a charged and chiral catalyst. The charged ...
References
{{reflist Chemical bonding it:Coppia ionica