Intimate ion pair
Encyclopedia
In chemistry
the intimate ion pair concept introduced by Saul Winstein describes the interactions between a cation, anion and surrounding solvent
molecules. In ordinary aqueous solutions of inorganic salts 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 or intimate or contact ion pair there are no solvent molecules between the two ions. When solvation increases, ionic bonding decreases and a loose or solvent-shared ion pair results. The ion pair concept explains stereochemistry
in solvolysis
.
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 a 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 should lead to a racemic
product.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
the intimate ion pair concept introduced by Saul Winstein describes the interactions between a cation, anion and surrounding solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
molecules. In ordinary aqueous solutions of inorganic salts an ion is completely solvated and shielded from the counterion. In less polar
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
solvents two ions can still be connected to some extent. In a tight or intimate or contact ion pair there are no solvent molecules between the two ions. When solvation increases, ionic bonding decreases and a loose or solvent-shared ion pair results. The ion pair concept explains stereochemistry
Stereochemistry
Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules....
in solvolysis
Solvolysis
Solvolysis is a special type of nucleophilic substitution or elimination where the nucleophile is a solvent molecule. For certain nucleophiles, there are specific terms for the type of solvolysis reaction...
.
The concept of intimate ion pairs is used to explain the slight tendency for inversion of stereochemistry during an SN1 reaction
SN1 reaction
The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is unimolecular...
. 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 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 should lead to a racemic
Racemic
In chemistry, a racemic mixture, or racemate , is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture was "racemic acid", which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.- Nomenclature :A...
product.