Walden inversion is the inversion of a chiral

Chirality (chemistry)

A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

 center in a molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

 in a chemical reaction

Chemical reaction

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

. Since a molecule can form two enantiomer

Enantiomer

In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...

s around a chiral center, the Walden inversion converts the configuration of the molecule from one enantiomeric form to the other. For example, in a S_N2 reaction

SN2 reaction

The SN2 reaction is a type of nucleophilic substitution, where a lone pair from a nucleophile attacks an electron deficient electrophilic center and bonds to it, expelling another group called a leaving group. Thus the incoming group replaces the leaving group in one step...

, Walden inversion occurs at a tetrahedral carbon atom. It can be visualized by imagining an umbrella

Umbrella

An umbrella or parasol is a canopy designed to protect against rain or sunlight. The term parasol usually refers to an item designed to protect from the sun; umbrella refers to a device more suited to protect from rain...

 turned inside-out in a gale

Gale

A gale is a very strong wind. There are conflicting definitions of how strong a wind must be to be considered a gale. The U.S. government's National Weather Service defines a gale as 34–47 knots of sustained surface winds. Forecasters typically issue gale warnings when winds of this strength are...

.

It was first observed by chemist Paul Walden

Paul Walden

Paul Walden was a Latvian-German chemist known for his work in stereochemistry and history of chemistry. In particular he invented the stereochemical reaction known as Walden inversion and synthesized the first room-temperature ionic liquid, ethylammonium nitrate.-Early years:Walden was born in...

 in 1896.