Walden inversion is the inversion of a stereogenic center in a
chiral
Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object.
An object or a system is ''chiral'' if it is distinguishable from i ...
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
in a
chemical reaction
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...
. Since a molecule can form two
enantiomer
In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical anti ...
s around a stereogenic center, the Walden inversion converts the configuration of the molecule from one enantiomeric form to the other. For example, in an SN2 reaction, Walden inversion occurs at a tetrahedral carbon atom. It can be visualized by imagining an
umbrella
An umbrella or parasol is a folding canopy supported by wooden or metal ribs that is usually mounted on a wooden, metal, or plastic pole. It is designed to protect a person against rain or sunlight. The term ''umbrella'' is traditionally use ...
turned inside-out in a
gale
A gale is a strong wind; the word is typically used as a descriptor in nautical contexts. The U.S. National Weather Service defines a gale as sustained surface winds moving at a speed of between 34 and 47 knots (, or ).SN2 reaction gives rise to a product whose configuration is opposite to the reactant. Therefore, during SN2 reaction, 100% inversion of product takes place. This is known as Walden inversion.
It was first observed by chemist
Paul Walden
Paul Walden ( lv, Pauls Valdens; russian: Павел Иванович Вальден; german: Paul von Walden; 26 July 1863 – 22 January 1957) was a Russian, Latvian and German chemist known for his work in stereochemistry and history of chem ...
in 1896. He was able to convert one enantiomer of a chemical compound into the other enantiomer and back again in a so-called Walden cycle which went like this: (+) chlorosuccinic acid (1 in the illustration) was converted to (+) malic acid 2 by action of
silver oxide
Silver oxide is the chemical compound with the formula Ag2O. It is a fine black or dark brown powder that is used to prepare other silver compounds.
Preparation
Silver oxide can be prepared by combining aqueous solutions of silver nitrate and a ...
in water with retention of configuration. In the next step the
hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydrox ...
group was replaced by
chlorine
Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...
to the other isomer of chlorosuccinic acid 3 by reaction with
phosphorus pentachloride
Phosphorus pentachloride is the chemical compound with the formula PCl5. It is one of the most important phosphorus chlorides, others being PCl3 and POCl3. PCl5 finds use as a chlorinating reagent. It is a colourless, water-sensitive and mois ...
. A reaction with silver oxide yielded (-) malic acid 4 and finally a reaction with PCl5 returned the cycle to its starting point.
:
In this reaction, the silver oxide in the first step acts as a
hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. ...
donor while the silver ion plays no role in the reaction. The intermediates are the carboxyl dianion A which gives an intramolecularnucleophilic substitution by the β-carboxylate anion to produce a four-membered β- lactone ring B. The α-carboxyl group is also reactive but
in silico
In biology and other experimental sciences, an ''in silico'' experiment is one performed on computer or via computer simulation. The phrase is pseudo-Latin for 'in silicon' (correct la, in silicio), referring to silicon in computer chips. It ...
data suggests that the
transition state
In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked ...
for the formation of the three-membered α-lactone is very high. A hydroxyde ion ring-opens the lactone to form the alcohol C and the net effect of two counts of inversion is retention of configuration.
See also
* Another demonstration of the Walden cycle in the
Brook rearrangement In organic chemistry the Brook rearrangement refers to any ,''n''carbon to oxygen silyl migration. The rearrangement was first observed in the late 1950s by Canadian chemist Adrian Gibbs Brook (1924–2013), after which the reaction is named. The ...