What is Walden Inversion?
The phenomenon of inversion of configuration during a chemical reaction is known as Walden Inversion. Generally Walden inversion is referred to as optical inversion. The inversion of configuration may or may not lead to the change in direction of rotation.
Walden’s inversion is the reversal of a chiral center in a molecule in a chemical reaction. Since the molecule can form two enantiomers around the chiral center, the Walden inversion transforms the structure of the molecule from one enantiomeric form to another.
Walden Inversion Reaction
In 1896, Walden reported an inversion of optical rotation in the conversion of malic to chlorosuccinic acid by reaction with phosphorus pentachloride.
Walden inversion has been extensively studied with numerous reactants and optically active substances. Perhaps the most satisfactory explanation for the change in configuration was suggested by Werner in 1911 and is commonly known as the opposite face mechanism for the Walden inversion.
A walden inversion occurs at a tetrahedral carbon atom during an SN2 reaction when the entry of the reagent and the departure of the leaving group are synchronous. The result is an inversion of configuration at the centre under attack.
Walden Inversion Mechanism
The stereochemical course of an SN2 reaction’s inversion of configuration is explained below.
Nearly 100 years ago, Paul Walden demonstrated that (+) malic acid could be converted to either (+) or (-) chlorosuccinic acid (2-chlorobutanedioic acid) with different reagents. Although the absolute configuration of each substance was not known at the time, it was clear that one of these processes occurred by inversion of configuration at the stereocenter and the other by retention.
A series of investigations definitely showed that the stereochemistry of a chiral substance is normally inverted during the course of an SN2 reaction.
To commend Walden for his work on the stereochemical course of substitution reactions, we sometimes refer to the stereochemistry of an SN2 reaction as Walden inversion.
The presence or absence of an asymmetric or a chiral carbon atom in a molecule is not only a criterion of dis-symmetry or chirality and hence, enantiomerism but it is certain that most of the molecules which have chiral carbon atoms are optically active.
Walden inversion was to develop a scheme for deciding by which mechanism a particular reaction had proceeded or would proceed. Ingold and co-workers showed how to distinguish whether a substitution occurred by the synchronous or the sequential route by kinetic criteria. They then went on to explore the structural features and reaction conditions that promoted one or other of these mechanistic routes.
Basically, which mechanism occurs is decided by which transition state is lower in energy. This can be analysed in structural terms, concerning things such as the energetic cost of breaking the initial bond, the steric environment of the transition states and the potential stabilizing effects of the proposed solvent etc. Typically, in synthesis one wants to employ the stereospecific SN2 mechanism because it gives a single predictable product.
Walden Inversion Explanation
Stereochemistry refers to the arrangement of atoms in space, and most particularly to the “handedness” of chemical compounds. When a carbon atom is bound to four distinct groups, those groups are arranged about the carbon as if they were at the vertices of a tetrahedron with the carbon at its center. As a result, there are two distinct such arrangements possible – these are non-superimposable mirror images of one another called enantiomers. Enantiomers have very similar chemical structures and so their chemical and physical behaviour is also very similar.
However, there are some chemical differences between them, and they have different optical activities, they rotate plane polarized light in opposite directions. Walden has shown that it was possible by a series of reactions to convert an optically active substance into its enantiomer. Chemists at the time had assumed that substitution reactions of the sort Walden employed would proceed in a way that either kept the spatial arrangement the same, or perhaps randomized the spatial arrangement.
Walden showed that at some point during his series of reactions, the groups around carbon had to systematically shift from one spatial arrangement to another. In the forty years after the discovery of Walden inversion, many additional inversions were discovered, however, not all substitution reactions on optically active starting products led to inversion, some resulted in the retention of configuration and others led to a mix of enantiomers products.
Frequently Asked Questions – FAQs
What is an inversion reaction?
Inversion, in chemistry, of the spatial re-arrangement of atoms or groups of atoms in a dissymmetric molecule, gives rise to a molecular structure form that is a mirror image of the original molecule.
What is retention and inversion?
Retention refers to the phase in which the molecular composition is preserved during the reaction. Inversion refers to the mechanism in which the structure of the molecules is changed during the course of the reaction. The result is thus of the opposite enantiomeric form relative to the reactant.
What do you mean by Walden inversion?
Walden’s inversion is the reversal of a chiral center in a molecule in a chemical reaction. Since a molecule can form two enantiomers around a chiral center, the Walden inversion converts the configuration of the molecule from one enantiomeric form to another.
Why does inversion occur in sn2?
Configuration reversal usually occurs when organic compounds undergo a nucleophilic substitution reaction by an SN2 mechanism. A Nucleophile (an electron-rich species with an affinity to an electron-deficient center) can strike the Stereocenter in two directions, such as from the front or from the back.
Why is iodine a better leaving group than chlorine?
The idea that the iodide ion is a strong base / nucleophile as it is quite capable of contributing a single pair of electrons to an electron deficiency site often correlates with the idea that it is a strong leaving party. This means it’s a stronger acid than HF, HCl, and HBr, so iodine is better off.
What is retention in configuration?
A system in which an atom’s relative configuration or absolute configuration is preserved. If the atom in question is a stereo core, structure retention normally (but not always) shifts the absolute configuration of R to R, and S to S.
What is relative configuration?
The location in space of atoms or groups with respect to (i.e. relative to) everything else in the molecule. Compare this with an absolute structure that is independent of atoms or groups in the molecule elsewhere.
What is difference between conformation and configuration?
Conformation is the set of potential forms that a molecule can only create by way of single bond rotation. Configuration is the relative location in a molecule of the atoms and can be altered purely by cleaving and creating new chemical bonds.