Inversion Chemical Reaction

Inversion is the process of shifting the molecule orientation by 180 degrees angle of a chiral centre in a molecule during a chemical reaction. Since a molecule can form two enantiomeric forms around a chiral nucleus, the Walden inversion shifts the configuration of a molecule from one enantiomeric form to the other. In an SN2 reaction, for example, Walden inversion occurs at a tetrahedral carbon atom. To get a sense of it, imagine an umbrella turned inside out in a gale. The Walden inversion occurs when the nucleophile attacks the backside of the reactant in an SN2 reaction, resulting in a product with the opposite structure as the reactant. As a consequence, during the SN2 reaction, the product inversion is 100%. This is known as Walden inversion. 

In 1896, chemist Paul Walden was the first to find it. He was able to convert one enantiomer of a chemical compound into the other enantiomer and back in a so-called Walden loop: 

• Chlorosuccinic acid was converted to malic acid (+).

• The hydroxyl group (OH) was substituted by chlorine molecules to the other isomer of chlorosuccinic acid in the next step by silver oxide in water molecules with configuration retention. 

• A second reaction with silver oxide (AgO) yielded (-) malic acid after a reaction with phosphorus pentachloride (PCl₅).

• After that, a second reaction with PCl₅ (phosphorus pentachloride) brought the cycle back to where it started.

Reaction Showing Inversion

1. The silver oxide acts as a hydroxide donor in the first step of the reaction shown below, while the silver ion has no effect. The carboxyl dianion A undergoes intramolecular nucleophilic substitution by the carboxylate anion (COO^-), resulting in the formation of a four-membered lactone ring B. While the carboxyl group is reactive as well, in silico data indicates that the transition state for the formation of the three-membered lactone is extremely strong. The lactone is opened by a hydroxide ion ring to form the alcohol C, and the net result of two counts of inversion is configuration retention.

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2. Nitrogen Inversion

In chemistry, nitrogen inversion (also known as umbrella inversion) is a fluxional phase in which the molecule "turns inside out" in nitrogen and amines. It's a rapid oscillation of the nitrogen atom and its substituents, with the nitrogen "running" through the plane created by the substituents (though the substituents still shift, but in the opposite direction); the molecule is in a planar transition state. 

Nitrogen inversion provides a low-energy mechanism for racemization for a compound that would otherwise be chiral due to a nitrogen stereocenter, rendering chiral resolution unlikely.

Nitrogen inversion is a form of pyramidal inversion that affects carbanions, phosphines, arsines, stibines, and sulfoxides, among other things.

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Energy Barrier in Inversion Process

At room temperature, the ammonia interconversion is extremely fast, inverting 30 billion times per second. The rapidity of the inversion is due to two factors: a low energy barrier (24.2 kJ/mol; 5.8 kcal/mol) and the barrier's narrow width, which allows for regular quantum tunnelling. At room temperature, however, phosphine (PH₃) inverts very slowly (energy barrier: 132 kJ/mol).

Quantum Effect on Inversion Process

Quantum tunnelling occurs in ammonia due to a thin tunnelling barrier rather than thermal excitation. Energy level splitting occurs when two states are superimposed, which is used in ammonia masers.

Mechanism of Inversion

When an organic compound undergoes a Nucleophilic substitution reaction through the SN₂ mechanism, inversion of configuration is common.

A Nucleophile (an electron-rich species with an affinity for an electron-deficient centre) may strike the Stereocenter from both the front and back sides.

The nucleophile attacks the stereocenter from the same side as the leaving party in a frontside assault. This results in Configuration Retention. During a chemical reaction or transformation, retention of configuration refers to the continuity of the spatial arrangement of bonds to an asymmetric core. With the help of an example, it's easy to understand:

The nucleophile strikes the stereocenter from the opposite side of the leaving party in a backside attack. As a consequence, a product with Inversion Of Configuration is developed.

Consider the following equation to understand how Retention of Configuration and Inversion of Configuration occur in an organic molecule undergoing nucleophilic substitution.

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The nucleophile strikes the stereocenter from the opposite side of the leaving party in a backside attack. As a consequence, a product with Inversion Of Configuration is developed.

Consider the following equation to understand how Retention of Configuration and Inversion of Configuration occur in an organic molecule undergoing nucleophilic substitution.

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The incoming nucleophile ‘Y' targeting the stereocenter Carbon here substitutes for halogen.

a) On the one hand, ‘Y' attacks from the front, resulting in a product with Retention Of Configuration as shown in structure A,  

b) On the other hand, ‘Y' attacks from the back, resulting in a product with Inversion Of Configuration as shown in structure B.

Let's take a closer look at the SN₂ reaction, which causes Inversion Of Configuration.

Nucleophilic Substitution-Bimolecular Reaction Mechanism (SN₂)

This form of nucleophilic substitution reaction has a second order kinetics, which means that the 

rate is determined by the concentrations of both the nucleophile and the haloalkane.

Rate = K [ R-X] [-OH]

Order = 1+1 = 2 (sum of power of concentration terms in rate law)

Some Important Features of SN₂ Mechanism

1.The reaction will be completed in a single step, which will be the Rate Determining Step. As a result, it is classified as 2nd order kinetics.

2.The configuration is inverted.

3.There is no carbocation formation. However, the creation of the transition state is involved.

4.The SN₂ mechanism is favoured by the presence of a primary alkyl group and a nonpolar solvent.

Hydrolysis of Sucrose

Sucrose is converted to glucose and fructose after hydrolysis breaks the glycosidic bond. However, since hydrolysis is so sluggish, sucrose solutions can sit for years with little change. However, if the enzyme sucrase is added, the reaction will proceed quickly. Hydrolysis can also be accelerated by using weak acids like cream of tartar or lemon juice. Similarly, during digestion, gastric acidity transforms sucrose to glucose and fructose, with an acetal bond between them that can be broken by an acid.

Hydrolysis releases about 1.0 kcal (4.2 kJ) per mole of sucrose, or about 3 small calories per gramme of substance, given (higher) heats of combustion of 1349.6 kcal/mol for sucrose, 673.0 for glucose, and 675.6 for fructose.

Let's look at why the hydrolysis of sucrose is called inversion. Sucrose has a dextrorotatory effect. However, following the hydrolysis reaction, glucose (which is dextrorotatory) and fructose (which is laevorotatory) are formed. As a result, sucrose hydrolysis is an inversion reaction.

C₁₂H₂₂O₁₁     +      H₂O  \[\xrightarrow[\Delta ]{H}\]     C₆H₁₂O₆      +       C₆H₁₂O₆   

Sucrose                            D-(+)-Glucose      D-(-)-Fructose

Did You Know that?

• In 1934, microwave spectroscopy was used for the first time to detect ammonia inversion.

• In one analysis, putting the nitrogen atom near a phenolic alcohol group delayed the inversion of an aziridine by a factor of 50 relative to the oxidised hydroquinone. The device interconverts through oxygen oxidation and sodium dithionite reduction.