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Inversion is the process in which the structure of the molecules is changed during the course of the reaction whereas retention refers to the phase in which the molecular composition is preserved during the reaction. There is number of different mechanisms which are included such as acid-catalyzed rearrangement, ring-opening metathesis, and chiral inversion. Inversion can also be explained as the rearrangement of atoms or groups of atoms in a dissymmetric molecule, giving rise to a product with a molecular configuration which is like a mirror image of the original molecule. The phenomenon of inversion where the reaction of an atom or group of atoms are replaced by another atom or group is known as Walden inversion in 1895, by a German chemist, Paul Walden.
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Key terms: Inversion, chemical reaction, Walden Inversion, SN2, Nucleophile, Phenomenon, Acid, Molecule, Organic molecule
What is Walden Inversion?
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During an SN2 reaction a Walden inversion occurs at a tetrahedral carbon atom. In a molecule, Walden inversion is the reversal of a chiral center in a chemical reaction. Around this chiral center molecule can form two enantiomers, Walden inversion transforms the structure of the molecule from one enantiomeric form to another. At the reaction center, Walter inversion has an inversion stereochemistry.
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Walden Inversion Reaction
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In 1896 inversion reaction was discovered and named after Russian, Latvian, and German chemist Paul Walden reported that an inversion of optical rotation in the conversion of malic to chlorosuccinic acid by reaction with phosphorus pentachloride.
This inversion by Walden was deeply studied and explained for the change in configuration suggested by Werner in 1911 and is commonly known as the opposite face mechanism for the Walden inversion.
What is Walden Inversion Mechanism?
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The mechanism of Walden Inversion is the same as an SN2 reaction where Nucleophilic attack on the carbon atom forms an intermediate. This step which is followed by the displacement of the leaving group resulting in the product is shown below:
What is Nucleophilic Substitution-Bimolecular Reaction Mechanism (SN2)?
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There are two mechanistic models where an alkyl halide can undergo nucleophilic substitution. In 1st step of its reaction where bond-forming and bond-breaking occur simultaneously. In all figures below 'X' will indicate a halogen substituent.
This is called as SN2 mechanism where S stands for 'substitution', the subscript N stands for 'nucleophilic', and the number 2 refers to the fact that this is a bimolecular reaction:
During the reaction mechanism where nucleophilic substitution reaction of the leaving group (which generally consists of halide groups or other electron-withdrawing groups) with a nucleophile in a given organic compound.
At a transition state, three 'R' substituents and the electrophilic carbon all lie on the same plane.
Features of SN2
- This reaction I completed in a single step.
- Inverted configuration
- No carbocation formation whereas the creation of the transition state is involved.
What do you understand about Nitrogen Inversion?
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In case of Nitrogen Inversion where the chemical reaction inverts the configuration of the nitrogen atom in a molecule. In this chemical reaction, an electron lone pair pushes the three bonded groups from one side to the other and inverts the configuration. In the N-H form, the nitrogen atom is bonded to a hydrogen atom in the N-L form, and the nitrogen atom is bonded to a lone pair of electrons.
What is the Quantum Effect on Inversion Process?
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Quantum tunneling is a process that occurs in ammonia due to a thin tunnelling barrier rather than thermal excitation.
Things to Remember
- Nitrogen inversion which is also known as umbrella inversion is a fluxional phase in which the molecule "turns inside out" in nitrogen and amines.
- SN2 reaction is a nucleophilic substitution where the bond is broken and another is formed synchronously.
- There are two ways in which a nucleophile can attack the stereocenter of the substrate i.e. frontside attack and backside attack.
- SN1 is unimolecular, carbocation can form as an intermediate and is a two-stage system whereas SN2 is bimolecular, carbocation cannot be formed and is a one-stage process.
- With the increase of concentration of nucleophiles and substrates that will increase the chance of collision.
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Sample Questions
Ques. Select the correct answer from the following statements: (1 mark)
(a) Retention refers to the phase in which the structure of the molecules is changed during the course of the reaction.
(b) Inversion refers to the mechanism in which the structure of the molecules is changed during the course of the reaction.
(c) Retention refers to the phase in which the molecular structure is preserved during the reaction.
Ans. (b) Inversion refers to the mechanism in which the structure of the molecules is changed during the course of the reaction.
Ques. Which of the following is the correct mechanism for the following image? (2 marks)
(a) Nucleophilic Substitution-Bimolecular Reaction Mechanism (SN2)
(b) Walden Inversion Mechanism
(c) Quantum Effect on Inversion
(d) None of these
Ans. (b) Walden Inversion Mechanism
Walden Inversion Mechanism of Walden Inversion is the same as an SN2 reaction where Nucleophilic attack on the carbon atom forms an intermediate.
Ques. Which of the following are the correct inversion chemical reactions? (1 mark)
a) Nucleophilic attack by SN2 mechanism
b) Hydrolysis of sucrose
c) Both of these
d) None of these
Ans. (c) Both of these
Ques. Out of the two which will undergo SN2 reaction faster in the following pair and why? (2 marks)
Ans. CH3CH2Br reacts faster because it is a primary halide (1° halide). It stands for Ethyl bromide. It appears as a colorless volatile liquid which is slightly soluble in water and denser than water. Flash point below 0°F.
Ques. Out of the two, in the following pairs of substances undergoes SN2 substitution reaction faster and why? (2 marks)
Ans.
It is a molecular formula that consists of the chemical symbols for the constituent elements followed by numeric subscripts describing the number of atoms of each element present in the molecule.
Ques: Why does inversion occur in SN2? (3 marks)
Ans: When organic compounds undergo a nucleophilic substitution reaction by an SN2 mechanism configuration reversal usually occurs. A Nucleophile is a chemical species that forms bonds with electrophiles by donating an electron pair that strikes the Stereocenter in two directions, such as from the front or from the back.
Ques: What is relative configuration? (2 marks)
Ans: The position of atoms or groups in space in relation to (i.e., relative to) something else in the molecule compared to an absolute structure that is independent of atoms or groups in the molecule elsewhere.
Ques: What are the factors responsible for favoring elimination/substitution reactions? (3 marks)
Ans: The factors which are responsible for favoring eliminations/ substitution reactions are:
- Nature of the substrate present in the reaction
- Size and strength of the base/nucleophile- If the nucleophile is highly reactive there is a possibility that the reaction taking place will be a substitution reaction (SN2).
- Conditions involved in reaction (solvent used in the reaction).
Ques: How do Haloalkanes react with metals? (2 marks)
Ans: Haloalkanes on reaction with metals form a special class of compounds – organometallic compounds. Alkyl Halides or R-X reacts with certain metals to produce compounds containing C-metal bonds.
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