Table of Contents
Introduction
A new carbon-carbon bond is created when molecules of ester-containing at least one hydrogen atoms are exposed to alkoxide ions, resulting in the creation of keto ester. Claisen condensation is the popular name for this reaction. The Claisen condensation process is an organic coupling reaction that produces a C-C bond between two esters or a single ester and one carbonyl molecule. When a strong base is present, the reaction continues, yielding a beta-keto ester or a beta-diketone as the end product.
Rainer Ludwig Claisen, a German chemist, invented the reaction and named it after him. This reaction is depicted in the following diagram. The Claisen condensation reaction occurs when an alpha proton is removed by a strong base, resulting in the creation of an enolate ion.
The Claisen condensation reaction is an organic coupling process that results in the creation of a C-C bond between two esters or between a single ester and one carbonyl molecule. When a strong base is present, the reaction occurs, yielding a beta-keto ester or a beta-diketone as a result.
Claisen Condensation Mechanism
Using a strong base, an alpha proton can be extracted. As a result of this reaction, the enolate ion forms. This enolate anion is relatively stable due to the negative charge delocalization (electrons).
The enolate anion now attacks the carbonyl carbon of the second ester reactant with a nucleophilic assault. The alkoxy group is removed, and the alcohol’s conjugate base is regenerated. The doubly alpha proton that is generated is removed by this alkoxide ion, resulting in the formation of a new enolate anion that is now resonance stabilised.
To neutralise the negative charge on the enolate anion as well as any remaining bases, an aqueous acid (phosphoric acid or sulphuric acid, for example) is added. This produces a beta-diketone or beta-keto ester, which may be separated right away.
As a result, the ester (or carbonyl compound and ester) reactants become beta-keto esters or beta-diketones. The Claisen condensation reaction has been tweaked into a few different versions, which are explained in the following section.
Aldol condensation
In organic chemistry, an aldol condensation occurs when an enol or enolate ion combines with a carbonyl chemical to generate a -hydroxyaldehyde or -hydroxy ketone (an aldol reaction), which is then dehydrated to provide a conjugated enone. Aldol condensation is also a term that is often used in biochemistry to refer to the initial (addition) stage of the process—the aldol reaction—as performed by aldolases. The aldol reaction, on the other hand, is not technically a condensation reaction because it does not result in the loss of a tiny molecule.
Condensation of crossed aldols
When two different carbonyl compounds containing -hydrogen(s) undergo aldol condensation, the outcome is a crossed aldol condensation. Because either carbonyl molecule can operate as the nucleophile and self-condensation is possible, this normally results in four potential products, making the mixture synthetically worthless. However, if one of the molecules does not include a -hydrogen, making it non-enolizable, this difficulty can be avoided. The ketone acts as the nucleophile in an aldol condensation between an aldehyde and a ketone because its carbonyl carbon does not have a high electrophilic character due to the +I effect and steric hindrance.
Types of condensation
- It’s critical to distinguish aldol condensation from other carbonyl compound addition processes.
- A Knoevenagel condensation occurs when the base is an amine and the active hydrogen compound is sufficiently activated.
- The aldehyde in a Perkin reaction is aromatic, and the enolate is formed from an anhydride.
- Claisen–Schmidt condensation occurs when a -hydrogen aldehyde or ketone reacts with an aromatic carbonyl molecule that lacks a -hydrogen.
- Two ester compounds are involved in a Claisen condensation.
- Dieckmann condensation produces a cyclic compound by combining two ester groups in the same molecule.
- An aldehyde and an aliphatic nitro molecule are used in the Henry reaction.
- An,-unsaturated ketone and a carbonyl group are involved in a Robinson annulation, which begins with a Michael reaction before the aldol condensation.
- An aldehyde generated in situ from an alcohol self-condenses to the dimerized alcohol in the Guerbet reaction.
- Water is eliminated from the Japp–Maitland condensation by a nucleophilic displacement rather than an elimination process.
Variations
The following are some of the most important versions of this organic coupling reaction.
The Claisen condensation process is a reaction that takes place between two molecules of an ester-containing chemical that is enolizable.
The intramolecular reactions of two ester groups belonging to the same molecule are involved in the Dieckmann condensation reaction. A cyclic beta-keto ester is formed as a result of the reaction.
The crossed Claisen condensation is a process that occurs when an enolizable ester/ketone reacts with a non-enolizable ester.
The Stobbe condensation process, which can proceed with relatively weaker bases, is another variation of this reaction. It usually includes the usage of a succinic acid-derived diethyl ester.
FAQs
Q. What are the several Claisen condensation types?
Ans: The Claisen condensation process has several different types, which are listed below.
- The ‘classic’ Claisen condensation reaction, in which two molecules contain an enolizable ester self-condensate.
- One of the molecules in the ‘crossed’ Claisen condensation reaction (also known as mixed Claisen condensation) is an enolizable ester, whereas the other is a ketone or a non-enolizable ester.
- An intramolecular reaction in which a molecule with two ester groups undergoes the Dieckmann condensation reaction.
In Claisen reactions, what products are formed?
Claisen condensations typically produce -ketoesters, which are desired. In the field of organic chemistry, they are well-known for being particularly valuable synthetic intermediates. The necessary products are formed when an ester enolate combines with another ester molecule.
How does the Claisen condensation reaction work?
The very basic environment first removes a -proton, allowing the synthesis of an enolate ion to proceed. The delocalization of electrons stabilizes this enolate ion. The enolate ion then attacks the carbonyl carbon of the second ester with a nucleophilic assault. The alkoxy group is then removed, resulting in the production of a new enolate that is stabilized by resonance. Following the addition of a sufficient acid to neutralize the enolate, the beta-keto ester or beta-diketone can be separated.
