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Key Highlights
- Claisen condensation reaction is also known as the organic coupling reaction that proceeds in the presence of a strong base.
- The products formed after this reaction are beta-keto ester or beta-diketone.
- A strong base such as sodium ethoxide, potassium ethoxide, etc. helps in the removal of alpha protons.
- Nucleophiles are called electron-donating species and electrophiles are called electron-accepting species.
- Classic, Crossed, and Dieckman are the three types of Claisen condensation reactions.
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Key Terms: Claisen condensation, Claisen condensation reaction, Alpha hydrogen, Ester
When a single carbon is formed between either two ester compounds or in one ester and carbonyl compound, the reaction is called the Claisen condensation reaction.
Claisen Condensation Definition
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Claisen condensation reaction is defined as the coupling reaction in which beta ketone or beta ester is formed by the deprotonation of alpha hydrogen.
- In the presence of a strong base, self-condensation of esters also takes place.
- The formation of an enolate anion marks the beginning of the Claisen condensation reaction.
Example of Claisen Condensation Reaction
Claisen Condensation Reagents
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The reagents used in the Claisen Condensation reaction are:
- A strong base such as sodium ethoxide is used in this reaction.
- Enolizable (with alpha hydrogen )and non-enolizable (without alpha hydrogen) esters are utilized in the reaction.
Mechanism of Claisen Condensation
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The Claisen Condensation reaction occurs in three steps that are explained below:
Step-1
In the first step of the reaction, the removal of the alpha proton (hydrogen adjacent to the carbon) takes place with the help of a strong base. Enolate ion is formed in this step. The reaction can be given as:
RCOOR’ + OR– (Base) → R’--COO– + R–OH
Step-2
A tetrahedral alkoxide intermediate product is formed by the nucleophilic attack on the carbonyl carbon of the second ester compound. Here, the enolate ion serves the role of nucleophile in the reaction. A generalized reaction can explain this:
R’--C(=O)O– + R”--CO–R”’ → R’--C(=O)--O–R” + R”---C(=O)--R”’
Step-3
The negative charge of the enolate ion is neutralized by the addition of dilute acid (dilute phosphoric acid). This is followed by the formation of beta--keto ester or beta-diketone as the product of the reaction. Alkoxide acting as a leaving group is removed in this step.
R’--C(=O)--O–R” → R’--C(=O)--R” + ROH
Variations of Claisen Condensation Reaction
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Three main types of Claisen condensation reactions are as follows:
- Classic Claisen Condensation- A type of reaction in which self-condensation of two ester compounds occurs is called classic condensation.
- Crossed Claisen Condensation- The reaction in which an enolizable ester reacts with a non-enolizable ester is called crossed Claisen condensation.
- Dieckmann Condensation- In this reaction, a cyclic beta-keto ester is formed by the intermolecular reaction between two ester compounds.
Examples of Claisen Condensation
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The applications of Claisen condensation can be seen in our day-to-day lives. Some of the examples are:
- The products of the Claisen condensation reaction are used in the synthesis of long-chain fatty acids, and other macromolecules.
- This is utilized in the manufacture of drugs, and medicines such as the anticonvulsant drug phenytoin.
- The Claisen condensation is utilized in the synthesis of the core of antibiotics such as A ring in erythromycin antibiotic.
Sample Questions
Ques. What are the examples of enolizable and nonenolizable esters? (2 marks)
Ans. The presence of alpha hydrogen in enolizable esters distinguishes it from non-enolizable esters.
- Ethyl acetate and methyl propionate are the two common examples of enolizable esters.
- An example of a non-enolizable ester is benzoyl chloride.
Ques. What are nucleophiles and electrophiles? (2 marks)
Ans. An electron-rich species known as a nucleophile transfers electron pairs to species that lack electrons.
- Ammonia, water, and carbanions are a few examples of nucleophiles.
- On the other hand, electrophiles are electron-deficient species and can accept a pair of electrons from species with abundant electrons.
- Carbocations and carbonyl compounds are two examples of electrophiles.
Ques. What are the factors that affect the Claisen condensation reaction? (3 marks)
Ans. Four important factors affect the reaction rates:
- Reaction Temperature- The optimum temperature at which a reaction is carried is called reaction temperature.
- Base- Deprotonation and enolate formation are the crucial steps in the Claisen condensation reaction. Therefore, a strong base is always preferred for this reaction.
- Concentration- The concentration of reactants should be neither too much nor too little as this impacts the reaction rate.
Ques. What do you understand by the condensation reaction? (2 marks)
Ans. The reaction in which a single large molecule is formed by the combination of two molecules is called a condensation reaction. In organic chemistry, two types of condensation reactions are mostly studied- Aldol condensation & Claisen condensation reaction.
Ques. What are the differences between Claisen condensation and aldol condensation? (3 marks)
Ans. The differences between Claisen condensation and aldol condensation are as follows:
| Claisen Condensation | Aldol Condensation |
|---|---|
| In the Claisen condensation reaction, a single carbon bond is formed between ester compounds. | When an enolate ion combines with a carbonyl compound, either aldehyde or ketone is formed. This reaction is called the aldol condensation reaction. |
| The reaction is catalyzed by only base (strong base). | This can be catalyzed by both acid and base. |
| Beta-keto ester or beta-diketone is formed at the end of this reaction. | The beta-hydroxy aldehyde or beta-hydroxy ketone is formed as an end product. |
Ques. Why is the Claisen condensation reaction called a reversible reaction? (2 marks)
Ans. Claisen condensation reaction takes place under equilibrium conditions. Moreover, the presence of alpha hydrogen and a carbonyl group in the ester compound makes this reaction reversible.
Ques. Who discovered the Claisen condensation coupling reaction? (1 mark)
Ans. Rainer Ludwig Claisen was the scientist who discovered and explained the Claisen condensation coupling reaction.
Ques. Can non-enolizable esters undergo a Claisen condensation reaction? (2 marks)
Ans. The presence of alpha hydrogen is necessary for the Claisen condensation reaction. However, non-enolizable esters such as methyl formate lack alpha hydrogen. Therefore, they can not under Claisen condensation.
Ques. What is the importance of Claisen condensation? (2 marks)
Ans. The products of Claisen condensation are widely used in:
- Biosynthesis of fatty acids, and terpenes,
- Formation of polymers,
- Manufacturing pharmaceutical, and agrochemical products, etc.
Ques. What are the strong bases that are commonly used in Claisen condensation? (1 mark)
Ans. The two most commonly used strong bases are sodium ethoxide and potassium tert-butoxide. These bases help in the easy removal of alpha hydrogen.
Ques. What do you understand about retro Claisen condensation? (2 marks)
Ans. This reaction is the opposite of Claisen condensation. In this, beta-keto ester or beta-diketone is rearranged and broken into two ester compounds. This is preceded by the help of strong acids.
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