Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

25. Condensation Chemistry

Claisen Condensation

Organic Chemistry

25. Condensation Chemistry

Claisen Condensation

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5:25 minutes

Problem 35

Textbook Question

Esters with only one α hydrogen generally give poor yields in the Claisen condensation. Propose a mechanism for the Claisen condensation of ethyl isobutyrate, and explain why a poor yield is obtained.

Verified step by step guidance

Step 1: Begin by understanding the Claisen condensation mechanism. The Claisen condensation involves the reaction of two ester molecules in the presence of a strong base, typically an alkoxide, to form a β-keto ester. The reaction requires at least one α-hydrogen on the ester to proceed effectively.

Step 2: Analyze the structure of ethyl isobutyrate. Ethyl isobutyrate has only one α-hydrogen attached to the carbon adjacent to the ester group. This α-hydrogen is necessary for enolate ion formation, which is the key intermediate in the Claisen condensation.

Step 3: Describe the enolate formation step. The base abstracts the α-hydrogen from ethyl isobutyrate, forming an enolate ion. The enolate ion is resonance-stabilized, with the negative charge delocalized between the oxygen atom of the ester group and the α-carbon. Represent this step using the formula:

{CH}_{3} {CH}_{2} {CO}_{2} Et \to {CH}_{2} {CH}_{2} {CO}_{2} Et -

Step 4: Explain the nucleophilic attack step. The enolate ion attacks another molecule of ethyl isobutyrate at the carbonyl carbon, forming a tetrahedral intermediate. This intermediate then collapses, expelling the alkoxide group and forming the β-keto ester product.

Step 5: Discuss why the yield is poor. Ethyl isobutyrate has only one α-hydrogen, which limits the formation of enolate ions. Additionally, steric hindrance from the isobutyrate group reduces the efficiency of the nucleophilic attack. These factors contribute to the poor yield in the Claisen condensation of ethyl isobutyrate.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Claisen Condensation

Claisen condensation is a reaction between two esters or an ester and a carbonyl compound in the presence of a strong base, leading to the formation of a β-keto ester or a β-diketone. The mechanism involves the deprotonation of an α-hydrogen by the base, generating an enolate ion that attacks the carbonyl carbon of another ester, resulting in the formation of a new carbon-carbon bond.

α Hydrogen

α hydrogens are the hydrogen atoms attached to the carbon atom adjacent to a carbonyl group. Their presence is crucial in reactions like the Claisen condensation, as they can be deprotonated to form enolate ions. Esters with fewer α hydrogens, such as those with bulky or branched groups, are less likely to form enolates, leading to lower reactivity and poor yields in the condensation reaction.

Steric Hindrance

Steric hindrance refers to the repulsion between bulky groups in a molecule that can impede reactions. In the case of ethyl isobutyrate, the presence of a bulky isobutyl group can hinder the approach of the base to the α hydrogen, making it more difficult to form the enolate. This steric effect contributes to the poor yield observed in the Claisen condensation of esters with limited α hydrogens.

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Related Practice

Textbook Question

Propose mechanisms for the following reactions.

(b)

Textbook Question

Propose a mechanism for the self-condensation of methyl 3-phenylpropionate promoted by sodium methoxide.

Textbook Question

Predict the products of the following Claisen condensations.

(c)

Textbook Question

Ethoxide is used as the base in the condensation of ethyl acetate to avoid some unwanted side reactions. Show what side reactions would occur if the following bases were used.

(a) sodium methoxide

(b) sodium hydroxide

Textbook Question

Predict the products of self-condensation of the following esters.

(a) methyl propanoate + NaOCH₃

(b) ethyl phenylacetate + NaOCH2CH₃

Textbook Question

Predict the products from crossed Claisen condensation of the following pairs of esters. Indicate which combinations are poor choices for crossed Claisen condensations.

(a)

(b)

Textbook Question

Predict the products from crossed Claisen condensation of the following pairs of esters. Indicate which combinations are poor choices for crossed Claisen condensations.

(c)

(d)

Textbook Question

Show how you would use an aldol, Claisen, or another type of condensation to make each compound.

(d)

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