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

24. Enolate Chemistry: Reactions at the Alpha-Carbon

Enolate

Organic Chemistry

24. Enolate Chemistry: Reactions at the Alpha-Carbon

Enolate

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5:15 minutes

Problem 46c

Textbook Question

Show the resonance forms for the enolate ions that result when the following compounds are treated with a strong base.
(c) ethyl α-cyanoacetate

Verified step by step guidance

Identify the acidic hydrogen in ethyl α-cyanoacetate. The most acidic hydrogen is the one on the α-carbon (the carbon adjacent to both the ester group and the nitrile group) because it is flanked by two electron-withdrawing groups, which stabilize the resulting enolate ion.

Treat the compound with a strong base (e.g., hydroxide or alkoxide). The base will abstract the acidic hydrogen from the α-carbon, forming the enolate ion. This results in a negative charge on the α-carbon.

Draw the first resonance form of the enolate ion. In this form, the negative charge resides on the α-carbon, and the double bond remains between the α-carbon and the carbonyl carbon of the ester group.

Draw the second resonance form by delocalizing the negative charge. Move the lone pair of electrons from the α-carbon to form a double bond with the carbonyl carbon, and push the π-electrons of the carbonyl group onto the oxygen atom. This places the negative charge on the oxygen atom of the ester group.

Draw the third resonance form by delocalizing the negative charge further. Move the lone pair of electrons from the α-carbon to form a double bond with the nitrile carbon, and push the π-electrons of the nitrile group onto the nitrogen atom. This places the negative charge on the nitrogen atom of the nitrile group.

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Key Concepts

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

Enolate Ion Formation

Enolate ions are formed when a strong base abstracts a proton from the alpha carbon of a carbonyl compound, resulting in a resonance-stabilized anion. This process is crucial in reactions involving carbonyl compounds, as it allows for nucleophilic attack on electrophiles. In the case of ethyl α-cyanoacetate, the presence of the cyano group influences the stability and reactivity of the enolate formed.

Resonance Structures

Resonance structures are different Lewis structures for the same molecule that illustrate the delocalization of electrons. In the context of enolate ions, resonance allows for the distribution of negative charge across multiple atoms, enhancing stability. Understanding how to draw these structures is essential for predicting the behavior of enolate ions in chemical reactions.

Alpha-Substituted Carbonyl Compounds

Alpha-substituted carbonyl compounds, like ethyl α-cyanoacetate, have functional groups attached to the carbon adjacent to the carbonyl group. These substituents can significantly affect the acidity of the alpha hydrogen and the stability of the resulting enolate ion. Recognizing the influence of these substituents is key to understanding the reactivity and resonance forms of the enolate ions generated.

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Related Practice

Textbook Question

Give the important resonance forms for the possible enolate ions of the following:

(a) acetone

(b) cyclopentanone

Textbook Question

In Chapter 20, we studied the aldol reaction. Although not discussed at the time, this reaction is stereospecific, proceeding through the Zimmerman–Traxler transition state shown here.

(a) Show an arrow-pushing mechanism for this concerted reaction.

(b) Why is this a favorable mechanism?

Open Question

LDA can be used to form enolates on esters and nitriles. Predict the product of these alkylation reactions.

(a)

Textbook Question

Identify the enolate(s) that would form on treatment of each of the following carbonyls with base. [When there are two possibilities, draw both.]

(a)

Textbook Question

Show the resonance forms for the enolate ions that result when the following compounds are treated with a strong base.

(d) nitroacetone

Textbook Question

For each molecule shown below,

1. indicate the most acidic hydrogens.

2. draw the important resonance contributors of the anion that results from removal of the most acidic hydrogen.

(g)

Textbook Question

For each molecule shown below,

1. indicate the most acidic hydrogens.

2. draw the important resonance contributors of the anion that results from removal of the most acidic hydrogen.

(h)

Textbook Question

Give the important resonance forms for the possible enolate ions of the following:

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Show the resonance forms for the enolate ions that result when the following compounds are treated with a strong base.(c) ethyl α-cyanoacetate

Key Concepts

Enolate Ion Formation

Resonance Structures

Alpha-Substituted Carbonyl Compounds

Watch next

Show the resonance forms for the enolate ions that result when the following compounds are treated with a strong base.
(c) ethyl α-cyanoacetate