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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7:57 minutes

Problem 21a

Textbook Question

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

Verified step by step guidance

Step 1: Identify the role of LDA (Lithium Diisopropylamide). LDA is a strong, non-nucleophilic base commonly used to deprotonate the alpha-hydrogen of carbonyl compounds, forming an enolate intermediate.

Step 2: Locate the alpha-hydrogens in the ester compound. Alpha-hydrogens are the hydrogens attached to the carbon adjacent to the carbonyl group. In this case, the alpha-carbon is the one next to the carbonyl group in the ethyl ester.

Step 3: Deprotonation by LDA. LDA will abstract one of the alpha-hydrogens, forming an enolate ion. The enolate is stabilized by resonance between the oxygen of the carbonyl group and the alpha-carbon.

Step 4: Alkylation reaction. The enolate ion acts as a nucleophile and attacks the electrophilic alkyl halide (ethyl chloride, ClCH2CH3). This results in the formation of a new carbon-carbon bond at the alpha-carbon.

Step 5: Predict the product. The final product will be the ester with an ethyl group added to the alpha-carbon. The structure of the product will reflect the new substitution at the alpha-carbon position.

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Key Concepts

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

Enolate Formation

Enolates are nucleophilic species formed by the deprotonation of an alpha carbon adjacent to a carbonyl group. In the presence of a strong base like LDA (Lithium diisopropylamide), the hydrogen atom on the alpha carbon is abstracted, resulting in the formation of an enolate ion. This enolate can then act as a nucleophile in subsequent reactions, such as alkylation.

Alkylation Reactions

Alkylation involves the introduction of an alkyl group into a molecule, typically through a nucleophilic substitution reaction. In this context, the enolate formed from the ester can attack an alkyl halide, such as ethyl chloride, leading to the formation of a new carbon-carbon bond. This process is crucial for building complex organic molecules and expanding carbon skeletons.

Reactivity of Esters and Nitriles

Esters and nitriles can both undergo enolate formation, but their reactivity differs due to the electronic effects of their functional groups. Esters, with their electron-withdrawing carbonyl, can stabilize the enolate formed, making them suitable substrates for alkylation. Nitriles, while also capable of forming enolates, may exhibit different reactivity patterns due to their triple bond and the nature of the leaving group in alkylation reactions.

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Textbook Question

For each molecule shown below,

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

(a)

(b)

(c)

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1. Rank the following compounds in order of increasing acidity.

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Textbook Question

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

(a) acetone

(b) cyclopentanone

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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?

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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)

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Textbook Question

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

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