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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2:48 minutes

Problem 1a,b

Textbook Question

Phenylacetone can form two different enols.
(a) Show the structures of these enols.
(b) Predict which enol will be present in the larger concentration at equilibrium.

Verified step by step guidance

Step 1: Understand the structure of phenylacetone. Phenylacetone is a ketone with the structure C6H5-CH2-CO-CH3. It contains an alpha-carbon (the carbon adjacent to the carbonyl group) with hydrogens that can be deprotonated to form enols.

Step 2: Identify the possible enols. Enols are formed by the keto-enol tautomerization process, where a proton is transferred from the alpha-carbon to the oxygen of the carbonyl group, resulting in a double bond between the alpha-carbon and the carbonyl carbon. For phenylacetone, there are two alpha-carbons: one adjacent to the phenyl group and one adjacent to the methyl group. Draw the two possible enol structures: (1) enol with the double bond between the phenyl group and the carbonyl carbon, and (2) enol with the double bond between the methyl group and the carbonyl carbon.

Step 3: Analyze the stability of the enols. The stability of enols is influenced by factors such as conjugation and hydrogen bonding. The enol with the double bond adjacent to the phenyl group will benefit from conjugation with the aromatic ring, which stabilizes the structure. The enol with the double bond adjacent to the methyl group does not have this stabilization.

Step 4: Predict the equilibrium distribution. At equilibrium, the more stable enol will be present in a larger concentration. Since the enol with the double bond adjacent to the phenyl group is stabilized by conjugation, it is expected to be the predominant enol at equilibrium.

Step 5: Summarize the findings. The two enols of phenylacetone are (1) the enol with the double bond adjacent to the phenyl group and (2) the enol with the double bond adjacent to the methyl group. The enol with the double bond adjacent to the phenyl group will be present in a larger concentration at equilibrium due to its greater stability from conjugation with the aromatic ring.

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Key Concepts

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

Enol Tautomerism

Enol tautomerism is a chemical equilibrium between a keto form and its corresponding enol form. In this process, a hydrogen atom shifts from the carbon adjacent to the carbonyl group to the carbonyl carbon, resulting in the formation of an alkene with a hydroxyl group. Understanding this concept is crucial for predicting the structures of enols formed from phenylacetone.

Stability of Enols

The stability of enols is influenced by factors such as sterics, electronic effects, and resonance. Enols that can stabilize their double bond through resonance or that are less sterically hindered tend to be more stable and thus present in higher concentrations at equilibrium. Analyzing these factors helps in predicting which enol will dominate in phenylacetone.

Equilibrium Concentration

Equilibrium concentration refers to the relative amounts of reactants and products present in a chemical reaction at equilibrium. In the context of enols, the concentration of each enol at equilibrium can be predicted by considering their relative stabilities. The more stable enol will generally be favored, leading to a higher concentration compared to the less stable form.

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Related Practice

Textbook Question

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

(a) ethyl acetoacetate

(b) pentane-2,4-dione

Textbook Question

When cis-2,4-dimethylcyclohexanone is dissolved in aqueous ethanol containing a trace of NaOH, a mixture of cis and trans isomers results. Propose a mechanism for this isomerization.

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.

(d)

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.

(e)

Textbook Question

For each molecule shown below,

1. indicate the most acidic hydrogens.

(a)

(b)

(c)

Textbook Question

Draw the products of the following reactions:

Textbook Question

Draw the products of the following reactions:

Textbook Question

Alkylation of the following compound with methyl iodide under two different conditions forms two different ketoesters (A and B). Each ketoester forms a cyclic diketone (C and D) when treated with methoxide ion in methanol. a. Draw the structures of A and B, and indicate the conditions used in the alkylation reaction that cause that ketoester to be formed.

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Phenylacetone can form two different enols.(a) Show the structures of these enols.(b) Predict which enol will be present in the larger concentration at equilibrium.

Key Concepts

Enol Tautomerism

Stability of Enols

Equilibrium Concentration

Watch next

Phenylacetone can form two different enols.
(a) Show the structures of these enols.
(b) Predict which enol will be present in the larger concentration at equilibrium.