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ALPHA CARBON CHEMISTRY: ENOLS AND ENOLATES

A STUDENT SHOULD BE ABLE TO:

1. Identify α-carbons, α-hydrogens, and the bases strong enough to deprotonate the α-

carbons.

2. Draw the mechanisms for acid-catalyzed and base-catalyzed keto-enol tautomerization.

3. Predict products of and propose syntheses using the following types of reactions:

Acid-catalyzed halogenation of ketones at the α-position

Hell-Volhard-Zelinski reaction

Haloform reaction

Aldol reaction and aldol condensation. Aldol reaction products are formed

without heat (room temperature or lower). Aldol condensation products occur

under high temperatures or when extended conjugation within the product can

be achieved.

Claisen condensation

Alkylation of enolates

Acetoacetic ester synthesis (alkylation at the α-carbon)

Michael additions

4. Propose mechanisms for, and predict and interpret experimental results on the basis of

mechanism, for the following reactions:

Keto-enol tautomerism

Aldol, crossed aldol, and related condensations

Decarboxylation of beta-keto acids

Claisen condensation

Acetoacetic ester synthesis

Michael additions

5. Predict and interpret spectra to identify functional groups with α-carbons and hydrogens.

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To best prepare for this module, please work appropriate Skill Builder problems in the textbook.

A STUDENT WHO HAS MASTERED THE OBJECTIVES FOR THIS UNIT SHOULD BE

ABLE TO SOLVE THE FOLLOWING PROBLEMS AND RELATED ONES:

1.1 In the following acid-catalyzed tautomerization, identify the keto and enol forms. For the

keto form, circle the -carbons and hydrogens.

1.2 Which of these bases deprotonates acetone irreversibly? Use pKa data from your book.

1.3 Which of the bases shown in Problem 1.2 deprotonates acetone reversibly?

2.1 Draw the enol product. Then, show the mechanism for the base-catalyzed tautomerization

of acetone, using arrows and including lone pairs.

2.2 Draw the structure of the keto tautomer of phenol.

3.1 Predict the product(s) of each of the following reactions.

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3.1

3.2 Propose a synthesis of each of the compounds shown.

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3.2

3.3 Predict the product(s) of the following reactions of α,β-unsaturated ketones.

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4. Propose a mechanism for each of the following reactions. Include all lone pairs,

intermediates, and arrows.

5. Identify the following unknown from the given spectroscopic information.

Chemical Formula: C5H8O

IR: peak at 1715 cm-1

1H NMR:

doublet, δ 2.1, 3H

triplet, δ 3.1, 2H

multiplet, δ 5.6, 1H

multiplet, δ 5.7, 1H

triplet, δ 9.7, 1H

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SOLUTIONS TO SAMPLE PROBLEMS:

1.1

1.2 pKa for conjugate acid, with data from Table 3.1. If exact molecule is not available, the

closest example is given.

50 4.75 ~ -7 ~38 ~16

The pKa for acetone is 19.2. Only the bases with significantly weaker conjugate acids

will deprotonate acetone irreversibly: a and d

1.3 Using the data above, only bases with conjugate acids of similar pKa to acetone will

deprotonate acetone reversibly: e

2.1

2.2 The keto tautomer of phenol:

3.1 Predict the product(s):

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3.1 continued

3.2 Propose a synthesis:

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3.2 continued

3.3

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3.3 continued

4.

5. 3-pentenal

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Name ______________________________________________ Eighth Drill Test (Sample A)

Organic Chemistry 2220D Answer All Questions

1. Circle all carbons, and draw in all hydrogens.

2. Predict the product(s) of each of the following reactions (if any):

3. Propose a mechanism for each of the following reactions:

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Name ______________________________________________ Eighth Drill Test (Sample B)

Organic Chemistry 2220D Answer All Questions

1. Circle the bases that are often used to generate the enolate from CH3CH=O.

a) NaOH b) CH3ONa c) NaCl d) NaH e) NaNH2

Which of the bases above generate the enolate from CH3CH=O reversibly?

2. Which of the following compounds do not have protons?

a) H2C=O b) C6H5CH=O c) (CH3)3CCH=O d) CH3CH2CH2CH=O

3. Predict the major organic product(s) of the following reactions.

4. Draw the enol tautomer of the ketone shown.

5. Propose a mechanism for the following reactions.

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5.

6. Which of the following is an intermediate in the mechanism of the reaction shown?

7. Which of the following compounds is NOT a product of the aldol reaction shown?