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Chapter 22: Carbonyl Alpha-Substitution Reactions

Every problem in the chapter beside the book’s answer, a chapter at a time as you scroll.

Chapter 22: Carbonyl Alpha-Substitution Reactions

16 problems · 24 figures

22-1 §22.1 source ↗

Draw structures for the enol tautomers of the following compounds:
(a)
Cyclopentanone
(b)
Methyl thioacetate
(c)
Ethyl acetate
(d)
Propanal
(e)
Acetic acid
(f)
Phenylacetone

Answer (6 parts)

(a)
(b)
(c)
(d)
(e)
(f)

22-2 §22.1 source ↗

How many acidic hydrogens does each of the molecules listed in Problem 22-1 have? Identify them.

Answer

(a) 4 (b) 3 (c) 3 (d) 2 (e) 4 (f) 5

22-3 §22.1 source ↗

Draw structures for all monoenol forms of the following molecule. Which would you expect to be most stable? Explain.

Answer

22-4 §22.3 source ↗

Write the complete mechanism for the deuteration of acetone on treatment with D3O+.

Answer

Acid-catalyzed formation of an enol is followed by deuteronation of the enol double bond and dedeuteronation of oxygen.

22-5 §22.3 source ↗

Show how you might prepare 1-penten-3-one from 3-pentanone.

Answer

1. Br2; 2. Pyridine, heat

22-6 §22.4 source ↗

If methanol rather than water is added at the end of a Hell–Volhard–Zelinskii reaction, an ester rather than an acid is produced. Show how you would carry out the following transformation, and propose a mechanism for the ester-forming step.

Answer

The intermediate α-bromo acid bromide undergoes a nucleophilic acyl substitution reaction with methanol to give an α-bromo ester.

22-7 §22.5 source ↗

Identify the most acidic hydrogens in each of the following molecules:
(a)
CH3CH2CHO
(b)
(CH3)3C–COCH3
(c)
CH3CO2H
(d)
Benzamide
(e)
CH3CH2CH2CN
(f)
CH3CON(CH3)2

Answer (6 parts)

(a)
CH3CH2CHO
(b)
(CH3)3CCOCH3
(c)
CH3CO2H
(d)
PhCONH2
(e)
CH3CH2CH2CN
(f)
CH3CON(CH3)2

22-8 §22.5 source ↗

Draw a resonance structure of the acetonitrile anion, –:CH2C ≡ N, and account for the acidity of nitriles.

Answer

22-9 §22.6 source ↗

Why do you suppose ketone halogenations in acidic media are referred to as being acid-catalyzed, whereas halogenations in basic media are base-promoted? In other words, why is a full equivalent of base required for halogenation?

Answer

Acid is regenerated, but base is used stoichiometrically.

22-10 §22.7 source ↗

How could you use a malonic ester synthesis to prepare the following compounds? Show all steps.
(a)
(b)
(c)

Answer (3 parts)

(a)
1. Na+ −OEt; 2. PhCH2Br; 3. H3O+
(b)
1. Na+ −OEt; 2. CH3CH2CH2Br; 3. Na+ −OEt; 4. CH3Br; 5. H3O+
(c)
1. Na+ −OEt; 2. (CH3)2CHCH2Br; 3. H3O+

22-11 §22.7 source ↗

Monoalkylated and dialkylated acetic acids can be prepared by malonic ester synthesis, but trialkylated acetic acids (R3C–CO2H) can’t be prepared. Explain.

Answer

Malonic ester has only two acidic hydrogens to be replaced.

22-12 §22.7 source ↗

How could you use a malonic ester synthesis to prepare the following compound?

Answer

1. Na+ −OEt; 2. (CH3)2CHCH2Br; 3. Na+ −OEt; 4. CH3Br; 5. H3O+

22-13 §22.7 source ↗

What alkyl halides would you use to prepare the following ketones by an acetoacetic ester synthesis?
(a)
(b)

Answer (2 parts)

(a)
(CH3)2CHCH2Br
(b)
PhCH2CH2Br

22-14 §22.7 source ↗

Which of the following compounds can’t be prepared by an acetoacetic ester synthesis? Explain.

(a) Phenylacetone (b) Acetophenone (c) 3,3-Dimethyl-2-butanone

Answer

None can be prepared.

22-15 §22.7 source ↗

How would you prepare the following compound using an acetoacetic ester synthesis?

Answer

1. 2 Na+ −OEt; 2. BrCH2CH2CH2CH2Br; 3. H3O+

22-16 §22.7 source ↗

Show how you might prepare the following compounds using an alkylation reaction as the key step:
(a)
(b)
(c)
(d)
(e)
(f)

Answer (6 parts)

(a)
Alkylate phenylacetone with CH3I
(b)
Alkylate pentanenitrile with CH3CH2I
(c)
Alkylate cyclohexanone with H2C ═ CHCH2Br
(d)
Alkylate cyclohexanone with excess CH3I
(e)
Alkylate C6H5COCH2CH3 with CH3I
(f)
Alkylate methyl 3-methylbutanoate with CH3CH2I

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Problems and answers are adapted from Organic Chemistry: A Tenth Edition by John McMurry, OpenStax, and are used under the CC BY-NC-SA 4.0 licence. Access for free at https://openstax.org/books/organic-chemistry/pages/1-why-this-chapter. Illustrations: Copyright Rice University, OpenStax, under CC BY-NC-SA 4.0 license. This adaptation is released under the same licence, is non-commercial, and is neither affiliated with nor endorsed by OpenStax or Rice University.