Textbook Question
Propose a mechanism for the acid-catalyzed bromination of pentan-3-one.
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Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
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Wade 9th EditionCh. 22 - Condensations and Alpha Substitutions of Carbonyl CompoundsProblem 13
Wade 9th EditionCh. 22 - Condensations and Alpha Substitutions of Carbonyl CompoundsProblem 13Chapter 22, Problem 13
Predict the products of the following reactions.
(a) cyclopentyl methyl ketone + excess Cl2 + excess NaOH
(b) 1-cyclopentylethanol + excess I2 + excess NaOH
(c) propiophenone + excess Br2 + excess NaOH
Verified step by step guidance1
Step 1: Recognize the type of reaction. All three reactions involve halogenation in the presence of a strong base (NaOH). This is a classic example of the haloform reaction, which occurs with methyl ketones or alcohols that can be oxidized to methyl ketones.
Step 2: For reaction (a), cyclopentyl methyl ketone reacts with excess Cl2 and NaOH. The methyl group adjacent to the carbonyl group undergoes multiple halogenations, replacing all three hydrogens with chlorine atoms to form a trichloromethyl group. This group is then cleaved by the base to form chloroform (CHCl3) and a carboxylate ion derived from the remaining part of the molecule.
Step 3: For reaction (b), 1-cyclopentylethanol is a secondary alcohol. In the presence of excess I2 and NaOH, the alcohol is first oxidized to a ketone (cyclopentyl methyl ketone). The ketone then undergoes the haloform reaction as described in step 2, producing iodoform (CHI3) and a carboxylate ion.
Step 4: For reaction (c), propiophenone (C6H5-CH2-CO-CH3) contains a methyl ketone group. In the presence of excess Br2 and NaOH, the methyl group adjacent to the carbonyl undergoes halogenation to form a tribromomethyl group. This group is cleaved by the base to produce bromoform (CHBr3) and a carboxylate ion derived from the remaining part of the molecule.
Step 5: Summarize the products for each reaction: (a) chloroform (CHCl3) and the carboxylate ion of cyclopentane carboxylic acid, (b) iodoform (CHI3) and the carboxylate ion of cyclopentane carboxylic acid, (c) bromoform (CHBr3) and the carboxylate ion of benzoic acid.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Halogenation of Ketones
Halogenation of ketones involves the reaction of ketones with halogens in the presence of a base, leading to the formation of α-halo ketones. The base facilitates the deprotonation of the ketone, allowing the halogen to add to the α-carbon. Understanding this mechanism is crucial for predicting the products of reactions involving cyclopentyl methyl ketone and propiophenone.
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Oxidation of Alcohols
The oxidation of alcohols, particularly secondary alcohols like 1-cyclopentylethanol, can be achieved using iodine in the presence of a base. This reaction typically leads to the formation of ketones or aldehydes, depending on the structure of the alcohol. Recognizing the oxidation state changes and the role of the reagents is essential for predicting the products in this reaction.
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Reactivity of α-Halo Compounds
α-Halo compounds, formed from the halogenation of ketones, can undergo further reactions such as nucleophilic substitution or elimination. In the presence of a strong base like NaOH, these compounds can react to form enolates or undergo dehydrohalogenation. Understanding the reactivity of these intermediates is key to predicting the final products of the reactions presented.
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Related Practice
Textbook Question
Propose a mechanism to show how acetophenone undergoes base-promoted chlorination to give trichloroacetophenone.
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Textbook Question
Which compounds will give positive iodoform tests?
(a) 1-phenylethanol
(b) pentan-2-one
(c) pentan-2-ol
(d) pentan-3-one
(e) acetone
(f) isopropyl alcohol
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Textbook Question
Propose a mechanism for the reaction of cyclohexyl methyl ketone with excess bromine in the presence of sodium hydroxide.
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Textbook Question
Show how you would accomplish each conversion using an enamine synthesis with pyrrolidine as the secondary amine.
(c)
Textbook Question
Acid-catalyzed halogenation is synthetically useful for converting ketones to α,β-unsaturated ketones, which are useful in Michael reactions (Section 22-18). Propose a method for converting cyclohexanone to cyclohex-2-en-1-one, an important synthetic starting material.
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