Textbook Question
Propose mechanisms for the following reactions.
(a)
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Ch. 7 - Structure and Synthesis of Alkenes; Elimination
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
Chapter 7, Problem 34b
Propose mechanisms for the following reactions.
(b) 
Verified step by step guidance1
Step 1: Recognize that the reaction involves the dehydration of pentan-2-ol under acidic conditions (H2SO4 and heat). Dehydration reactions typically proceed via an E1 mechanism, where the alcohol is protonated to form a good leaving group (water).
Step 2: Protonation of the hydroxyl group (-OH) occurs first. The sulfuric acid donates a proton (H⁺) to the hydroxyl group, converting it into a better leaving group, water (H₂O). This step forms a positively charged intermediate.
Step 3: The water molecule leaves, generating a carbocation intermediate at the second carbon. This carbocation is secondary and relatively stable, but it can undergo rearrangement if a more stable carbocation is possible. In this case, no rearrangement occurs.
Step 4: Elimination of a proton (H⁺) from a β-carbon adjacent to the carbocation occurs, forming a double bond. The elimination can happen in two different directions: (a) elimination from the first carbon forms pent-1-ene, and (b) elimination from the third carbon forms pent-2-ene. Pent-2-ene exists as cis and trans isomers due to restricted rotation around the double bond.
Step 5: The final products are pent-1-ene and pent-2-ene (cis and trans). The reaction follows Zaitsev's rule, favoring the formation of the more substituted alkene (pent-2-ene) as the major product.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Elimination Reactions
Elimination reactions involve the removal of atoms or groups from a molecule, resulting in the formation of a double bond. In this case, the reaction of pentan-2-ol with sulfuric acid under heat leads to the elimination of water, producing alkenes. Understanding the mechanism of elimination is crucial for predicting the products formed in this reaction.
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Recognizing Elimination Reactions.
E1 Mechanism
The E1 mechanism is a two-step elimination process where the first step involves the formation of a carbocation intermediate after the leaving group departs. In the presence of heat and an acid like H2SO4, pentan-2-ol undergoes dehydration to form a carbocation, which then loses a proton to form the alkene. Recognizing this mechanism helps in understanding the regioselectivity and stereochemistry of the products.
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Regioselectivity and Stereochemistry
Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others, while stereochemistry deals with the spatial arrangement of atoms in molecules. In the dehydration of pentan-2-ol, both pent-1-ene and pent-2-ene (cis and trans) are formed, showcasing how the reaction can lead to different products based on the stability of intermediates and the orientation of the double bond.
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Related Practice
Textbook Question
Propose mechanisms for the following reactions.
(d)
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Textbook Question
Propose mechanisms for the following reactions.
(c)
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Textbook Question
Predict the products and mechanisms of the following reactions. When more than one product or mechanism is possible, explain which are most likely.
g. 1−bromo−1−methylcyclopentane + NaOEt in ethanol
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Textbook Question
Predict the products and mechanisms of the following reactions. When more than one product or mechanism is possible, explain which are most likely.
h. 1-bromo-1-methylcyclopentane heated in methanol
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Textbook Question
Show the product(s) you expect from dehydration of the following alcohols when they are heated in sulfuric or phosphoric acid. In each case, use a mechanism to show how the products are formed.
(a)
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