Textbook Question
Propose mechanisms for the following reactions.
(a)
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 34c
Propose mechanisms for the following reactions.
(c) 
Verified step by step guidance1
Step 1: Protonation of the alcohol group - The reaction begins with the protonation of the hydroxyl (-OH) group of 2-methylcyclohexanol by sulfuric acid (H2SO4). This converts the hydroxyl group into a better leaving group, forming water (H2O). The intermediate is now a protonated alcohol.
Step 2: Formation of the carbocation - The water molecule leaves, generating a carbocation intermediate. The carbocation forms at the carbon where the hydroxyl group was originally attached. In this case, the carbocation is at the 2-position of the cyclohexane ring.
Step 3: Carbocation rearrangement - The carbocation may undergo hydride or methyl shifts to form more stable carbocations. For example, a hydride shift can occur to move the positive charge to a tertiary carbon, which is more stable than a secondary carbocation.
Step 4: Elimination to form alkenes - The carbocation undergoes elimination (E1 mechanism) to form double bonds. A proton is removed from a β-carbon (adjacent to the carbocation), resulting in the formation of alkenes. Depending on which β-hydrogen is removed, different alkenes are formed: 1-methylcyclohexene, 3-methylcyclohexene, and methylenecyclohexane.
Step 5: Product distribution - The reaction produces a mixture of alkenes due to the possibility of forming different double bond positions. The major product is typically the more substituted alkene (Zaitsev's rule), but all three products (1-methylcyclohexene, 3-methylcyclohexene, and methylenecyclohexane) are observed.
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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 dehydration of 2-methylcyclohexanol leads to the formation of alkenes. The reaction typically requires an acid catalyst, such as sulfuric acid, and heat to facilitate the elimination of water.
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Recognizing Elimination Reactions.
Regioselectivity
Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple possibilities exist. In the dehydration of 2-methylcyclohexanol, the formation of different alkenes (1-methylcyclohexene, 3-methylcyclohexene, and methylenecyclohexane) illustrates how the stability of the resulting double bond influences the major products formed.
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Carbocation Stability
Carbocation stability is a key factor in determining the outcome of elimination reactions. More stable carbocations, such as tertiary or secondary, are favored during the reaction pathway. In this case, the formation of alkenes from 2-methylcyclohexanol involves the generation of carbocations, where the stability of these intermediates influences which alkene is predominantly formed.
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Related Practice
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.
(b)
Textbook Question
Propose mechanisms for the following reactions.
(d)
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Textbook Question
Propose mechanisms for the following reactions.
(b)
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)
5
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