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)
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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 35c
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.
(c) 
Verified step by step guidance1
Step 1: Recognize that the reaction involves the dehydration of an alcohol. Dehydration typically occurs in the presence of a strong acid, such as sulfuric or phosphoric acid, and heat. The goal is to remove a water molecule and form an alkene.
Step 2: Protonation of the alcohol group occurs first. The hydroxyl group (-OH) is protonated by the acid, converting it into a better leaving group (water). This step increases the reactivity of the alcohol.
Step 3: The protonated alcohol undergoes elimination. The water molecule leaves, forming a carbocation intermediate. Analyze the structure to determine the stability of the carbocation. In this case, the carbocation forms at the carbon attached to the hydroxyl group.
Step 4: Consider carbocation rearrangement if applicable. Check if the carbocation can rearrange to a more stable position (e.g., tertiary carbocation). In this molecule, the carbocation is already stable due to resonance with the double bond in the ring.
Step 5: A base (often the conjugate base of the acid used) removes a proton from a neighboring carbon atom, leading to the formation of a double bond. This step results in the formation of the alkene product. Use Zaitsev's rule to predict the major product, which favors the more substituted alkene.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Dehydration of Alcohols
Dehydration of alcohols involves the elimination of a water molecule from the alcohol, typically in the presence of an acid catalyst like sulfuric or phosphoric acid. This process leads to the formation of alkenes. The reaction can proceed via either an E1 or E2 mechanism, depending on the structure of the alcohol and the reaction conditions.
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01:23
General Reaction of Dehydration with POCl3
E1 and E2 Mechanisms
The E1 mechanism is a two-step process where the alcohol first loses a proton to form a carbocation intermediate, followed by the loss of a water molecule. In contrast, the E2 mechanism is a one-step process where the base abstracts a proton while the leaving group (water) departs simultaneously. The choice between these mechanisms depends on the substrate's structure and the reaction conditions.
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09:36Drawing the E2 Mechanism.
Regioselectivity in Elimination Reactions
Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others. In dehydration reactions, the formation of alkenes can lead to different products based on the stability of the resulting double bond. More substituted alkenes are generally favored due to their greater stability, following Zaitsev's rule.
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00:40Recognizing Elimination Reactions.
Related Practice
Textbook Question
For practice in recognizing mechanisms, classify each reaction according to the type of mechanism you expect:
1. Free-radical chain reaction
2. Reaction involving strong bases and strong nucleophiles
3. Reaction involving strong acids and strong electrophiles
(b)
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Textbook Question
The dehydrogenation of butane to trans-but-2-ene has ΔH° = +116 kJ/mol (+27.6 kcal/mol) and ΔS° = +117J/kelvin-mol (+28.0 cal/kelvin-mol).
a. Compute the value of ΔG° for dehydrogenation at room temperature (25 °C or 298 °K). Is dehydrogenation favored or disfavored?
HINT: When you are doing synthesis problems, avoid using these high-temperature industrial methods. They require specialized equipment, and they produce variable mixtures of products.
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Textbook Question
Propose mechanisms for the following reactions.
(d)
3464
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Textbook Question
For practice in recognizing mechanisms, classify each reaction according to the type of mechanism you expect:
1. Free-radical chain reaction
2. Reaction involving strong bases and strong nucleophiles
3. Reaction involving strong acids and strong electrophiles
(a)
1539
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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)
2172
views