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)
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 36a
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.
Verified step by step guidance1
Step 1: Recall the Gibbs free energy equation: ΔG° = ΔH° - TΔS°. This equation relates the change in enthalpy (ΔH°), the change in entropy (ΔS°), and the temperature (T) to determine the spontaneity of a reaction.
Step 2: Convert all units to be consistent. Ensure ΔH° is in joules (J) and ΔS° is in joules per kelvin (J/K). For ΔH° = +116 kJ/mol, convert to joules: 116 kJ/mol × 1000 J/kJ = 116,000 J/mol.
Step 3: Use the given temperature, T = 298 K (room temperature), and substitute the values into the Gibbs free energy equation: ΔG° = ΔH° - TΔS°. Substituting, ΔG° = 116,000 J/mol - (298 K × 117 J/K·mol).
Step 4: Perform the multiplication for the TΔS° term: TΔS° = 298 K × 117 J/K·mol. Subtract this value from ΔH° to calculate ΔG°.
Step 5: Analyze the sign of ΔG°. If ΔG° is negative, the reaction is thermodynamically favored (spontaneous). If ΔG° is positive, the reaction is thermodynamically disfavored (non-spontaneous).
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Gibbs Free Energy (ΔG°)
Gibbs Free Energy (ΔG°) is a thermodynamic potential that indicates the spontaneity of a reaction at constant temperature and pressure. It is calculated using the equation ΔG° = ΔH° - TΔS°, where ΔH° is the change in enthalpy, T is the temperature in Kelvin, and ΔS° is the change in entropy. A negative ΔG° value suggests that the reaction is spontaneous, while a positive value indicates non-spontaneity.
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Breaking down the different terms of the Gibbs Free Energy equation.
Enthalpy (ΔH°)
Enthalpy (ΔH°) is a measure of the total heat content of a system and reflects the energy required to break and form bonds during a chemical reaction. In the context of the dehydrogenation of butane, a positive ΔH° value (+116 kJ/mol) indicates that the reaction is endothermic, meaning it absorbs heat from the surroundings. This factor can influence the spontaneity of the reaction when combined with entropy changes.
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Entropy (ΔS°)
Entropy (ΔS°) is a measure of the disorder or randomness in a system. A positive ΔS° value (+117 J/K·mol) suggests that the products of the reaction have greater disorder compared to the reactants. In chemical reactions, an increase in entropy often favors spontaneity, as systems tend to evolve towards states of higher disorder. The interplay between ΔH° and ΔS° is crucial for determining the overall spontaneity of a reaction.
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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.
(c)
4
views
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)
1
views
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
(c)
5
views
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)
1
views
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
views