Textbook Question
a. Which reacts faster in an E2 reaction: 3-bromocyclohexene or bromocyclohexane?
b. Which reacts faster in an E1 reaction?
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Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides
Bruice8th EditionOrganic ChemistryISBN: 9780135213711
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Table of contents
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 1)31
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 2)65
- Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry84
- Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure183
- Ch. 4 - Isomers: The Arrangement of Atoms in Space121
- Ch. 5 - Alkenes: Structure, Nomenclature, and an Introduction to Reactivity • Thermodynamics and Kinetics83
- Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions175
- Ch. 7 - The Reactions of Alkynes • An Introduction to Multistep Synthesis119
- Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene148
- Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides212
- Ch. 10 - Reactions of Alcohols, Ethers, Epoxides, Amines, and Sulfur-Containing Compounds131
- Ch. 11 - Organometallic Compounds60
- Ch. 12 - Radicals90
- Ch. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis Spectroscopy62
- Ch. 14 - NMR Spectroscopy95
- Ch. 15 - Reactions of Carboxylic Acids and Carboxylic Acid Derivatives123
- Ch. 16 - Reactions of Aldehydes and Ketones • More Reactions of Carboxylic Acid Derivatives141
- Ch. 17 - Reactions at the Alpha-Carbon101
- Ch. 18 - Reactions of Benzene and Substituted Benzenes144
- Ch. 19 - More About Amines • Reactions of Heterocyclic Compounds49
- Ch. 20 - The Organic Chemistry of Carbohydrates80
- Ch. 21 - Amino Acids, Peptides, and Proteins57
- Ch. 22 - Catalysis in Organic Reactions and in Enzymatic Reactions35
- Ch. 23 - The Organic Chemistry of the Coenzymes—Compounds Derived from Vitamins1
- Ch. 28 - Pericyclic Reactions58
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Bruice 8th EditionCh. 9 - Substitution and Elimination Reactions of Alkyl HalidesProblem 91
Bruice 8th EditionCh. 9 - Substitution and Elimination Reactions of Alkyl HalidesProblem 91Chapter 10, Problem 91
When the following reactions are carried out under the same conditions, the rate constant for the first reaction (kH) is found to be 7 times greater than the rate constant for the second reaction (kD). What does that tell you about the mechanism of the reaction? (Hint: a C—D bond is 1.2 kcal/mol stronger than a C—H bond.)
Verified step by step guidance1
Step 1: Analyze the given reactions. The first reaction involves a substrate with C—H bonds, while the second reaction involves a substrate with C—D bonds. Both reactions proceed under identical conditions and produce similar products, but the rate constant for the reaction with C—H bonds (kH) is 7 times greater than the rate constant for the reaction with C—D bonds (kD).
Step 2: Understand the hint provided. The hint states that a C—D bond is 1.2 kcal/mol stronger than a C—H bond. This means that breaking a C—D bond requires more energy compared to breaking a C—H bond. This difference in bond strength is a key factor influencing the reaction rate.
Step 3: Consider the mechanism of the reaction. The reaction likely involves the breaking of the C—H or C—D bond in the rate-determining step. Since the C—D bond is stronger, it is harder to break, leading to a slower reaction rate for the second reaction compared to the first.
Step 4: Relate the observed rate constant difference to the kinetic isotope effect. The kinetic isotope effect occurs when the substitution of an isotope (e.g., hydrogen with deuterium) significantly affects the reaction rate. The fact that kH is 7 times greater than kD suggests a primary kinetic isotope effect, indicating that the bond to hydrogen or deuterium is being broken in the rate-determining step.
Step 5: Conclude that the mechanism of the reaction involves the cleavage of the C—H or C—D bond in the rate-determining step. The observed rate constant difference and the kinetic isotope effect provide strong evidence for this conclusion.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Kinetic Isotope Effect (KIE)
The kinetic isotope effect refers to the change in reaction rate that occurs when one atom in a molecule is replaced by one of its isotopes. In this case, the C—D bond is stronger than the C—H bond, leading to a slower reaction rate when breaking the C—D bond. The observed rate constant difference (kH > kD) indicates that the C—H bond is more easily broken, suggesting that the bond cleavage is a significant step in the reaction mechanism.
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Reaction Mechanism
A reaction mechanism is a step-by-step description of the pathway taken by reactants to form products. Understanding the mechanism helps in predicting the rate of reaction and the influence of various factors, such as bond strengths. In this case, the difference in rate constants suggests that the mechanism likely involves a transition state where the C—H bond is broken, confirming that bond strength plays a crucial role in determining the reaction's rate.
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Bond Strength and Stability
Bond strength refers to the energy required to break a bond between two atoms. In this scenario, the C—D bond is 1.2 kcal/mol stronger than the C—H bond, which means that reactions involving C—D bonds will generally proceed more slowly due to the increased energy required to break these bonds. This concept is essential for understanding why the rate constant for the reaction involving C—D bonds is lower compared to that involving C—H bonds.
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Related Practice
Textbook Question
Starting with an alkyl halide, how could the following compounds be prepared?
b. 1-methoxybutane
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1
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Textbook Question
For each of the following reactions, draw the major elimination product; if the product can exist as stereoisomers, indicate which stereoisomer is obtained in greater yield.
e. 3-bromo-3-methylpentane + high concentration of CH3CH2O−
f. 3-bromo-3-methylpentane + CH3CH2OH
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Textbook Question
Starting with an alkyl halide, how could the following compounds be prepared?
a. 2-methoxybutane
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Textbook Question
Starting with an alkyl halide, how could the following compounds be prepared?
c. butylmethylamine
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Textbook Question
For each of the following reactions, draw the major elimination product; if the product can exist as stereoisomers, indicate which stereoisomer is obtained in greater yield.
d. trans-1-chloro-3-methylcyclohexane + high concentration of CH3O−
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