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Ch. 12 - Radicals
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 12 - RadicalsProblem 35
Chapter 13, Problem 35

The deuterium kinetic isotope effect for the halogenation of an alkane is defined in the following equation, where X･ = Cl･ or Br･

Predict whether chlorination or bromination would have a greater deuterium kinetic isotope effect.

Verified step by step guidance
1
Step 1: Understand the concept of the deuterium kinetic isotope effect. It is a measure of the difference in reaction rates between the cleavage of a C-H bond and a C-D bond. The heavier isotope (deuterium) forms stronger bonds, which require more energy to break compared to the lighter isotope (hydrogen).
Step 2: Analyze the halogenation process. Halogenation involves the homolytic cleavage of the C-H or C-D bond by a halogen radical (X•, where X = Cl or Br). The rate of bond cleavage depends on the bond dissociation energy and the reactivity of the halogen radical.
Step 3: Compare the reactivity of chlorine and bromine radicals. Chlorine radicals (Cl•) are more reactive than bromine radicals (Br•) due to their higher electronegativity and smaller size, which allows them to interact more effectively with the C-H or C-D bond.
Step 4: Consider the bond dissociation energy. The C-H bond has a lower bond dissociation energy compared to the C-D bond, making it easier to cleave. Since chlorine radicals are more reactive, they will exhibit a greater difference in the cleavage rates of C-H and C-D bonds, leading to a larger deuterium kinetic isotope effect.
Step 5: Predict the outcome. Based on the higher reactivity of chlorine radicals and the stronger C-D bond, chlorination is expected to have a greater deuterium kinetic isotope effect compared to bromination.

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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 organic reactions, this effect is particularly significant for hydrogen isotopes, such as deuterium, which is heavier than protium. The KIE can provide insights into the transition state of a reaction, as heavier isotopes typically lead to slower reaction rates due to their increased mass.
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Halogenation of Alkanes

Halogenation is a substitution reaction where a halogen atom (like chlorine or bromine) replaces a hydrogen atom in an alkane. This process typically involves radical mechanisms, where halogen radicals react with alkanes to form alkyl halides. The nature of the halogen affects the reaction's kinetics and the stability of the transition state, which is crucial for understanding the KIE in this context.
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Radical Stability

Radical stability is a key factor in determining the reactivity of halogen radicals during the halogenation of alkanes. Bromine radicals are generally more stable than chlorine radicals due to their larger size and lower bond dissociation energy. This stability influences the rate of reaction and the extent of the kinetic isotope effect, as more stable radicals lead to a more favorable transition state, impacting the overall reaction kinetics.
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Related Practice
Textbook Question

When 2-methylpropane is monochlorinated in the presence of light at room temperature, 36% of the product is 2-chloro-2-methylpropane and 64% is 1-chloro-2-methylpropane. From these data, calculate how much easier it is to remove a hydrogen atom from a tertiary carbon than from a primary carbon under these conditions.

Textbook Question

Propose a mechanism to account for the products formed in the following reaction:

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Textbook Question

How many monochlorination products would be obtained if all stereoisomers are included?

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Textbook Question

What is the major product of the following reactions? Disregard stereoisomers:

d.

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Textbook Question

How many monochlorination products can be obtained from the radical chlorination of methylcyclohexane? Disregard stereoisomers.

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Textbook Question

Which product would be obtained in greatest yield? Explain.