Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions

Problem 29

Chapter 10, Problem 29

In the following reaction, which C―H bond would be most likely to react with a bromine radical?

Verified step by step guidance

Identify the type of reaction: This problem involves a radical halogenation reaction, specifically with bromine radicals. Bromine radicals are selective and prefer to react with the most stable radical intermediates.

Consider the stability of radical intermediates: In radical halogenation, the stability of the radical formed after hydrogen abstraction is crucial. Tertiary radicals are more stable than secondary radicals, which are more stable than primary radicals.

Analyze the structure of the molecule: Examine the molecule to identify the different types of C―H bonds present. Determine which carbon atoms are primary, secondary, or tertiary.

Evaluate the bond dissociation energies: Generally, the C―H bond at a tertiary carbon will have a lower bond dissociation energy compared to secondary or primary C―H bonds, making it more likely to react with a bromine radical.

Select the most likely C―H bond: Based on the stability of the radical intermediates and the bond dissociation energies, choose the C―H bond at the tertiary carbon as the most likely to react with a bromine radical.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Radical Halogenation

Radical halogenation is a reaction where a halogen, such as bromine, reacts with hydrocarbons to form haloalkanes. The process involves the formation of radicals, which are highly reactive species with unpaired electrons. Understanding the mechanism of radical formation and propagation is crucial for predicting which bonds are likely to react.

Bond Strength and Reactivity

In radical halogenation, the reactivity of C-H bonds is influenced by their bond strength. Weaker C-H bonds, typically found in tertiary carbons, are more susceptible to radical attack due to lower energy required for bond cleavage. Identifying the weakest C-H bond in a molecule helps predict the site of radical attack.

Stability of Radicals

The stability of the resulting radical is a key factor in determining which C-H bond will react. More stable radicals, such as tertiary radicals, are favored due to their ability to better distribute the unpaired electron. This stability is often achieved through hyperconjugation and resonance, making certain C-H bonds more reactive.

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The radical stability trend.

Related Practice

Textbook Question

In the following molecules, identify the carbon where the radical is most likely to form in the first propagation step.

(c)

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Textbook Question

Identify the allylic carbon(s) in the following molecules.

(c)

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Textbook Question

Identify the allylic carbon(s) in the following molecules.

(b)

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Textbook Question

In the following molecules, identify the carbon where the radical is most likely to form in the first propagation step.

(d)

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Textbook Question

In the following allylic radicals, identify the carbon where the new C–Br bond is most likely to form in the second propagation step.

(a)

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Textbook Question

We show in Chapter 12 that C― Br bonds can break to give a carbocation and a bromide anion. For which of the organohalides (A or B) would you expect this process to be fastest? Explain.

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