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Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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Tutti i libri di testoMullins 1st EditionCh. 11 - Properties and Synthesis of Alkyl Halides: Radical ReactionsProblema 34
Capitolo 10, Problema 34

(a) Using bond-dissociation energies (Table 5.6), which of the indicated bonds should break most easily?
(b) How does that help you explain the results shown in Figure 11.40?
Comparison of H-Br and Br-Br bonds, with arrows indicating the bonds in question.

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Step 1: Understand the concept of bond-dissociation energy (BDE). Bond-dissociation energy is the energy required to break a specific bond in a molecule in the gas phase, forming two radicals. A lower BDE indicates that the bond is weaker and easier to break.
Step 2: Refer to Table 5.6 for the bond-dissociation energies of the bonds in question. Look up the BDE values for the H―Br bond and the Br―Br bond. For example, the BDE for H―Br is approximately 366 kJ/mol, while the BDE for Br―Br is approximately 193 kJ/mol.
Step 3: Compare the BDE values. Since the BDE for Br―Br is significantly lower than that for H―Br, the Br―Br bond is weaker and should break more easily.
Step 4: Relate this to the results shown in Figure 11.40. The figure likely demonstrates a reaction or process where the Br―Br bond breaks more readily than the H―Br bond, consistent with the lower BDE of Br―Br. This could explain why bromine radicals (Br•) are generated more easily from Br―Br dissociation.
Step 5: Conclude that the bond-dissociation energy values provide a quantitative explanation for the observed behavior in Figure 11.40, where the Br―Br bond breaks more easily than the H―Br bond due to its lower energy requirement.

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Bond-Dissociation Energy

Bond-dissociation energy (BDE) is the energy required to break a specific bond in a molecule, resulting in the formation of two radicals. It is a crucial concept in understanding the stability of chemical bonds; higher BDE values indicate stronger bonds that are less likely to break. In the context of the question, comparing the BDEs of H-Br and Br-Br bonds will help determine which bond is more easily broken.
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How to calculate enthalpy using bond dissociation energies.

Radical Stability

Radical stability refers to the relative stability of free radicals, which are species with unpaired electrons. The stability of a radical can be influenced by factors such as the presence of electron-donating groups or the degree of substitution. In the question, understanding the stability of the radicals formed from breaking H-Br and Br-Br bonds is essential for explaining the observed results in Figure 11.40.
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The radical stability trend.

Reaction Mechanisms

A reaction mechanism describes the step-by-step sequence of elementary reactions by which overall chemical change occurs. It provides insight into how bonds are broken and formed during a reaction. In this case, analyzing the mechanism involving the breaking of H-Br versus Br-Br bonds will clarify the differences in reactivity and help explain the results shown in Figure 11.40.
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Heck Reaction Mechanism
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