Textbook Question
Calculate ∆H° for the following reactions.
(b) CH3Br + HCl → CH3Cl + HBr
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6. Thermodynamics and Kinetics
Enthalpy
Problem 38c
Textbook Question
Using bond-dissociation energies, identify the most stable radical. Justify the difference in stability based on the structure.
(c) 
Verified step by step guidance1
Step 1: Understand the concept of radical stability. Radicals are stabilized by factors such as resonance, hyperconjugation, and inductive effects. Resonance is particularly significant because it delocalizes the unpaired electron over multiple atoms, reducing its energy.
Step 2: Analyze the structure of the radicals. The first radical is an alkyl radical (ethyl radical), while the second radical is a benzyl radical. The benzyl radical has the unpaired electron adjacent to a benzene ring, which allows for resonance stabilization.
Step 3: Evaluate resonance stabilization in the benzyl radical. The unpaired electron on the benzyl radical can delocalize into the π-system of the benzene ring, forming multiple resonance structures. This delocalization significantly increases the stability of the benzyl radical.
Step 4: Compare the ethyl radical. The ethyl radical lacks resonance stabilization because the unpaired electron is localized on the carbon atom. It can only be stabilized through hyperconjugation with adjacent C-H bonds, which is less effective than resonance.
Step 5: Justify the difference in stability using bond-dissociation energies. Bond-dissociation energy is lower for the bond that forms the benzyl radical compared to the ethyl radical, indicating that the benzyl radical is more stable. This difference arises due to the resonance stabilization in the benzyl radical.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Bond-Dissociation Energy
Bond-dissociation energy (BDE) is the energy required to break a bond in a molecule, resulting in the formation of radicals. Higher BDE values indicate stronger bonds, which typically correlate with lower stability of the resulting radicals. Understanding BDE is crucial for predicting the stability of radicals, as radicals formed from weaker bonds are generally more stable due to lower energy requirements for bond cleavage.
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Radical Stability
Radical stability refers to the relative stability of radical species, which can be influenced by factors such as hybridization, resonance, and steric effects. Generally, tertiary radicals are more stable than secondary, which are more stable than primary radicals. This stability is often due to the ability of surrounding groups to donate electron density to the unpaired electron, thus stabilizing the radical.
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Resonance Effects
Resonance effects occur when a molecule can be represented by multiple valid Lewis structures, allowing for the delocalization of electrons. In the context of radicals, resonance can significantly enhance stability by spreading out the unpaired electron over multiple atoms. This delocalization reduces the energy of the radical, making it more stable compared to localized radicals that lack such resonance stabilization.
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