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Ch.4 - The Study of Chemical Reactions
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.4 - The Study of Chemical ReactionsProblem 42
Chapter 4, Problem 42

Use the information in Table 4-2 (p. 167) to rank the following radicals in decreasing order of stability.

Verified step by step guidance
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Step 1: Identify the radicals provided in the image. These include: methyl radical (•CH₃), ethyl radical (CH₃CH₂•), benzyl radical (C₆H₅CH₂•), tertiary butyl radical ((CH₃)₃C•), isopropyl radical ((CH₃)₂CH•), and allyl radical (CH₂=CH—CH₂•).
Step 2: Recall the factors that influence radical stability. Stability is determined by resonance stabilization, hyperconjugation, and inductive effects. Radicals that are resonance-stabilized are generally more stable than those stabilized by hyperconjugation or inductive effects.
Step 3: Analyze each radical for resonance stabilization. The benzyl radical (C₆H₅CH₂•) and allyl radical (CH₂=CH—CH₂•) are resonance-stabilized, making them more stable than the others. Benzyl radical has extensive resonance due to the aromatic ring, while allyl radical has resonance across the π-bond.
Step 4: Consider hyperconjugation and inductive effects for the remaining radicals. The tertiary butyl radical ((CH₃)₃C•) is highly stabilized by hyperconjugation due to the presence of three methyl groups. The isopropyl radical ((CH₃)₂CH•) is next, followed by the ethyl radical (CH₃CH₂•), and finally the methyl radical (•CH₃), which has no hyperconjugation or resonance stabilization.
Step 5: Rank the radicals in decreasing order of stability based on the analysis: Benzyl radical (C₆H₅CH₂•) > Allyl radical (CH₂=CH—CH₂•) > Tertiary butyl radical ((CH₃)₃C•) > Isopropyl radical ((CH₃)₂CH•) > Ethyl radical (CH₃CH₂•) > Methyl radical (•CH₃).

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Key Concepts

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

Radical Stability

Radical stability refers to the relative stability of free radicals, which are species with unpaired electrons. The stability of a radical is influenced by factors such as the degree of substitution (primary, secondary, tertiary) and resonance effects. Tertiary radicals are generally more stable than secondary, which are more stable than primary, due to hyperconjugation and the ability to delocalize the unpaired electron.
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Hyperconjugation

Hyperconjugation is a stabilizing interaction that occurs when the electrons in a sigma bond (C-H or C-C) interact with an adjacent empty p-orbital or a radical. This effect allows for the delocalization of the unpaired electron in radicals, enhancing their stability. The more alkyl groups attached to the carbon bearing the radical, the greater the hyperconjugation, leading to increased stability.
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Resonance

Resonance is a concept in organic chemistry where a molecule can be represented by two or more valid Lewis structures, known as resonance structures. In the case of radicals, resonance can significantly enhance stability by allowing the unpaired electron to be delocalized over multiple atoms. For example, benzyl radicals benefit from resonance, making them more stable than non-resonance-stabilized radicals.
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Related Practice
Textbook Question

Use bond-dissociation enthalpies (Table 4-2, p. 167) to calculate values of ΔH° for the following reactions.

c. (CH3)3C—OH + HCl → (CH3)3C—Cl + H2O

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Textbook Question

Use bond-dissociation enthalpies (Table 4-2, p. 167) to calculate values of ΔH° for the following reactions.

d. CH3CH2CH3 + H2 → CH3CH3 + CH4

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Textbook Question

For each alkane,

1. draw all the possible monochlorinated derivatives.

a. Cyclopentane

b. Methylcyclopentane

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Textbook Question

For each alkane, which monobrominated derivatives could you form in good yield by free-radical bromination?

a. Cyclopentane

b. Methylcyclopentnae

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Textbook Question

For each alkane,

2. determine whether free-radical chlorination would be a good way to make any of these monochlorinated derivatives. Will the reaction give mostly one major product?

a. Cyclopentane

b. Methylcyclopentane

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Textbook Question

Use bond-dissociation enthalpies (Table 4-2, p. 167) to calculate values of ΔH° for the following reactions.

b. CH3CH2Cl + HI → CH3CH2I + HCl

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