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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 28
Chapter 4, Problem 28

The triphenylmethyl cation is so stable that some of its salts can be stored for months. Explain why this cation is so stable.

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The triphenylmethyl cation is stabilized by resonance. Resonance occurs when the positive charge on the central carbon atom can be delocalized across the three phenyl rings attached to it.
Each phenyl ring has a conjugated π-electron system, which allows the positive charge to be distributed over multiple atoms, reducing the overall energy of the cation.
The delocalization of the positive charge creates multiple resonance structures, which increases the stability of the molecule. This is a key feature of aromatic systems and conjugated π-electron systems.
Additionally, the phenyl rings are planar and aromatic, which further contributes to the stability of the cation through aromatic stabilization.
The combination of resonance stabilization and aromaticity makes the triphenylmethyl cation exceptionally stable, allowing its salts to be stored for extended periods.

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Key Concepts

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

Carbocation Stability

Carbocations are positively charged species that are classified based on their stability. Stability increases with the degree of alkyl substitution, as more alkyl groups can donate electron density through hyperconjugation and inductive effects. The triphenylmethyl cation is exceptionally stable due to resonance stabilization, where the positive charge is delocalized over multiple carbon atoms.
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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 the triphenylmethyl cation, the positive charge can be delocalized across the aromatic rings, allowing for multiple resonance forms. This delocalization lowers the energy of the cation, contributing to its overall stability.
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Aromaticity

Aromaticity refers to the enhanced stability of cyclic compounds with conjugated pi electron systems that follow Hückel's rule (4n + 2 pi electrons). The tropylium ion, a related structure, is aromatic and contributes to the stability of the triphenylmethyl cation. The presence of aromatic rings in the structure allows for effective electron delocalization, further stabilizing the cation.
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Related Practice
Textbook Question

In the presence of a small amount of bromine, cyclohexene undergoes the following light-promoted reaction:

b. Draw the structure of the rate-limiting transition state.

c. Use Hammond's postulate to predict which intermediate most closely resembles this transition state.

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Textbook Question

Rank the following radicals in decreasing order of stability. Classify each as primary, secondary, or tertiary.

a. The isopentyl radical,

b. The 3-methyl-2-butyl radical,

c. The 2-methyl-2-butyl radical,

d.

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Textbook Question

Write an equation for the reaction of vitamin E with an oxidizing radical (RO•) to give ROH and a less reactive free radical.

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Textbook Question

In the presence of a small amount of bromine, cyclohexene undergoes the following light-promoted reaction:

a. Propose a mechanism for this reaction.

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Textbook Question

Acetonitrile (CH3C≡N) is deprotonated by very strong bases. Write resonance forms to show the stabilization of the carbanion that results.

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Textbook Question

Acetylacetone (pentane-2,4-dione) reacts with sodium hydroxide to give water and the sodium salt of a carbanion. Write a complete structural formula for the carbanion, and use resonance forms to show the stabilization of the carbanion.

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