Ch. 17 - Reactions of Aromatic Compounds

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 17 - Reactions of Aromatic Compounds

Problem 5b

Chapter 17, Problem 5b

(b) Explain why m-xylene undergoes nitration 100 times faster than p-xylene.

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Understand the nitration reaction: Nitration is an electrophilic aromatic substitution reaction where a nitro group (-NO₂) is introduced into the aromatic ring. The rate of this reaction depends on the electron density of the aromatic ring and the positions of substituents.

Analyze the structure of m-xylene and p-xylene: Both compounds are derivatives of xylene (dimethylbenzene). In m-xylene, the two methyl groups are positioned meta to each other, while in p-xylene, the two methyl groups are positioned para to each other.

Consider the electron-donating effect of methyl groups: Methyl groups are electron-donating through both inductive (+I) and hyperconjugation effects. This increases the electron density on the aromatic ring, making it more reactive toward electrophiles like the nitronium ion (NO₂⁺).

Examine the steric hindrance in p-xylene: In p-xylene, the two methyl groups are positioned opposite each other, which can create steric hindrance at the para position. This steric hindrance can slow down the approach of the electrophile, reducing the rate of nitration compared to m-xylene.

Conclude the reasoning: In m-xylene, the methyl groups are positioned meta to each other, which minimizes steric hindrance and allows for a higher electron density at the reactive positions. This makes m-xylene undergo nitration much faster than p-xylene.

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Key Concepts

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

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. The reaction is facilitated by the stability of the aromatic system, which allows for the formation of a resonance-stabilized carbocation intermediate. Understanding EAS is crucial for analyzing the reactivity of substituted aromatic compounds, such as m-xylene and p-xylene, in nitration reactions.

Substituent Effects

Substituent effects refer to how different groups attached to an aromatic ring influence its reactivity and orientation during electrophilic substitution. Electron-donating groups, like methyl groups in xylene, increase the electron density of the ring, enhancing its reactivity towards electrophiles. The position of these substituents (ortho, meta, para) also affects the rate of reaction, with m-xylene being more reactive than p-xylene due to the favorable resonance stabilization of the intermediate formed during nitration.

Resonance Stabilization

Resonance stabilization is a key concept in organic chemistry that describes how the electron density in a molecule can be delocalized over multiple atoms, leading to increased stability. In the case of m-xylene, the formation of a resonance-stabilized carbocation during nitration is more favorable compared to p-xylene. This increased stabilization in m-xylene's transition state results in a significantly faster reaction rate, making it more reactive towards nitration.

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Related Practice

Textbook Question

Step 2 of the iodination of benzene shows water acting as a base and removing a proton from the sigma complex. We did not consider the possibility of water acting as a nucleophile and attacking the carbocation, as in an electrophilic addition to an alkene. Draw the reaction that would occur if water reacted as a nucleophile and added to the carbocation. Explain why this type of addition is rarely observed.

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Textbook Question

Propose a mechanism for the aluminum chloride–catalyzed reaction of benzene with chlorine.

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Textbook Question

Styrene (vinylbenzene) undergoes electrophilic aromatic substitution much faster than benzene, and the products are found to be primarily ortho- and para-substituted styrenes. Use resonance forms of the intermediates to explain these results.

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Textbook Question

p-Xylene undergoes nitration much faster than benzene. Use resonance forms of the sigma complex to explain this accelerated rate

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Textbook Question

Propose a mechanism for the bromination of ethoxybenzene to give o- and p-bromoethoxybenzene.

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Textbook Question

When bromine is added to two beakers, one containing phenyl isopropyl ether and the other containing cyclohexene, the bromine color in both beakers disappears. What observation could you make while performing this test that would allow you to distinguish the alkene from the aryl ether?

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