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Ch. 18 - Reactions of Benzene and Substituted Benzenes
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
Chapter 19, Problem 55b

Rank each group of compounds from most reactive to least reactive toward electrophilic aromatic substitution:
b. 1-chloro-2,4-dinitrobenzene, 2,4-dinitrophenol, 1-methyl-2,4-dinitrobenzene

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1
Step 1: Understand the concept of electrophilic aromatic substitution (EAS). EAS is a reaction where an electrophile replaces a hydrogen atom on an aromatic ring. The reactivity of the aromatic compound depends on the substituents already present on the ring, which can either activate or deactivate the ring toward electrophilic attack.
Step 2: Analyze the substituents on each compound. Substituents can be electron-donating groups (EDGs) or electron-withdrawing groups (EWGs). EDGs increase the electron density on the aromatic ring, making it more reactive toward electrophiles, while EWGs decrease the electron density, making the ring less reactive.
Step 3: For 1-chloro-2,4-dinitrobenzene, the substituents are a chlorine atom (weakly electron-withdrawing due to its inductive effect but slightly donating via resonance) and two nitro groups (strongly electron-withdrawing due to their -NO₂ groups). The nitro groups significantly deactivate the ring toward EAS.
Step 4: For 2,4-dinitrophenol, the substituents are a hydroxyl group (-OH, a strong electron-donating group via resonance) and two nitro groups (strongly electron-withdrawing). The hydroxyl group partially counteracts the deactivating effect of the nitro groups, making this compound more reactive than 1-chloro-2,4-dinitrobenzene.
Step 5: For 1-methyl-2,4-dinitrobenzene, the substituents are a methyl group (-CH₃, a weak electron-donating group via hyperconjugation) and two nitro groups (strongly electron-withdrawing). The methyl group slightly increases the reactivity of the ring compared to 1-chloro-2,4-dinitrobenzene but less so than the hydroxyl group in 2,4-dinitrophenol. Rank the compounds based on these observations.

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Key Concepts

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

Electrophilic Aromatic Substitution (EAS)

Electrophilic Aromatic Substitution is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. The reactivity of the aromatic compound is influenced by the nature of substituents already present on the ring, which can either activate or deactivate the ring towards further substitution.
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Activating and Deactivating Groups

Substituents on an aromatic ring can be classified as activating or deactivating based on their electronic effects. Activating groups, such as -OH or -NH2, increase the electron density of the ring, making it more reactive towards electrophiles. In contrast, deactivating groups, like -NO2 or -Cl, withdraw electron density, reducing reactivity.
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Ortho/Para vs. Meta Directing

The position of substituents on an aromatic ring affects the orientation of new substituents during EAS. Activating groups typically direct new electrophiles to the ortho and para positions, while deactivating groups often direct them to the meta position. Understanding these directing effects is crucial for predicting the reactivity and product distribution in EAS reactions.
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Ortho, Para major products