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Ch. 18 - Reactions of Benzene and Substituted Benzenes
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Non è quello che usi tu?Cambia libro di testo
Capitolo 18, Problema 55a

Rank each group of compounds from most reactive to least reactive toward electrophilic aromatic substitution:
a. benzene, ethylbenzene, chlorobenzene, nitrobenzene, anisole

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1
Understand the concept of electrophilic aromatic substitution (EAS): This reaction involves the substitution of a hydrogen atom on an aromatic ring with an electrophile. The reactivity of the aromatic ring depends on the electronic effects of substituents already present on the ring, which can either activate or deactivate the ring toward EAS.
Classify the substituents on the aromatic rings: 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.
Analyze the substituents in the given compounds: - Benzene: No substituents, so it is the baseline for reactivity. - Ethylbenzene: The ethyl group is an EDG due to its inductive effect, slightly activating the ring. - Chlorobenzene: The chlorine atom is an EWG due to its inductive effect, but it also has a resonance effect that slightly activates the ring. - Nitrobenzene: The nitro group is a strong EWG, significantly deactivating the ring. - Anisole: The methoxy group (-OCH₃) is a strong EDG due to its resonance effect, highly activating the ring.
Rank the compounds based on their reactivity: The order of reactivity is determined by the net effect of the substituents. Strong EDGs (anisole) will make the ring most reactive, followed by weaker EDGs (ethylbenzene). Benzene, with no substituents, is the baseline. Weak EWGs (chlorobenzene) will deactivate the ring slightly, and strong EWGs (nitrobenzene) will make the ring least reactive.
Final ranking from most reactive to least reactive: anisole > ethylbenzene > benzene > chlorobenzene > nitrobenzene. This ranking is based on the electronic effects of the substituents and their influence on the aromatic ring's reactivity toward electrophilic aromatic substitution.

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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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EAS Review

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 -OCH3, 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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Activating and deactivating groups

Resonance and Inductive Effects

Resonance and inductive effects are key to understanding how substituents influence the reactivity of aromatic compounds. Resonance involves the delocalization of electrons across the aromatic system, while inductive effects refer to the electron-withdrawing or donating ability of substituents through sigma bonds. These effects determine the stability of the carbocation intermediate formed during EAS.
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Understanding the Inductive Effect.