Identify the correct order of reactivity in electrophilic...

Identify the correct order of reactivity in electrophilic substitution reactions of the following compounds. (a) $\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}$ (b) IV $>\mathrm{III}>\mathrm{II}>\mathrm{I}$ (c) $\mathrm{II}>\mathrm{I}>\mathrm{III}>\mathrm{IV}$ (d) $\mathrm{II}>\mathrm{III}>\mathrm{I}>\mathrm{IV}$

Identify the correct order of reactivity in electrophilic substitution reactions of the following compounds.
(a) $\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}$
(b) IV $>\mathrm{III}>\mathrm{II}>\mathrm{I}$
(c) $\mathrm{II}>\mathrm{I}>\mathrm{III}>\mathrm{IV}$
(d) $\mathrm{II}>\mathrm{III}>\mathrm{I}>\mathrm{IV}$

Key Concepts

Electrophilic Aromatic Substitution (EAS)

This is a fundamental reaction class in aromatic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. The reaction is governed by the generation of a positively charged sigma complex (arenium ion) and results in restoration of aromaticity, making the process highly dependent on the electronic nature of substituents already present on the ring.

Substituent Effects

Substituents on an aromatic ring can influence the reactivity of the ring toward electrophilic attack through their electron-donating or electron-withdrawing characteristics. These effects determine both the rate of reaction and the position at which the electrophile will attack (ortho, meta, or para).

Activating vs. Deactivating Groups

Activating groups increase the electron density on the aromatic ring, thereby enhancing its reactivity toward electrophilic attack, while deactivating groups withdraw electron density, making the aromatic ring less reactive. The overall reactivity order in EAS reactions is determined by comparing the ability of different substituents to donate or withdraw electrons.

Resonance and Inductive Effects

Resonance effects involve the delocalization of electrons through the ?-system of the aromatic ring, which can either stabilize positive charges in intermediates (activating) or destabilize them (deactivating). Inductive effects refer to the electron-withdrawing or donating power of substituents transmitted through sigma bonds, which also influences the reactivity in electrophilic aromatic substitution.

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