Electrophilic Aromatic Substitution and Reactivity of Aromatic Compounds

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Reactions of Aromatic Compounds

Electrophilic Aromatic Substitution (EAS)

Electrophilic Aromatic Substitution is a fundamental reaction type for aromatic compounds, where an electrophile replaces a hydrogen atom on the aromatic ring. The aromatic system is preserved throughout the process, making these reactions distinct from typical addition reactions to alkenes.

Arene (Ar-H): Generic term for aromatic hydrocarbons.
Aryl group (Ar): Derived from arenes by removal of a hydrogen atom.
Electrophile: Species with a full or partial positive charge that attacks the aromatic ring.

Summary of major EAS reactions

General Mechanism of EAS: Arenium Ion Intermediates

The mechanism involves two main steps:

Step 1: The aromatic ring uses two π electrons to react with the electrophile, forming an arenium ion (a delocalized cyclohexadienyl cation).
Step 2: A proton is removed from the arenium ion, regenerating the aromatic system.

Step 1: Formation of arenium ion Step 2: Regeneration of aromaticity

Energy Profile of EAS

The first step (formation of the arenium ion) is highly endothermic and rate-determining due to loss of aromaticity.
The second step (regeneration of aromaticity) is highly exothermic and fast.

Energy diagram for EAS

Major Electrophilic Aromatic Substitution Reactions

Halogenation of Benzene

Halogenation requires a Lewis acid catalyst (e.g., FeCl3 or FeBr3) to generate the electrophilic halogen species. Fluorination is too rapid, and iodination requires special conditions.

Chlorination and bromination of benzene Iodination of benzene

Mechanism of Bromination

Bromine reacts with FeBr3 to form a positive bromine ion.
The bromine ion attacks benzene, forming an arenium ion.
A proton is removed, regenerating aromaticity and FeBr3.

Mechanism of bromination

Nitration of Benzene

Nitration uses a mixture of concentrated nitric and sulfuric acids. The electrophile is the nitronium ion (NO2+).

Mechanism of nitration

Sulfonation of Benzene

Sulfonation is performed with fuming sulfuric acid (contains SO3) or concentrated sulfuric acid. The reaction is reversible and equilibrium-driven.

Mechanism of sulfonation Sulfonation equilibrium

Friedel-Crafts Alkylation

Alkylation of benzene is achieved using alkyl halides and a Lewis acid (AlCl3). Carbocation intermediates are generated, and primary alkyl halides may not form discrete carbocations.

General Friedel-Crafts alkylation Alkylation with propene and cyclohexene Alkylation with cyclohexanol

Friedel-Crafts Acylation

Acylation uses acid chlorides or anhydrides with AlCl3 to introduce acyl groups onto the aromatic ring. The electrophile is the acylium ion, stabilized by resonance.

Acetyl and benzoyl groups Acetophenone synthesis from acetyl chloride Preparation of acid chlorides Formation of acylium ion

Limitations of Friedel-Crafts Reactions

Carbocation rearrangement can occur, leading to mixtures of products.
Strong electron-withdrawing groups and amino groups reduce reactivity.
Aryl and vinyl halides do not undergo Friedel-Crafts reactions.
Polyalkylation is common, but polyacylation is not.

Carbocation rearrangement in alkylation Poor yields with certain substituents Aryl halides do not react Vinyl halides do not react Polyalkylation example

Synthetic Applications: Clemmensen Reduction

Clemmensen reduction converts phenyl ketones to methylene groups, allowing for the synthesis of unbranched alkylbenzenes.

Clemmensen reduction of ethyl phenyl ketone General Clemmensen reduction Synthesis of α-tetralone Synthesis of 3-benzoylpropanoic acid

Effects of Substituents on Reactivity and Orientation

Activating and Deactivating Groups

Activating groups: Increase reactivity and are ortho-para directors.
Deactivating groups: Decrease reactivity; strong deactivators are meta directors.
Halides: Mildly deactivating but ortho-para directors.

Methyl group as activating group Nitration of toluene Bromination of aniline and phenol

Product Distribution Table

Reaction	Ortho Product (%)	Para Product (%)	Total Ortho and Para (%)	Meta Product (%)
Chlorination	39	55	94	6
Bromination	11	87	98	2
Nitration	30	70	100	0
Sulfonation	100	0	100	0

Chlorobenzene substitution Bromobenzene substitution

Classification of Substituents

Ortho–Para Directors	Meta Directors
Strongly Activating: –NH2, –NHR, –NR2, –OH, –O– Moderately Activating: –NHCOCH3, –NHCOOR, –OCH3, –OR Weakly Activating: –CH3, –C2H5, –R, –C6H5 Weakly Deactivating: –F, –Cl, –Br, –I	Moderately Deactivating: –C≡N, –SO3H, –CO2H, –CO2R, –CHO, –COR Strongly Deactivating: –NO2, –NR3+, –CF3, –CCl3

Classification of substituents

Theory of Substituent Effects

Reactivity: Electron-Releasing vs. Electron-Withdrawing Groups

Electron-releasing groups: Stabilize the transition state and arenium ion, lower activation energy, and increase reaction rate.
Electron-withdrawing groups: Destabilize the transition state and arenium ion, raise activation energy, and decrease reaction rate.

Effect of substituents on transition state Free-energy profiles for substituent effects

Inductive and Resonance Effects

Inductive effect: Polarized bonds to substituents (e.g., halogens) can withdraw electron density, slowing the reaction.
Resonance effect: Lone pairs on substituents can stabilize the arenium ion by contributing additional resonance forms.

Inductive effect example Resonance effect example

Meta-Directing Groups: Mechanism

Meta-directing groups have a partial or full positive charge on the atom attached to the ring, destabilizing ortho and para arenium ions.
Meta substitution is favored due to less destabilization.

Ortho attack with CF3 group Para attack with CF3 group Meta-directing group example

Ortho-Para Directing Groups

Groups with lone pairs (e.g., –NH2, –OH, –Cl) stabilize the arenium ion in ortho and para substitution via resonance.
These groups are activating and ortho-para directors.

Groups with lone pairs Ortho attack with NH2 group Meta attack with NH2 group

Summary Table: Substituent Effects

Substituent	Effect	Direction
–CH3, –NH2, –OH	Activating	Ortho-Para
–NO2, –COOH, –SO3H	Deactivating	Meta
–Cl, –Br	Deactivating	Ortho-Para

Additional info:

All mechanisms and tables are expanded with academic context for clarity.
Images are included only when directly relevant to the explanation.