8.3 Reactivity of Benzene: Substitution, Addition, and Oxidation Reactions | Hydrocarbons | Chemistry 12

Benzene is a unique organic compound characterized by its highly stable delocalized pi ( $π$ ) electron ring. This aromatic stability influences its reactivity, leading to distinct types of reactions compared to typical unsaturated hydrocarbons like alkenes. Benzene primarily undergoes electrophilic substitution reactions, but can also participate in addition and oxidation reactions under specific, often vigorous, conditions.

8.3.1 Electrophilic Aromatic Substitution (EAS) Reactions

Electrophilic aromatic substitution reactions are the most characteristic reactions of benzene. They involve an electrophile attacking the high electron density of the delocalized $π$ -electron ring, resulting in the replacement of a hydrogen atom on the benzene ring by the electrophile. The delocalized $π$ -electron system is restored in the final product, preserving the aromatic stability.

General Mechanism of Electrophilic Aromatic Substitution Reactions:

EAS reactions involve three main steps:

Generation of the Electrophile: An electrophile ( $E^{+}$ ) is generated from a suitable reagent, often with the help of a Lewis acid catalyst (e.g., $Fe X_{3}$ or $Al X_{3}$ ).
Attack of Electrophile on Benzene (Formation of Arenium Ion): The $π$ -electrons of the benzene ring attack the electrophile, forming a positively charged intermediate known as an arenium ion (also called a sigma ( $σ$ ) complex). In this step, the aromaticity of the benzene ring is temporarily broken.
Removal of Hydrogen (Deprotonation): A Lewis base (often the conjugate base of the catalyst or the catalyst itself) removes a hydrogen ion ( $H^{+}$ ) from the arenium ion. This step restores the delocalized $π$ -electron system, regenerating the aromatic ring and forming the substituted benzene product. The catalyst is also regenerated.

Key Information: The delocalized $π$ -electron system in benzene is far more stable than the localized $π$ -bonds in alkenes. This is why benzene always requires a catalyst for electrophilic substitution reactions, unlike alkenes which readily undergo addition reactions.

Specific Electrophilic Aromatic Substitution Reactions

Halogenation

Halogenation is the reaction where a hydrogen atom on the benzene ring is replaced by a halogen atom (e.g., $- Cl$ , $- Br$ ). This reaction typically requires a Lewis acid catalyst, such as iron(III) halide ( $Fe X_{3}$ ) or aluminum halide ( $Al X_{3}$ ).

General Reaction:

Figure 8.3.1: General halogenation reaction of benzene.

Example: Bromination of Benzene When benzene reacts with bromine in the presence of iron(III) bromide ( $FeB r_{3}$ ), bromobenzene is formed.

Mechanism of Bromination:

Generation of Electrophile ( $Br^{+}$ ): Bromine ( $B r_{2}$ ) acts as a Lewis base, donating electrons to the Lewis acid $FeB r_{3}$ , generating the highly reactive bromonium ion ( $Br^{+}$ ).
Formation of Arenium Ion ( $σ$ -Complex): The $π$ -electrons of the benzene ring attack the bromonium ion ( $Br^{+}$ ), forming a carbocation intermediate (arenium ion), which has lost its aromaticity temporarily.
Deprotonation/Regeneration of Catalyst: The $FeB r_{4}^{-}$ ion (a Lewis base) abstracts a proton ( $H^{+}$ ) from the arenium ion, restoring the aromatic ring and regenerating the $FeB r_{3}$ catalyst.

Chlorination of Benzene: Similar to bromination, chlorination occurs with chlorine gas in the presence of a catalyst like $AlC l_{3}$ , producing chlorobenzene and hydrogen chloride.

Mechanism follows the same three steps as bromination, substituting $Br$ with $Cl$ and $FeB r_{3}$ with $AlC l_{3}$ .

Halogenation of Alkylarenes (e.g., Methylbenzene): Alkyl substituents (like methyl groups) are ring activating and ortho/para directing. They increase electron density at positions 2 (ortho) and 4 (para) on the benzene ring, making these positions more susceptible to electrophilic attack. Hydroxyl ( $- O H$ ) and amino ( $- N H_{2}$ ) groups similarly activate the ring.

Example: Chlorination of methylbenzene (toluene) produces a mixture of 2-chlorotoluene and 4-chlorotoluene.

In excess chlorine, multiple substitutions can occur, leading to products like 2,4-dichlorotoluene, 2,6-dichlorotoluene, and 2,4,6-trichlorotoluene.

Side Chain Halogenation: In contrast to ring halogenation, if chlorine reacts with an alkyl side chain of an alkylbenzene in the presence of ultraviolet (UV) light (or strong sunlight), a free-radical substitution mechanism occurs on the side chain, not the ring.

With excess chlorine, all hydrogen atoms on the methyl side chain can be replaced by chlorine atoms.

Nitration

Nitration introduces a nitro group ( $- N O_{2}$ ) to the benzene ring. The electrophile for this reaction is the nitronium ion ( $N O_{2}^{+}$ ), generated from a mixture of concentrated nitric acid ( $HN O_{3}$ ) and concentrated sulfuric acid ( $H_{2} S O_{4}$ ), known as the 'nitrating mixture'. The reaction is typically carried out by refluxing benzene at $25 - 6 0^{\circ} C$ .

General Reaction:

Nitration of benzene — Figure 8.3.3: General nitration reaction of benzene.

Mechanism of Nitration:

Generation of Nitronium Ion ( $N O_{2}^{+}$ ): Sulfuric acid protonates nitric acid, which then dehydrates to form the nitronium ion. $HN O_{3} + H_{2} S O_{4} ⇌ H_{2} N O_{3}^{+} + HS O_{4}^{-}$ $H_{2} N O_{3}^{+} ⇌ N O_{2}^{+} + H_{2} O$ Or more generally:

Figure 8.3.4: Formation of nitronium ion.

Formation of Arenium Ion ( $σ$ -Complex): The benzene $π$ -electrons attack the nitronium ion, forming the arenium ion intermediate. This intermediate is resonance stabilized.
Deprotonation/Regeneration of Catalyst: A base (typically $HS O_{4}^{-}$ ) removes a proton from the arenium ion, restoring the aromaticity and forming nitrobenzene. Sulfuric acid is regenerated.

Friedel-Crafts Reactions

Discovered by Charles Friedel and James Crafts in 1877, these reactions introduce alkyl or acyl groups to the benzene ring, catalyzed by Lewis acids like aluminum chloride ( $AlC l_{3}$ ).

i. Alkylation (Friedel-Crafts Alkylation)

Introduction of an alkyl group (e.g., methyl, ethyl) into the benzene ring by reacting benzene with a chloroalkane (alkyl halide) in the presence of a Lewis acid catalyst like $AlC l_{3}$ .

General Reaction:

Figure 8.3.5: General Friedel-Crafts alkylation reaction.

Example: Reaction of benzene with chloromethane to form methylbenzene (toluene).

Figure 8.3.6: Formation of methylbenzene from benzene.

Mechanism of Friedel-Crafts Alkylation:

Generation of Electrophile ( $R^{+}$ ): The alkyl halide reacts with the Lewis acid catalyst ( $AlC l_{3}$ ) to generate a carbocation (alkyl cation), which acts as the electrophile. Example: Generation of methyl cation ( $C H_{3}^{+}$ ). $C H_{3} Cl + AlC l_{3} \to C H_{3}^{+} + [AlC l_{4}]^{-}$
Formation of Arenium Ion: The benzene $π$ -electrons attack the alkyl cation, forming the arenium ion. This ion is resonance stabilized.
Deprotonation/Regeneration of Catalyst: The $[AlC l_{4}]^{-}$ ion removes a proton from the arenium ion, restoring aromaticity and regenerating $AlC l_{3}$ . $[AlC l_{4}]^{-} + H^{+} \to AlC l_{3} + HCl$

ii. Acylation (Friedel-Crafts Acylation)

Introduction of an acyl group ( $R - CO -$ ) into the benzene ring by reacting benzene with an acyl halide ( $RCOCl$ ) in the presence of a Lewis acid catalyst ( $Al X_{3}$ ). The product is a phenyl ketone.

Mechanism of Friedel-Crafts Acylation:

Generation of Electrophile (Acylium Ion): The acyl halide reacts with the Lewis acid catalyst ( $AlC l_{3}$ ) to generate a resonance-stabilized acylium ion ( $R - C \equiv O^{+}$ ), which is the electrophile. $RCOCl + AlC l_{3} \to R - C \equiv O^{+} + [AlC l_{4}]^{-}$
Formation of Arenium Ion: The benzene $π$ -electrons attack the acylium ion, forming the arenium ion intermediate.
Deprotonation/Regeneration of Catalyst: A base ( $[AlC l_{4}]^{-}$ ) removes a proton from the arenium ion, restoring aromaticity and forming the phenyl ketone. $AlC l_{3}$ is regenerated. $[AlC l_{4}]^{-} + H^{+} \to AlC l_{3} + HCl$

Key Information: Aluminum chloride ( $AlC l_{3}$ ) is often referred to as a halogen carrier in Friedel-Crafts reactions because it helps generate the electrophilic halogen species.

8.3.2 Addition Reactions

Benzene, despite its apparent unsaturation, is generally reluctant to undergo addition reactions under normal conditions. This is due to the inherent stability of its delocalized $π$ -electron aromatic system. Breaking this stable system requires harsh conditions (high temperature, high pressure, strong reagents).

Hydrogenation

Hydrogenation is the addition of hydrogen molecules across the carbon atoms of the benzene ring, converting it into cyclohexane. This reaction breaks the aromaticity and requires specific conditions:

Catalysts: Nickel ( $\mathrm{Ni$ ), Platinum ( $Pt$ ), or Palladium ( $Pd$ )
Temperature: $150 - 20 0^{\circ} C$
Pressure: $20 - 30 atm$

General Reaction: $C_{6} H_{6} + 3 H_{2} Ni, 150 - 20 0^{\circ} C, 20 - 30atm C_{6} H_{12}$

Role of Catalyst: The catalyst provides a surface area that breaks hydrogen molecules into individual hydrogen atoms. It also interacts with the benzene molecule, weakening its $π$ -electron system and making it more reactive. Six hydrogen atoms then add to the six carbon atoms of the benzene molecule, forming cyclohexane and releasing it from the catalyst surface.

Example: Hydrogenation of methylbenzene yields methylcyclohexane under similar conditions.

Difference Between Addition Reactions and Electrophilic Substitution Reactions

The fundamental difference lies in the fate of the intermediate carbocation formed after the initial electrophilic attack:

Feature	Electrophilic Aromatic Substitution	Addition Reactions (e.g., Alkenes)
Initial Attack	Electrophile attacks $π$ -system	Electrophile attacks $π$ -system
Intermediate	Arenium ion (retains some unsaturation)	Bridged halonium ion or carbocation (localized positive charge)
Second Step	Loss of $H^{+}$ to restore aromaticity	Attack by nucleophile to form a saturated product
Aromaticity	Restored in the final product	Destroyed (if aromatic) or leads to saturated product
Product	Substituted aromatic compound	Adduct (saturated compound)
Conditions for Benzene	Requires catalyst, relatively mild	Requires harsh conditions (high T/P, strong reagents)

8.3.3 Oxidation Reactions

Benzene and other alkanes are generally resistant to oxidation by common oxidizing agents. However, they can be oxidized under certain conditions. In alkylbenzenes, the aliphatic side chain is often more susceptible to oxidation than the aromatic ring itself, especially under prolonged heating or strong oxidizing conditions.

The presence of the benzene ring activates the alkyl side chain towards oxidation. For instance, while alkanes are not easily oxidized by potassium manganate(VII), the methyl side-chain of methylbenzene is readily oxidized by hot alkaline or hot acidic potassium manganate(VII) ( $KMn O_{4}$ ) to produce benzoic acid.

The oxidation typically targets the carbon atom of the alkyl side-chain closest to the benzene ring (the benzylic carbon), converting it into a carboxylic acid group ( $- COOH$ ). Longer alkyl chains will have the benzylic carbon oxidized to a carboxylic acid, and other carbons may be oxidized to carbon dioxide.

Example: Oxidation of Methylbenzene to Benzoic Acid

For alkyl chains longer than methyl, all carbon atoms in the side chain, except the benzylic carbon (which forms the carboxylic acid group), are typically oxidized to carbon dioxide ( $C O_{2}$ ).

Mechanism of Methylbenzene Oxidation to Benzoic Acid:

The mechanism involves a series of free-radical and carbocation intermediates:

Hydrogen Abstraction & Benzyl Cation Formation: Hot alkaline $KMn O_{4}$ abstracts a benzylic hydrogen atom as a free radical, forming a benzyl radical. This radical then reacts with an odd-electron intermediate of $KMn O_{4}$ to form a benzyl cation.
Hydroxyl Group Addition: The benzyl cation abstracts a hydroxyl group ( $- O H$ ) from potassium hydrogenmanganate, forming an unstable alcohol.
Further Oxidation: The alkaline $KMn O_{4}$ continues to oxidize the benzylic carbon, first to a diol, then to benzaldehyde, and finally to benzoic acid.

8.3.1 Electrophilic Aromatic Substitution (EAS) Reactions

General Mechanism of Electrophilic Aromatic Substitution Reactions:

EAS reactions involve three main steps:

Generation of the Electrophile: An electrophile ( $E^{+}$ ) is generated from a suitable reagent, often with the help of a Lewis acid catalyst (e.g., $Fe X_{3}$ or $Al X_{3}$ ).
Attack of Electrophile on Benzene (Formation of Arenium Ion): The $π$ -electrons of the benzene ring attack the electrophile, forming a positively charged intermediate known as an arenium ion (also called a sigma ( $σ$ ) complex). In this step, the aromaticity of the benzene ring is temporarily broken.
Removal of Hydrogen (Deprotonation): A Lewis base (often the conjugate base of the catalyst or the catalyst itself) removes a hydrogen ion ( $H^{+}$ ) from the arenium ion. This step restores the delocalized $π$ -electron system, regenerating the aromatic ring and forming the substituted benzene product. The catalyst is also regenerated.

Specific Electrophilic Aromatic Substitution Reactions

Halogenation

General Reaction:

Example: Bromination of Benzene When benzene reacts with bromine in the presence of iron(III) bromide ( $FeB r_{3}$ ), bromobenzene is formed.

Mechanism of Bromination:

Generation of Electrophile ( $Br^{+}$ ): Bromine ( $B r_{2}$ ) acts as a Lewis base, donating electrons to the Lewis acid $FeB r_{3}$ , generating the highly reactive bromonium ion ( $Br^{+}$ ).
Formation of Arenium Ion ( $σ$ -Complex): The $π$ -electrons of the benzene ring attack the bromonium ion ( $Br^{+}$ ), forming a carbocation intermediate (arenium ion), which has lost its aromaticity temporarily.
Deprotonation/Regeneration of Catalyst: The $FeB r_{4}^{-}$ ion (a Lewis base) abstracts a proton ( $H^{+}$ ) from the arenium ion, restoring the aromatic ring and regenerating the $FeB r_{3}$ catalyst.

Chlorination of Benzene: Similar to bromination, chlorination occurs with chlorine gas in the presence of a catalyst like $AlC l_{3}$ , producing chlorobenzene and hydrogen chloride.

Mechanism follows the same three steps as bromination, substituting $Br$ with $Cl$ and $FeB r_{3}$ with $AlC l_{3}$ .

Example: Chlorination of methylbenzene (toluene) produces a mixture of 2-chlorotoluene and 4-chlorotoluene.

In excess chlorine, multiple substitutions can occur, leading to products like 2,4-dichlorotoluene, 2,6-dichlorotoluene, and 2,4,6-trichlorotoluene.

With excess chlorine, all hydrogen atoms on the methyl side chain can be replaced by chlorine atoms.

Nitration

General Reaction:

Mechanism of Nitration:

Generation of Nitronium Ion ( $N O_{2}^{+}$ ): Sulfuric acid protonates nitric acid, which then dehydrates to form the nitronium ion. $HN O_{3} + H_{2} S O_{4} ⇌ H_{2} N O_{3}^{+} + HS O_{4}^{-}$ $H_{2} N O_{3}^{+} ⇌ N O_{2}^{+} + H_{2} O$ Or more generally:

Formation of Arenium Ion ( $σ$ -Complex): The benzene $π$ -electrons attack the nitronium ion, forming the arenium ion intermediate. This intermediate is resonance stabilized.
Deprotonation/Regeneration of Catalyst: A base (typically $HS O_{4}^{-}$ ) removes a proton from the arenium ion, restoring the aromaticity and forming nitrobenzene. Sulfuric acid is regenerated.

Friedel-Crafts Reactions

Discovered by Charles Friedel and James Crafts in 1877, these reactions introduce alkyl or acyl groups to the benzene ring, catalyzed by Lewis acids like aluminum chloride ( $AlC l_{3}$ ).

i. Alkylation (Friedel-Crafts Alkylation)

Introduction of an alkyl group (e.g., methyl, ethyl) into the benzene ring by reacting benzene with a chloroalkane (alkyl halide) in the presence of a Lewis acid catalyst like $AlC l_{3}$ .

General Reaction:

Example: Reaction of benzene with chloromethane to form methylbenzene (toluene).

Mechanism of Friedel-Crafts Alkylation:

Generation of Electrophile ( $R^{+}$ ): The alkyl halide reacts with the Lewis acid catalyst ( $AlC l_{3}$ ) to generate a carbocation (alkyl cation), which acts as the electrophile. Example: Generation of methyl cation ( $C H_{3}^{+}$ ). $C H_{3} Cl + AlC l_{3} \to C H_{3}^{+} + [AlC l_{4}]^{-}$
Formation of Arenium Ion: The benzene $π$ -electrons attack the alkyl cation, forming the arenium ion. This ion is resonance stabilized.
Deprotonation/Regeneration of Catalyst: The $[AlC l_{4}]^{-}$ ion removes a proton from the arenium ion, restoring aromaticity and regenerating $AlC l_{3}$ . $[AlC l_{4}]^{-} + H^{+} \to AlC l_{3} + HCl$

ii. Acylation (Friedel-Crafts Acylation)

Mechanism of Friedel-Crafts Acylation:

Generation of Electrophile (Acylium Ion): The acyl halide reacts with the Lewis acid catalyst ( $AlC l_{3}$ ) to generate a resonance-stabilized acylium ion ( $R - C \equiv O^{+}$ ), which is the electrophile. $RCOCl + AlC l_{3} \to R - C \equiv O^{+} + [AlC l_{4}]^{-}$
Formation of Arenium Ion: The benzene $π$ -electrons attack the acylium ion, forming the arenium ion intermediate.
Deprotonation/Regeneration of Catalyst: A base ( $[AlC l_{4}]^{-}$ ) removes a proton from the arenium ion, restoring aromaticity and forming the phenyl ketone. $AlC l_{3}$ is regenerated. $[AlC l_{4}]^{-} + H^{+} \to AlC l_{3} + HCl$

Key Information: Aluminum chloride ( $AlC l_{3}$ ) is often referred to as a halogen carrier in Friedel-Crafts reactions because it helps generate the electrophilic halogen species.

8.3.2 Addition Reactions

Hydrogenation

Hydrogenation is the addition of hydrogen molecules across the carbon atoms of the benzene ring, converting it into cyclohexane. This reaction breaks the aromaticity and requires specific conditions:

Catalysts: Nickel ( $\mathrm{Ni$ ), Platinum ( $Pt$ ), or Palladium ( $Pd$ )
Temperature: $150 - 20 0^{\circ} C$
Pressure: $20 - 30 atm$

General Reaction: $C_{6} H_{6} + 3 H_{2} Ni, 150 - 20 0^{\circ} C, 20 - 30atm C_{6} H_{12}$

Example: Hydrogenation of methylbenzene yields methylcyclohexane under similar conditions.

Difference Between Addition Reactions and Electrophilic Substitution Reactions

The fundamental difference lies in the fate of the intermediate carbocation formed after the initial electrophilic attack:

Feature	Electrophilic Aromatic Substitution	Addition Reactions (e.g., Alkenes)
Initial Attack	Electrophile attacks $π$ -system	Electrophile attacks $π$ -system
Intermediate	Arenium ion (retains some unsaturation)	Bridged halonium ion or carbocation (localized positive charge)
Second Step	Loss of $H^{+}$ to restore aromaticity	Attack by nucleophile to form a saturated product
Aromaticity	Restored in the final product	Destroyed (if aromatic) or leads to saturated product
Product	Substituted aromatic compound	Adduct (saturated compound)
Conditions for Benzene	Requires catalyst, relatively mild	Requires harsh conditions (high T/P, strong reagents)

8.3.3 Oxidation Reactions

Example: Oxidation of Methylbenzene to Benzoic Acid

For alkyl chains longer than methyl, all carbon atoms in the side chain, except the benzylic carbon (which forms the carboxylic acid group), are typically oxidized to carbon dioxide ( $C O_{2}$ ).

Mechanism of Methylbenzene Oxidation to Benzoic Acid:

The mechanism involves a series of free-radical and carbocation intermediates:

Hydrogen Abstraction & Benzyl Cation Formation: Hot alkaline $KMn O_{4}$ abstracts a benzylic hydrogen atom as a free radical, forming a benzyl radical. This radical then reacts with an odd-electron intermediate of $KMn O_{4}$ to form a benzyl cation.
Hydroxyl Group Addition: The benzyl cation abstracts a hydroxyl group ( $- O H$ ) from potassium hydrogenmanganate, forming an unstable alcohol.
Further Oxidation: The alkaline $KMn O_{4}$ continues to oxidize the benzylic carbon, first to a diol, then to benzaldehyde, and finally to benzoic acid.