An arene is a hydrocarbon that contains a benzene ring as part of its structure. Benzene itself (C 6 H 6 ) is the simplest arene.

Why does benzene not readily react with bromine water, unlike alkenes?

In alkenes, the localised π bond creates a region of high electron density that polarises Br 2 , enabling electrophilic addition. In benzene, the electrons are delocalised across the ring, so there is no localised high electron density to polarise Br 2 . A halogen carrier (e.g. AlBr 3 ) is needed to generate an electrophile before benzene will react.

True or False? All carbon–carbon bonds in benzene are the same length.

True. Because the electrons are delocalised across the ring, all C–C bonds have equal single and double bond character. Bond length measurements confirm all six bonds are identical — this is key evidence against the Kekulé model.

What is the Kekulé structure of benzene?

The Kekulé structure is a model of benzene proposed by Kekulé in which six carbon atoms form a hexagonal ring with alternating single and double bonds . This model predicts benzene should react like an alkene, but experimental evidence shows it does not.

What hybridisation do the carbon atoms in benzene adopt, and what bond angles does this produce?

The carbon atoms in benzene are sp 2 hybridised , which produces bond angles of 120° and a planar ring structure. The remaining unhybridised p orbital on each carbon contributes to the delocalised π system .

The enthalpy change of hydrogenation of cyclohexene (one C=C) is .......... . If benzene had three isolated double bonds, the predicted value would be .......... . The actual measured value for benzene is .......... .

The enthalpy change of hydrogenation of cyclohexene (one C=C) is −120 kJ mol −1 . If benzene had three isolated double bonds, the predicted value would be −360 kJ mol −1 . The actual measured value for benzene is −208 kJ mol −1 . The difference (~152 kJ mol −1 ) is the delocalisation energy , showing benzene is more stable than the Kekulé structure predicts.

True or False? Benzene undergoes electrophilic addition with bromine.

False. Benzene undergoes electrophilic substitution , not addition. Addition would disrupt the stable delocalised π system . Maintaining the ring's delocalisation is energetically favourable, so substitution (retaining the ring) is preferred.

Define delocalisation in benzene.

Delocalisation In benzene, the p orbitals on each carbon atom overlap laterally with neighbouring p orbitals to form a π system spread over the entire ring. This means the electrons are not fixed between any two carbon atoms — they are free to move over all six carbons, producing two ring-shaped clouds of electron density above and below the plane of the ring.

What two types of experimental evidence support the delocalised model of benzene rather than the Kekulé structure?

Enthalpy of hydrogenation — the actual value (−208 kJ mol −1 ) is much less exothermic than the predicted value for three isolated double bonds (−360 kJ mol −1 ), showing benzene is more stable than the Kekulé model predicts. Bond lengths — all six C–C bonds in benzene are identical in length, intermediate between a single and double bond, consistent with delocalisation rather than alternating bonds.

What are the products when benzene undergoes complete combustion?

Complete combustion of benzene produces carbon dioxide and water : 2C 6 H 6 (l) + 15O 2 (g) → 12CO 2 (g) + 6H 2 O (g) Because benzene has a high carbon-to-hydrogen ratio, incomplete combustion is common, producing a smoky yellow flame .

What is nitration of benzene?

Nitration is a reaction in which a nitro group (–NO 2 ) replaces a hydrogen atom on the benzene ring. Benzene is reacted with a mixture of concentrated HNO 3 and concentrated H 2 SO 4 at 25–60 °C. The product is nitrobenzene (C 6 H 5 NO 2 ).

What is the role of the metal halide carrier in the halogenation of benzene?

The metal halide carrier (e.g. AlBr 3 or FeBr 3 ) acts as a catalyst . It reacts with the halogen (X 2 ) to generate a highly electrophilic X + ion, which attacks the benzene ring. The carrier is regenerated at the end of the reaction.

True or False? Benzene reacts with bromine via electrophilic addition.

False. Benzene undergoes electrophilic substitution , not addition. A halogen carrier such as AlBr 3 or FeBr 3 is required to generate the Br + electrophile. Addition would destroy the stable delocalised ring.

What are Friedel-Crafts reactions, and what two types exist?

Friedel-Crafts reactions are electrophilic substitution reactions used to introduce substituents onto a benzene ring, making it more reactive for further synthesis. The two types are: Friedel-Crafts alkylation — introduces an alkyl group Friedel-Crafts acylation — introduces an acyl group

In the halogenation of benzene, the halogen carrier AlCl 3 reacts with Cl 2 to generate the electrophile .......... , producing the byproduct .......... . The benzene ring is then attacked, and a hydrogen atom is replaced to give a product of formula .......... .

In the halogenation of benzene, the halogen carrier AlCl 3 reacts with Cl 2 to generate the electrophile Cl + , producing the byproduct AlCl 4 − . The benzene ring is then attacked, and a hydrogen atom is replaced to give a product of formula C 6 H 5 Cl .

What are the three steps common to all electrophilic substitution reactions of benzene?

Generating the electrophile — the reagent (or catalyst) forms a reactive electrophile Electrophilic attack on the benzene ring — the electrophile attacks the delocalised π system Regenerating aromaticity — a proton (H + ) is lost to restore the delocalised ring

True or False? Sulfuric acid acts as a catalyst in the nitration of benzene.

True. Concentrated H 2 SO 4 acts as a catalyst by protonating HNO 3 to generate the nitronium ion (NO 2 + ), which is the electrophile that attacks the benzene ring. The H 2 SO 4 is regenerated at the end of the reaction.

What conditions are required for the nitration of benzene, and why must the temperature be controlled?

Conditions: concentrated HNO 3 and concentrated H 2 SO 4 , at a temperature of 25–60 °C . The temperature must be kept below 60 °C to prevent further nitration, which would introduce a second (or third) nitro group and give unwanted polynitration products.

Define electrophilic substitution .

Electrophilic substitution is a reaction in which an electrophile replaces a hydrogen atom on the benzene ring. The delocalised π system is preserved because substitution (rather than addition) maintains the aromatic stability of the ring.

What are the three steps in the general mechanism for electrophilic substitution of benzene?

Generation of the electrophile — the reactive electrophile is formed in situ from the reagents Electrophilic attack — the electrophile is attracted to the delocalised π system and bonds to the ring, forming an arenium ion intermediate Regenerating aromaticity — a proton (H + ) is lost from the ring, restoring the delocalised π system

True or False? Electrophiles used in benzene reactions must always be added directly to the reaction mixture.

False. Electrophiles are usually too weak or unstable to be added directly. They are generated in situ from appropriate reagents, such as using AlCl 3 with Cl 2 to produce Cl + , or concentrated H 2 SO 4 with HNO 3 to produce NO 2 + .

In the Friedel-Crafts acylation mechanism, an .......... group is substituted onto the benzene ring. The electrophile is the .......... ion, generated by the acyl chloride reacting with .......... . The halogen carrier is regenerated at the end.

In the Friedel-Crafts acylation mechanism, an acyl group is substituted onto the benzene ring. The electrophile is the acylium ion (RCO + ), generated by the acyl chloride reacting with AlCl 3 . The halogen carrier is regenerated at the end.

How is the nitronium ion generated in the nitration of benzene?

Concentrated H 2 SO 4 acts as an acid and protonates concentrated HNO 3 , generating the nitronium ion (NO 2 + ): HNO 3 + H 2 SO 4 → NO 2 + + HSO 4 − + H 2 O NO 2 + is the electrophile that attacks the benzene ring.

An arenium ion is the positively charged intermediate formed when an electrophile bonds to the benzene ring during electrophilic substitution. At this stage, the delocalised π system is temporarily disrupted. The arenium ion then loses a proton (H + ) to regenerate aromaticity .

Why is aromaticity regenerated in the final step of electrophilic substitution rather than being permanently lost?

After the electrophile bonds to the ring to form the arenium ion intermediate, the ring is destabilised because the delocalised π system is disrupted. Loss of a proton (H + ) from the sp 3 carbon restores the delocalised ring, which is more stable than the arenium ion. This strong thermodynamic driving force means substitution always completes by regenerating aromaticity.

True or False? In Friedel-Crafts acylation, the product contains a C=O group directly attached to the benzene ring.

True. Acylation introduces an acyl group (RCO–) onto the ring. The product is a ketone where the carbonyl (C=O) group is directly bonded to the aromatic ring. This is more useful synthetically than alkylation because the product is less reactive toward further substitution.

How does benzene's electrophilic substitution differ mechanistically from an alkene's electrophilic addition?

In alkene electrophilic addition , the π bond breaks and the electrophile and nucleophile both add across the double bond — no atoms are lost. In benzene electrophilic substitution , the electrophile attacks the delocalised ring and a hydrogen atom is lost as H + to restore the aromatic π system . Aromaticity is preserved throughout.

A phenol is a compound in which an –OH group is directly attached to a benzene ring. Its molecular formula is C 6 H 5 OH. The –OH group interacts with the π system of the ring, making phenol more reactive toward electrophiles than benzene.

Why does phenol react more readily with electrophiles than benzene does?

The –OH group in phenol is an activating group . One of the lone pairs on the oxygen overlaps with the π bonding system of the ring, increasing the electron density of the aromatic ring. This makes the ring more susceptible to electrophilic attack compared to benzene.

True or False? Phenol requires a halogen carrier to react with bromine.

False. Unlike benzene, phenol reacts with bromine water at room temperature without a halogen carrier. The –OH group activates the ring sufficiently for the Br 2 molecule to act as the electrophile directly, without needing to generate Br + in situ.

To which positions on the benzene ring does the –OH group direct incoming electrophiles in phenol?

The –OH group directs electrophiles to the 2, 4, and 6 positions (ortho and para positions) on the benzene ring. This is because the overlap of the oxygen lone pair with the π system increases electron density most at these positions.

When phenol reacts with bromine water, the orange solution is .......... . A white precipitate of .......... is formed. Three bromine atoms substitute at the .......... positions.

When phenol reacts with bromine water, the orange solution is decolourised . A white precipitate of 2,4,6-tribromophenol is formed. Three bromine atoms substitute at the 2, 4, and 6 positions.

What are the observable changes when phenol reacts with bromine water?

The orange bromine solution is decolourised (turns colourless), and a white precipitate of 2,4,6-tribromophenol forms. Three hydrogen atoms are substituted by bromine atoms in a single reaction, reflecting how strongly the –OH group activates the ring.

True or False? The bromination of phenol is an electrophilic substitution reaction.

True. Bromine acts as the electrophile (or is polarised by the electron-rich ring) and substitutes hydrogen atoms at the 2, 4, and 6 positions. The –OH group activates the ring enough that Br 2 can react without a halogen carrier, but the reaction type is still electrophilic substitution .

How does the lone pair on the oxygen of phenol affect the benzene ring's reactivity?

The lone pair on the oxygen overlaps with the π bonding system of the benzene ring, donating electron density into the ring. This increases the electron density overall, particularly at the ortho and para positions (2, 4, 6), making the ring far more reactive toward electrophilic attack than benzene.

A LevelChemistryEdexcelFlashcards7. Advanced Organic ChemistryOrganic Chemistry III: Aromatic Chemistry

Organic Chemistry III: Aromatic Chemistry (Edexcel A Level Chemistry): Flashcards

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FrontBenzene Structure
What is an arene?

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What is an arene?
An arene is a hydrocarbon that contains a benzene ring as part of its structure.
Benzene itself (C₆H₆) is the simplest arene.
Why does benzene not readily react with bromine water, unlike alkenes?
In alkenes, the localised π bond creates a region of high electron density that polarises Br₂, enabling electrophilic addition.
In benzene, the electrons are delocalised across the ring, so there is no localised high electron density to polarise Br₂. A halogen carrier (e.g. AlBr₃) is needed to generate an electrophile before benzene will react.
True or False?
All carbon–carbon bonds in benzene are the same length.
True.
Because the electrons are delocalised across the ring, all C–C bonds have equal single and double bond character. Bond length measurements confirm all six bonds are identical — this is key evidence against the Kekulé model.
What is the Kekulé structure of benzene?
The Kekulé structure is a model of benzene proposed by Kekulé in which six carbon atoms form a hexagonal ring with alternating single and double bonds.
This model predicts benzene should react like an alkene, but experimental evidence shows it does not.
What hybridisation do the carbon atoms in benzene adopt, and what bond angles does this produce?
The carbon atoms in benzene are sp² hybridised, which produces bond angles of 120° and a planar ring structure.
The remaining unhybridised p orbital on each carbon contributes to the delocalised π system.
The enthalpy change of hydrogenation of cyclohexene (one C=C) is ........... If benzene had three isolated double bonds, the predicted value would be ........... The actual measured value for benzene is ...........
The enthalpy change of hydrogenation of cyclohexene (one C=C) is −120 kJ mol⁻¹. If benzene had three isolated double bonds, the predicted value would be −360 kJ mol⁻¹. The actual measured value for benzene is −208 kJ mol⁻¹.
The difference (~152 kJ mol⁻¹) is the delocalisation energy, showing benzene is more stable than the Kekulé structure predicts.
True or False?
Benzene undergoes electrophilic addition with bromine.
False.
Benzene undergoes electrophilic substitution, not addition. Addition would disrupt the stable delocalised π system. Maintaining the ring's delocalisation is energetically favourable, so substitution (retaining the ring) is preferred.
Define delocalisation in benzene.
Delocalisation
In benzene, the p orbitals on each carbon atom overlap laterally with neighbouring p orbitals to form a π system spread over the entire ring.
This means the electrons are not fixed between any two carbon atoms — they are free to move over all six carbons, producing two ring-shaped clouds of electron density above and below the plane of the ring.
What two types of experimental evidence support the delocalised model of benzene rather than the Kekulé structure?
1. Enthalpy of hydrogenation — the actual value (−208 kJ mol⁻¹) is much less exothermic than the predicted value for three isolated double bonds (−360 kJ mol⁻¹), showing benzene is more stable than the Kekulé model predicts.
1. Bond lengths — all six C–C bonds in benzene are identical in length, intermediate between a single and double bond, consistent with delocalisation rather than alternating bonds.
What are the products when benzene undergoes complete combustion?
Complete combustion of benzene produces carbon dioxide and water:
2C₆H₆ (l) + 15O₂ (g) → 12CO₂ (g) + 6H₂O (g)
Because benzene has a high carbon-to-hydrogen ratio, incomplete combustion is common, producing a smoky yellow flame.
What is nitration of benzene?
Nitration is a reaction in which a nitro group (–NO₂) replaces a hydrogen atom on the benzene ring.
Benzene is reacted with a mixture of concentrated HNO₃ and concentrated H₂SO₄ at 25–60 °C. The product is nitrobenzene (C₆H₅NO₂).
What is the role of the metal halide carrier in the halogenation of benzene?
The metal halide carrier (e.g. AlBr₃ or FeBr₃) acts as a catalyst. It reacts with the halogen (X₂) to generate a highly electrophilic X⁺ ion, which attacks the benzene ring. The carrier is regenerated at the end of the reaction.
True or False?
Benzene reacts with bromine via electrophilic addition.
False.
Benzene undergoes electrophilic substitution, not addition. A halogen carrier such as AlBr₃ or FeBr₃ is required to generate the Br⁺ electrophile. Addition would destroy the stable delocalised ring.
What are Friedel-Crafts reactions, and what two types exist?
Friedel-Crafts reactions are electrophilic substitution reactions used to introduce substituents onto a benzene ring, making it more reactive for further synthesis.
The two types are:
1. Friedel-Crafts alkylation — introduces an alkyl group
2. Friedel-Crafts acylation — introduces an acyl group
In the halogenation of benzene, the halogen carrier AlCl₃ reacts with Cl₂ to generate the electrophile .........., producing the byproduct .......... . The benzene ring is then attacked, and a hydrogen atom is replaced to give a product of formula .......... .
In the halogenation of benzene, the halogen carrier AlCl₃ reacts with Cl₂ to generate the electrophile Cl⁺, producing the byproduct AlCl₄⁻. The benzene ring is then attacked, and a hydrogen atom is replaced to give a product of formula C₆H₅Cl.
What are the three steps common to all electrophilic substitution reactions of benzene?
1. Generating the electrophile — the reagent (or catalyst) forms a reactive electrophile
2. Electrophilic attack on the benzene ring — the electrophile attacks the delocalised π system
3. Regenerating aromaticity — a proton (H⁺) is lost to restore the delocalised ring
True or False?
Sulfuric acid acts as a catalyst in the nitration of benzene.
True.
Concentrated H₂SO₄ acts as a catalyst by protonating HNO₃ to generate the nitronium ion (NO₂⁺), which is the electrophile that attacks the benzene ring. The H₂SO₄ is regenerated at the end of the reaction.
What conditions are required for the nitration of benzene, and why must the temperature be controlled?
Conditions: concentrated HNO₃ and concentrated H₂SO₄, at a temperature of 25–60 °C.
The temperature must be kept below 60 °C to prevent further nitration, which would introduce a second (or third) nitro group and give unwanted polynitration products.
Define electrophilic substitution.
Electrophilic substitution is a reaction in which an electrophile replaces a hydrogen atom on the benzene ring.
The delocalised π system is preserved because substitution (rather than addition) maintains the aromatic stability of the ring.
What are the three steps in the general mechanism for electrophilic substitution of benzene?
1. Generation of the electrophile — the reactive electrophile is formed in situ from the reagents
2. Electrophilic attack — the electrophile is attracted to the delocalised π system and bonds to the ring, forming an arenium ion intermediate
3. Regenerating aromaticity — a proton (H⁺) is lost from the ring, restoring the delocalised π system
True or False?
Electrophiles used in benzene reactions must always be added directly to the reaction mixture.
False.
Electrophiles are usually too weak or unstable to be added directly. They are generated in situ from appropriate reagents, such as using AlCl₃ with Cl₂ to produce Cl⁺, or concentrated H₂SO₄ with HNO₃ to produce NO₂⁺.
In the Friedel-Crafts acylation mechanism, an .......... group is substituted onto the benzene ring. The electrophile is the .......... ion, generated by the acyl chloride reacting with .......... . The halogen carrier is regenerated at the end.
In the Friedel-Crafts acylation mechanism, an acyl group is substituted onto the benzene ring. The electrophile is the acylium ion (RCO⁺), generated by the acyl chloride reacting with AlCl₃. The halogen carrier is regenerated at the end.
How is the nitronium ion generated in the nitration of benzene?
Concentrated H₂SO₄ acts as an acid and protonates concentrated HNO₃, generating the nitronium ion (NO₂⁺):
HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O
NO₂⁺ is the electrophile that attacks the benzene ring.
Define arenium ion.
An arenium ion is the positively charged intermediate formed when an electrophile bonds to the benzene ring during electrophilic substitution.
At this stage, the delocalised π system is temporarily disrupted. The arenium ion then loses a proton (H⁺) to regenerate aromaticity.
Why is aromaticity regenerated in the final step of electrophilic substitution rather than being permanently lost?
After the electrophile bonds to the ring to form the arenium ion intermediate, the ring is destabilised because the delocalised π system is disrupted.
Loss of a proton (H⁺) from the sp³ carbon restores the delocalised ring, which is more stable than the arenium ion. This strong thermodynamic driving force means substitution always completes by regenerating aromaticity.
True or False?
In Friedel-Crafts acylation, the product contains a C=O group directly attached to the benzene ring.
True.
Acylation introduces an acyl group (RCO–) onto the ring. The product is a ketone where the carbonyl (C=O) group is directly bonded to the aromatic ring. This is more useful synthetically than alkylation because the product is less reactive toward further substitution.
How does benzene's electrophilic substitution differ mechanistically from an alkene's electrophilic addition?
In alkene electrophilic addition, the π bond breaks and the electrophile and nucleophile both add across the double bond — no atoms are lost.
In benzene electrophilic substitution, the electrophile attacks the delocalised ring and a hydrogen atom is lost as H⁺ to restore the aromatic π system. Aromaticity is preserved throughout.
Define phenol.
A phenol is a compound in which an –OH group is directly attached to a benzene ring.
Its molecular formula is C₆H₅OH. The –OH group interacts with the π system of the ring, making phenol more reactive toward electrophiles than benzene.
Why does phenol react more readily with electrophiles than benzene does?
The –OH group in phenol is an activating group. One of the lone pairs on the oxygen overlaps with the π bonding system of the ring, increasing the electron density of the aromatic ring.
This makes the ring more susceptible to electrophilic attack compared to benzene.
True or False?
Phenol requires a halogen carrier to react with bromine.
False.
Unlike benzene, phenol reacts with bromine water at room temperature without a halogen carrier. The –OH group activates the ring sufficiently for the Br₂ molecule to act as the electrophile directly, without needing to generate Br⁺ in situ.
To which positions on the benzene ring does the –OH group direct incoming electrophiles in phenol?
The –OH group directs electrophiles to the 2, 4, and 6 positions (ortho and para positions) on the benzene ring.
This is because the overlap of the oxygen lone pair with the π system increases electron density most at these positions.
When phenol reacts with bromine water, the orange solution is .......... . A white precipitate of .......... is formed. Three bromine atoms substitute at the .......... positions.
When phenol reacts with bromine water, the orange solution is decolourised. A white precipitate of 2,4,6-tribromophenol is formed. Three bromine atoms substitute at the 2, 4, and 6 positions.
What are the observable changes when phenol reacts with bromine water?
The orange bromine solution is decolourised (turns colourless), and a white precipitate of 2,4,6-tribromophenol forms.
Three hydrogen atoms are substituted by bromine atoms in a single reaction, reflecting how strongly the –OH group activates the ring.
True or False?
The bromination of phenol is an electrophilic substitution reaction.
True.
Bromine acts as the electrophile (or is polarised by the electron-rich ring) and substitutes hydrogen atoms at the 2, 4, and 6 positions. The –OH group activates the ring enough that Br₂ can react without a halogen carrier, but the reaction type is still electrophilic substitution.
How does the lone pair on the oxygen of phenol affect the benzene ring's reactivity?
The lone pair on the oxygen overlaps with the π bonding system of the benzene ring, donating electron density into the ring.
This increases the electron density overall, particularly at the ortho and para positions (2, 4, 6), making the ring far more reactive toward electrophilic attack than benzene.

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