Define Friedel-Crafts reaction .

A Friedel-Crafts reaction is an electrophilic substitution reaction of a benzene ring with a halogenoalkane (alkylation) or an acyl chloride (acylation) in the presence of an anhydrous AlCl 3 catalyst.

What reagents and conditions are needed to nitrate benzene?

Benzene is mixed with concentrated nitric acid (HNO 3 ) and concentrated sulfuric acid (H 2 SO 4 ) at a temperature between 25 and 60°C. The H 2 SO 4 generates the NO 2 + electrophile .

True or False? Hydrogenation of benzene is a substitution reaction that preserves the aromatic ring.

False. Hydrogenation is an addition reaction that destroys the aromatic stabilisation of the benzene ring, producing cyclohexane.

In Friedel-Crafts alkylation, benzene reacts with a .......... in the presence of anhydrous AlCl 3 to substitute an .......... group onto the ring.

In Friedel-Crafts alkylation, benzene reacts with a chloroalkane in the presence of anhydrous AlCl 3 to substitute an alkyl group onto the ring.

What product is formed when alkyl side-chains in alkylarenes are completely oxidised?

The alkyl side-chains are oxidised to carboxylic acid groups . For example, ethylbenzene treated with hot alkaline KMnO 4 followed by acidification gives benzoic acid.

True or False? Alkylarenes are halogenated at the 2 and 4 positions because alkyl groups are electron-withdrawing.

False. Alkyl groups are electron-donating, not electron-withdrawing. They push electron density into the ring, activating the 2 and 4 positions towards electrophilic attack.

An acyl group is an alkyl group containing a carbonyl (C=O) group, with the general formula RCO–. It is introduced into a benzene ring by Friedel-Crafts acylation.

The complete oxidation of alkylarenes is carried out using hot, alkaline followed by acidification with dilute .

The complete oxidation of alkylarenes is carried out using hot, alkaline KMnO 4 followed by acidification with dilute H 2 SO 4 .

What catalyst and conditions are required for the hydrogenation of benzene?

Benzene is heated with hydrogen gas in the presence of a nickel or platinum catalyst to form cyclohexane . This is an addition reaction that destroys the aromaticity of the ring.

Define halogen carrier .

A halogen carrier is an anhydrous Lewis acid catalyst (such as AlCl 3 or AlBr 3 ) used in the halogenation of arenes. It accepts a lone pair from the halogen molecule to generate the electrophilic halogen cation (e.g. Cl + or Br + ).

What are the three steps of electrophilic substitution in arenes?

Generation of the electrophile (in situ from appropriate reagents). Electrophilic attack : a pair of electrons from the benzene ring is donated to the electrophile, disrupting aromaticity. Regeneration of aromaticity : heterolytic cleavage of the C–H bond returns electrons to the π system.

True or False? During electrophilic attack on benzene, the aromaticity of the ring is maintained throughout the reaction.

False. Aromaticity is temporarily lost during electrophilic attack. The ring only has four π electrons and carries a positive charge spread over five carbons. Aromaticity is restored only in the final step by heterolytic cleavage of the C–H bond.

The NO 2 + electrophile used in nitration is generated by reacting concentrated .......... with concentrated .......... .

The NO 2 + electrophile used in nitration is generated by reacting concentrated HNO 3 with concentrated H 2 SO 4 .

How is the Br + electrophile generated for the bromination of benzene?

Bromine forms a dative bond with the AlBr 3 halogen carrier by donating a lone pair from one of its bromine atoms into an empty 3p orbital of AlBr 3 . This polarises the Br–Br bond and generates Br + in situ.

Define heterolytic cleavage .

A heterolytic cleavage is the breaking of a covalent bond in which both electrons in the bond go to one of the two atoms, generating ions rather than radicals.

True or False? Arenes predominantly undergo addition reactions because the π electrons are easily available for attack.

False. Although arenes are electron-rich, they predominantly undergo substitution reactions because addition would destroy the thermodynamically stable delocalised aromatic π system. Substitution restores aromaticity.

During the halogenation of benzene, the halogen molecule forms a .......... bond with the AlBr 3 catalyst by donating a lone pair into an empty .......... orbital.

During the halogenation of benzene, the halogen molecule forms a dative bond with the AlBr 3 catalyst by donating a lone pair into an empty 3p orbital.

Why does hydrogenation of benzene lead to a less stable product than benzene itself?

Hydrogenation is an addition reaction that completely destroys the aromatic stabilisation of benzene. Cyclohexane lacks the delocalised π system, so it is energetically less stable than benzene.

Define free-radical substitution .

A free-radical substitution is a chain reaction mechanism in which radicals (species with unpaired electrons) react with a substrate, typically initiated by UV light, resulting in substitution of a hydrogen atom.

What conditions direct halogenation to the aromatic ring rather than the side chain in alkylarenes?

Halogenation occurs in the aromatic ring when a halogen and anhydrous halogen carrier catalyst (such as AlBr 3 or AlCl 3 ) are used. The catalyst generates the electrophile for electrophilic substitution.

True or False? Halogenation of the side chain of an alkylarene requires an anhydrous AlCl 3 catalyst.

False. Side-chain halogenation is a free-radical substitution reaction requiring the halogen to be passed into the boiling alkylarene under UV light. No AlCl 3 catalyst is needed.

Aryl halides are less reactive than halogenoalkanes because the carbon–halogen bond in aryl halides has a partial .......... character due to overlap of the halogen lone pair with the benzene .......... system.

Aryl halides are less reactive than halogenoalkanes because the carbon–halogen bond in aryl halides has a partial double bond character due to overlap of the halogen lone pair with the benzene π system.

What happens when excess halogen is used in the side-chain halogenation of an alkylarene?

With excess halogen, all hydrogen atoms on the alkyl side-chain are progressively replaced by halogen atoms. No substitution occurs in the benzene ring under these conditions.

True or False? The carbon–halogen bond in an aryl halide is stronger than in a halogenoalkane.

True. The lone pairs on the halogen in an aryl halide partially overlap with the benzene π system, giving the C–X bond partial double bond character and making it stronger and harder to break than the C–X bond in a halogenoalkane.

An aryl halide is an organic compound in which a halogen atom is bonded directly to a benzene ring. Aryl halides are formed by the electrophilic substitution of arenes with halogens.

Halogenation of an alkylarene in the side chain is initiated by .......... light and proceeds via a .......... mechanism.

Halogenation of an alkylarene in the side chain is initiated by UV light and proceeds via a free-radical substitution mechanism.

Why does the partial overlap of halogen lone pairs with the benzene π system affect the reactivity of aryl halides?

The lone pair overlap strengthens the C–X bond by giving it partial double bond character . This makes the bond harder to break, so aryl halides do not readily undergo nucleophilic substitution unlike halogenoalkanes.

Define electron-donating group .

An electron-donating group is a substituent that pushes electron density into the benzene ring, making it more reactive towards electrophilic substitution and directing incoming electrophiles to the 2, 4 and/or 6 positions.

Where does bromination occur on nitrobenzene, and why?

Bromination of nitrobenzene occurs at the 3 and/or 5 positions . The nitro group (–NO 2 ) is electron-withdrawing, removing electron density from the ring and directing the incoming Br + electrophile to the 3 and 5 positions.

True or False? Electron-withdrawing groups increase the reactivity of a benzene ring towards electrophilic substitution.

False. Electron-withdrawing groups remove electron density from the π system, deactivating the ring and making it less reactive towards electrophilic substitution.

Electron-donating substituents (e.g. –R, –OH, –NH 2 ) direct electrophilic attack to the .......... positions, while electron-withdrawing substituents (e.g. –NO 2 , –COOH) direct attack to the .......... positions.

Electron-donating substituents (e.g. –R, –OH, –NH 2 ) direct electrophilic attack to the 2, 4 and/or 6 positions, while electron-withdrawing substituents (e.g. –NO 2 , –COOH) direct attack to the 3 and/or 5 positions.

Define electron-withdrawing group .

An electron-withdrawing group is a substituent that removes electron density from the benzene ring, deactivating it towards electrophilic substitution and directing incoming electrophiles to the 3 and/or 5 positions.

What two products are formed when methylbenzene is brominated, and what directs the substitution to those positions?

2-bromomethylbenzene and 4-bromomethylbenzene are formed. The methyl group is electron-donating, pushing electron density into the ring and activating the 2 and 4 positions towards the Br + electrophile.

True or False? The –NH 2 group directs electrophilic substitution to the 3 and 5 positions of a benzene ring.

False. The –NH 2 group is an electron-donating substituent that activates the ring and directs electrophilic attack to the 2, 4 and/or 6 positions.

The –COR group is an electron- .......... substituent that .......... the benzene ring to electrophilic attack.

The –COR group is an electron- withdrawing substituent that deactivates the benzene ring to electrophilic attack.

How does an electron-donating group affect the position of electrophilic substitution on a benzene ring?

An electron-donating group increases electron density in the ring and activates it towards electrophilic substitution. It directs the incoming electrophile preferentially to the 2, 4 and/or 6 positions relative to itself.

A LevelChemistryCambridge (CIE)Flashcards30. HydrocarbonsArenes

Arenes (Cambridge (CIE) A Level Chemistry): Flashcards

Exam code: 9701

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Define Friedel-Crafts reaction.

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Define Friedel-Crafts reaction.
A Friedel-Crafts reaction is an electrophilic substitution reaction of a benzene ring with a halogenoalkane (alkylation) or an acyl chloride (acylation) in the presence of an anhydrous AlCl₃ catalyst.
What reagents and conditions are needed to nitrate benzene?
Benzene is mixed with concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄) at a temperature between 25 and 60°C. The H₂SO₄ generates the NO₂⁺ electrophile.
True or False?
Hydrogenation of benzene is a substitution reaction that preserves the aromatic ring.
False.
Hydrogenation is an addition reaction that destroys the aromatic stabilisation of the benzene ring, producing cyclohexane.
In Friedel-Crafts alkylation, benzene reacts with a .......... in the presence of anhydrous AlCl₃ to substitute an .......... group onto the ring.
In Friedel-Crafts alkylation, benzene reacts with a chloroalkane in the presence of anhydrous AlCl₃ to substitute an alkyl group onto the ring.
What product is formed when alkyl side-chains in alkylarenes are completely oxidised?
The alkyl side-chains are oxidised to carboxylic acid groups. For example, ethylbenzene treated with hot alkaline KMnO₄ followed by acidification gives benzoic acid.
True or False?
Alkylarenes are halogenated at the 2 and 4 positions because alkyl groups are electron-withdrawing.
False.
Alkyl groups are electron-donating, not electron-withdrawing. They push electron density into the ring, activating the 2 and 4 positions towards electrophilic attack.
Define acyl group.
An acyl group is an alkyl group containing a carbonyl (C=O) group, with the general formula RCO–. It is introduced into a benzene ring by Friedel-Crafts acylation.
The complete oxidation of alkylarenes is carried out using hot, alkaline followed by acidification with dilute .
The complete oxidation of alkylarenes is carried out using hot, alkaline KMnO₄ followed by acidification with dilute H₂SO₄.
What catalyst and conditions are required for the hydrogenation of benzene?
Benzene is heated with hydrogen gas in the presence of a nickel or platinum catalyst to form cyclohexane. This is an addition reaction that destroys the aromaticity of the ring.
Define halogen carrier.
A halogen carrier is an anhydrous Lewis acid catalyst (such as AlCl₃ or AlBr₃) used in the halogenation of arenes. It accepts a lone pair from the halogen molecule to generate the electrophilic halogen cation (e.g. Cl⁺ or Br⁺).
What are the three steps of electrophilic substitution in arenes?
1. Generation of the electrophile (in situ from appropriate reagents).
2. Electrophilic attack: a pair of electrons from the benzene ring is donated to the electrophile, disrupting aromaticity.
3. Regeneration of aromaticity: heterolytic cleavage of the C–H bond returns electrons to the π system.
True or False?
During electrophilic attack on benzene, the aromaticity of the ring is maintained throughout the reaction.
False.
Aromaticity is temporarily lost during electrophilic attack. The ring only has four π electrons and carries a positive charge spread over five carbons. Aromaticity is restored only in the final step by heterolytic cleavage of the C–H bond.
The NO₂⁺ electrophile used in nitration is generated by reacting concentrated .......... with concentrated .......... .
The NO₂⁺ electrophile used in nitration is generated by reacting concentrated HNO₃ with concentrated H₂SO₄.
How is the Br⁺ electrophile generated for the bromination of benzene?
Bromine forms a dative bond with the AlBr₃ halogen carrier by donating a lone pair from one of its bromine atoms into an empty 3p orbital of AlBr₃. This polarises the Br–Br bond and generates Br⁺ in situ.
Define heterolytic cleavage.
A heterolytic cleavage is the breaking of a covalent bond in which both electrons in the bond go to one of the two atoms, generating ions rather than radicals.
True or False?
Arenes predominantly undergo addition reactions because the π electrons are easily available for attack.
False.
Although arenes are electron-rich, they predominantly undergo substitution reactions because addition would destroy the thermodynamically stable delocalised aromatic π system. Substitution restores aromaticity.
During the halogenation of benzene, the halogen molecule forms a .......... bond with the AlBr₃ catalyst by donating a lone pair into an empty .......... orbital.
During the halogenation of benzene, the halogen molecule forms a dative bond with the AlBr₃ catalyst by donating a lone pair into an empty 3p orbital.
Why does hydrogenation of benzene lead to a less stable product than benzene itself?
Hydrogenation is an addition reaction that completely destroys the aromatic stabilisation of benzene. Cyclohexane lacks the delocalised π system, so it is energetically less stable than benzene.
Define free-radical substitution.
A free-radical substitution is a chain reaction mechanism in which radicals (species with unpaired electrons) react with a substrate, typically initiated by UV light, resulting in substitution of a hydrogen atom.
What conditions direct halogenation to the aromatic ring rather than the side chain in alkylarenes?
Halogenation occurs in the aromatic ring when a halogen and anhydrous halogen carrier catalyst (such as AlBr₃ or AlCl₃) are used. The catalyst generates the electrophile for electrophilic substitution.
True or False?
Halogenation of the side chain of an alkylarene requires an anhydrous AlCl₃ catalyst.
False.
Side-chain halogenation is a free-radical substitution reaction requiring the halogen to be passed into the boiling alkylarene under UV light. No AlCl₃ catalyst is needed.
Aryl halides are less reactive than halogenoalkanes because the carbon–halogen bond in aryl halides has a partial .......... character due to overlap of the halogen lone pair with the benzene .......... system.
Aryl halides are less reactive than halogenoalkanes because the carbon–halogen bond in aryl halides has a partial double bond character due to overlap of the halogen lone pair with the benzene π system.
What happens when excess halogen is used in the side-chain halogenation of an alkylarene?
With excess halogen, all hydrogen atoms on the alkyl side-chain are progressively replaced by halogen atoms. No substitution occurs in the benzene ring under these conditions.
True or False?
The carbon–halogen bond in an aryl halide is stronger than in a halogenoalkane.
True.
The lone pairs on the halogen in an aryl halide partially overlap with the benzene π system, giving the C–X bond partial double bond character and making it stronger and harder to break than the C–X bond in a halogenoalkane.
Define aryl halide.
An aryl halide is an organic compound in which a halogen atom is bonded directly to a benzene ring. Aryl halides are formed by the electrophilic substitution of arenes with halogens.
Halogenation of an alkylarene in the side chain is initiated by .......... light and proceeds via a .......... mechanism.
Halogenation of an alkylarene in the side chain is initiated by UV light and proceeds via a free-radical substitution mechanism.
Why does the partial overlap of halogen lone pairs with the benzene π system affect the reactivity of aryl halides?
The lone pair overlap strengthens the C–X bond by giving it partial double bond character. This makes the bond harder to break, so aryl halides do not readily undergo nucleophilic substitution unlike halogenoalkanes.
Define electron-donating group.
An electron-donating group is a substituent that pushes electron density into the benzene ring, making it more reactive towards electrophilic substitution and directing incoming electrophiles to the 2, 4 and/or 6 positions.
Where does bromination occur on nitrobenzene, and why?
Bromination of nitrobenzene occurs at the 3 and/or 5 positions. The nitro group (–NO₂) is electron-withdrawing, removing electron density from the ring and directing the incoming Br⁺ electrophile to the 3 and 5 positions.
True or False?
Electron-withdrawing groups increase the reactivity of a benzene ring towards electrophilic substitution.
False.
Electron-withdrawing groups remove electron density from the π system, deactivating the ring and making it less reactive towards electrophilic substitution.
Electron-donating substituents (e.g. –R, –OH, –NH₂) direct electrophilic attack to the .......... positions, while electron-withdrawing substituents (e.g. –NO₂, –COOH) direct attack to the .......... positions.
Electron-donating substituents (e.g. –R, –OH, –NH₂) direct electrophilic attack to the 2, 4 and/or 6 positions, while electron-withdrawing substituents (e.g. –NO₂, –COOH) direct attack to the 3 and/or 5 positions.
Define electron-withdrawing group.
An electron-withdrawing group is a substituent that removes electron density from the benzene ring, deactivating it towards electrophilic substitution and directing incoming electrophiles to the 3 and/or 5 positions.
What two products are formed when methylbenzene is brominated, and what directs the substitution to those positions?
2-bromomethylbenzene and 4-bromomethylbenzene are formed. The methyl group is electron-donating, pushing electron density into the ring and activating the 2 and 4 positions towards the Br⁺ electrophile.
True or False?
The –NH₂ group directs electrophilic substitution to the 3 and 5 positions of a benzene ring.
False.
The –NH₂ group is an electron-donating substituent that activates the ring and directs electrophilic attack to the 2, 4 and/or 6 positions.
The –COR group is an electron-.......... substituent that .......... the benzene ring to electrophilic attack.
The –COR group is an electron-withdrawing substituent that deactivates the benzene ring to electrophilic attack.
How does an electron-donating group affect the position of electrophilic substitution on a benzene ring?
An electron-donating group increases electron density in the ring and activates it towards electrophilic substitution. It directs the incoming electrophile preferentially to the 2, 4 and/or 6 positions relative to itself.

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