Define delocalised pi system in benzene

The delocalised pi system in benzene is formed by the lateral overlap of p orbitals on all six carbon atoms, creating two ring-shaped clouds of electron density above and below the plane of the ring, with all C-C bonds identical in length.

What bond length is observed for all C-C bonds in benzene, and what does this indicate?

All C-C bonds in benzene are 140 pm — intermediate between a single bond (154 pm) and a double bond (134 pm). This indicates that all bonds are identical , consistent with a delocalised ring rather than the Kekulé structure.

What enthalpy of hydrogenation does the Kekulé structure of benzene predict, and what value is actually measured?

The Kekulé structure predicts -360 kJ mol -1 (3 × -120 kJ mol -1 for three C=C bonds). The actual measured value is -208 kJ mol -1 , which is far less exothermic, indicating extra stability from delocalisation.

True or False? Benzene decolourises bromine water in the same way as cyclohexene.

False. Benzene does not decolourise bromine water. Unlike cyclohexene, benzene has no localised areas of high electron density to polarise the Br 2 molecule, so electrophilic addition does not occur.

Benzene does not show a C=C infrared peak at .......... cm -1 . Instead it shows peaks at 1450, 1500 and .......... cm -1 characteristic of arenes.

Benzene does not show a C=C infrared peak at 1650 cm -1 . Instead it shows peaks at 1450, 1500 and 1580 cm -1 characteristic of arenes.

Why can benzene not undergo electrophilic addition with Br 2 without a catalyst?

Benzene has no localised areas of high electron density , so it cannot polarise the bromine molecule to generate a Br + electrophile. A halogen carrier such as AlBr 3 is required to generate the electrophile.

What four types of evidence support the delocalised model of benzene over the Kekulé structure?

Enthalpy of hydrogenation (less exothermic than predicted) C-C bond lengths (all 140 pm, not alternating 134/154 pm) Saturation tests (benzene does not decolourise Br 2 water) Infrared spectroscopy (no peak at 1650 cm -1 )

True or False? Benzene is a planar molecule with bond angles of 120°.

True. Benzene is planar with bond angles of 120°. Each carbon is sp 2 hybridised, forming three sigma bonds in the plane of the ring.

What are the four main types of reaction that benzene undergoes?

Combustion (with oxygen) Halogenation (with halogen carrier) Nitration (with conc. HNO 3 /H 2 SO 4 ) Friedel-Crafts reactions (alkylation and acylation) Benzene also undergoes sulfonation with fuming sulfuric acid.

What is Friedel-Crafts acylation ?

Friedel-Crafts acylation is an electrophilic substitution reaction in which an acyl group (RCO-) replaces a hydrogen atom on the benzene ring, using an acyl chloride and an AlCl 3 catalyst.

What role does the metal halide carrier play in the halogenation of benzene?

The metal halide carrier (e.g. AlBr 3 , FeBr 3 ) reacts with the halogen to generate the electrophile (X + ) and is regenerated at the end of the reaction, fulfilling its role as a catalyst.

Benzene undergoes nitration with a mixture of concentrated .......... and concentrated .......... at a temperature between 25 and 60°C.

Benzene undergoes nitration with a mixture of concentrated nitric acid and concentrated sulfuric acid at a temperature between 25 and 60°C.

True or False? Friedel-Crafts reactions are examples of electrophilic substitution.

True. Both Friedel-Crafts alkylation and acylation are electrophilic substitution reactions in which the generated electrophile attacks the benzene ring and a hydrogen atom is substituted.

What is the overall equation for the combustion of benzene in excess oxygen?

2C 6 H 6 (l) + 15O 2 (g) → 12CO 2 (g) + 6H 2 O (g) If insufficient oxygen is available, incomplete combustion occurs and benzene may produce a smoky yellow flame .

What is sulfonation of benzene, and what reagent and conditions are required?

Sulfonation is the electrophilic substitution of a hydrogen atom on benzene by a sulfonyl group (-SO 3 H). It requires fuming sulfuric acid at 40°C for around 30 minutes.

The overall equation for bromination of benzene is C 6 H 6 + Br 2 → .......... + .......... in the presence of AlBr 3 .

The overall equation for bromination of benzene is C 6 H 6 + Br 2 → C 6 H 5 Br + HBr in the presence of AlBr 3 .

What are the three stages common to all electrophilic substitution mechanisms of benzene?

Generation of the electrophile Electrophilic attack on the benzene ring Regenerating aromaticity (heterolytic cleavage of C-H bond)

What electrophile is generated in the nitration of benzene, and how is it formed?

The electrophile is the nitronium ion (NO 2 + ). It is generated by reacting concentrated HNO 3 with concentrated H 2 SO 4 : HNO 3 + H 2 SO 4 → NO 2 + + HSO 4 - + H 2 O

How is aromaticity restored in Step 3 of the electrophilic substitution mechanism?

Aromaticity is restored by heterolytic cleavage of the C-H bond, where the bonding pair of electrons enters the pi bonding system, regenerating the delocalised ring and releasing H + .

True or False? In the electrophilic substitution mechanism, the curly arrow for electrophilic attack must start from the benzene ring.

True. The curly arrow must start from or within the circle/hexagon of the benzene ring and point to the electrophile. It shows electron donation from the pi system to the electrophile.

In the Friedel-Crafts acylation mechanism, the electrophile generated is the .......... ion, formed by reacting the acyl chloride with .......... .

In the Friedel-Crafts acylation mechanism, the electrophile generated is the acylium ion (e.g. CH 3 CO + ), formed by reacting the acyl chloride with AlCl 3 .

What intermediate structure is drawn in Step 2 of the electrophilic substitution mechanism, and what does it show?

A sigma complex (or carbocation intermediate) is drawn with a horseshoe covering at least 3 carbon atoms and a positive charge inside. It shows that aromaticity has been disrupted and only four pi electrons remain.

What are the reagent and conditions for Friedel-Crafts acylation of benzene with ethanoyl chloride?

Ethanoyl chloride (CH 3 COCl) with anhydrous AlCl 3 as catalyst, heated under reflux . The product is phenylethanone (C 6 H 5 COCH 3 ) and HCl.

In the halogenation of benzene, what reaction regenerates the halogen carrier catalyst in Step 4?

The H + ion released in Step 3 reacts with the complex ion: H + + [AlBr 4 ] - → HBr + AlBr 3 This regenerates the AlBr 3 catalyst and produces HBr as a by-product.

True or False? In nitration, H 2 SO 4 is consumed and not regenerated.

False. H 2 SO 4 acts as a catalyst . The H + released in Step 3 reacts with HSO 4 - to regenerate H 2 SO 4 , so it is not consumed overall.

Why does phenol react with bromine water without a catalyst, whereas benzene requires a halogen carrier?

In phenol, a lone pair of electrons on the oxygen atom of the -OH group overlaps with the pi system, increasing the electron density of the ring. This makes phenol more susceptible to electrophilic attack without the need for an activated electrophile.

What are the product and observations when phenol reacts with bromine water?

The orange bromine water is decolourised and a white precipitate of 2,4,6-tribromophenol is formed.

What is 2,4,6-tribromophenol ?

2,4,6-tribromophenol is the product of phenol reacting with bromine water, formed by electrophilic substitution at the 2, 4 and 6 positions of the ring, which are activated by the -OH group.

True or False? The -OH group in phenol directs incoming electrophiles to the 2, 4 and 6 positions of the benzene ring.

True. The -OH group is an activating group. It donates electron density into the ring via lone pair overlap, directing electrophilic attack to the 2, 4 and 6 positions .

Phenol reacts with bromine water at .......... temperature, producing a white precipitate of .......... and decolourising the orange solution.

Phenol reacts with bromine water at room temperature, producing a white precipitate of 2,4,6-tribromophenol and decolourising the orange solution.

International A Level (IAL)ChemistryEdexcelFlashcards5. Transition Metals & Organic Nitrogen ChemistryOrganic Chemistry: Arenes

Organic Chemistry: Arenes (Edexcel International A Level (IAL) Chemistry): Flashcards

Exam code: YCH11

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FrontBenzene - Structure & Stability
Define delocalised pi system in benzene

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Define delocalised pi system in benzene
The delocalised pi system in benzene is formed by the lateral overlap of p orbitals on all six carbon atoms, creating two ring-shaped clouds of electron density above and below the plane of the ring, with all C-C bonds identical in length.
What bond length is observed for all C-C bonds in benzene, and what does this indicate?
All C-C bonds in benzene are 140 pm — intermediate between a single bond (154 pm) and a double bond (134 pm). This indicates that all bonds are identical, consistent with a delocalised ring rather than the Kekulé structure.
What enthalpy of hydrogenation does the Kekulé structure of benzene predict, and what value is actually measured?
The Kekulé structure predicts -360 kJ mol^-1 (3 × -120 kJ mol^-1 for three C=C bonds). The actual measured value is -208 kJ mol^-1, which is far less exothermic, indicating extra stability from delocalisation.
True or False?
Benzene decolourises bromine water in the same way as cyclohexene.
False.
Benzene does not decolourise bromine water. Unlike cyclohexene, benzene has no localised areas of high electron density to polarise the Br₂ molecule, so electrophilic addition does not occur.
Benzene does not show a C=C infrared peak at .......... cm^-1. Instead it shows peaks at 1450, 1500 and .......... cm^-1 characteristic of arenes.
Benzene does not show a C=C infrared peak at 1650 cm^-1. Instead it shows peaks at 1450, 1500 and 1580 cm^-1 characteristic of arenes.
Why can benzene not undergo electrophilic addition with Br₂ without a catalyst?
Benzene has no localised areas of high electron density, so it cannot polarise the bromine molecule to generate a Br⁺ electrophile. A halogen carrier such as AlBr₃ is required to generate the electrophile.
What four types of evidence support the delocalised model of benzene over the Kekulé structure?
1. Enthalpy of hydrogenation (less exothermic than predicted)
2. C-C bond lengths (all 140 pm, not alternating 134/154 pm)
3. Saturation tests (benzene does not decolourise Br₂ water)
4. Infrared spectroscopy (no peak at 1650 cm^-1)
True or False?
Benzene is a planar molecule with bond angles of 120°.
True.
Benzene is planar with bond angles of 120°. Each carbon is sp² hybridised, forming three sigma bonds in the plane of the ring.
What are the four main types of reaction that benzene undergoes?
1. Combustion (with oxygen)
2. Halogenation (with halogen carrier)
3. Nitration (with conc. HNO₃/H₂SO₄)
4. Friedel-Crafts reactions (alkylation and acylation)
Benzene also undergoes sulfonation with fuming sulfuric acid.
What is Friedel-Crafts acylation?
Friedel-Crafts acylation is an electrophilic substitution reaction in which an acyl group (RCO-) replaces a hydrogen atom on the benzene ring, using an acyl chloride and an AlCl₃ catalyst.
What role does the metal halide carrier play in the halogenation of benzene?
The metal halide carrier (e.g. AlBr₃, FeBr₃) reacts with the halogen to generate the electrophile (X⁺) and is regenerated at the end of the reaction, fulfilling its role as a catalyst.
Benzene undergoes nitration with a mixture of concentrated .......... and concentrated .......... at a temperature between 25 and 60°C.
Benzene undergoes nitration with a mixture of concentrated nitric acid and concentrated sulfuric acid at a temperature between 25 and 60°C.
True or False?
Friedel-Crafts reactions are examples of electrophilic substitution.
True.
Both Friedel-Crafts alkylation and acylation are electrophilic substitution reactions in which the generated electrophile attacks the benzene ring and a hydrogen atom is substituted.
What is the overall equation for the combustion of benzene in excess oxygen?
2C₆H₆ (l) + 15O₂ (g) → 12CO₂ (g) + 6H₂O (g)
If insufficient oxygen is available, incomplete combustion occurs and benzene may produce a smoky yellow flame.
What is sulfonation of benzene, and what reagent and conditions are required?
Sulfonation is the electrophilic substitution of a hydrogen atom on benzene by a sulfonyl group (-SO₃H). It requires fuming sulfuric acid at 40°C for around 30 minutes.
The overall equation for bromination of benzene is C₆H₆ + Br₂ → .......... + .......... in the presence of AlBr₃.
The overall equation for bromination of benzene is C₆H₆ + Br₂ → C₆H₅Br + HBr in the presence of AlBr₃.
What are the three stages common to all electrophilic substitution mechanisms of benzene?
1. Generation of the electrophile
2. Electrophilic attack on the benzene ring
3. Regenerating aromaticity (heterolytic cleavage of C-H bond)
What electrophile is generated in the nitration of benzene, and how is it formed?
The electrophile is the nitronium ion (NO₂⁺). It is generated by reacting concentrated HNO₃ with concentrated H₂SO₄:
HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄^- + H₂O
How is aromaticity restored in Step 3 of the electrophilic substitution mechanism?
Aromaticity is restored by heterolytic cleavage of the C-H bond, where the bonding pair of electrons enters the pi bonding system, regenerating the delocalised ring and releasing H⁺.
True or False?
In the electrophilic substitution mechanism, the curly arrow for electrophilic attack must start from the benzene ring.
True.
The curly arrow must start from or within the circle/hexagon of the benzene ring and point to the electrophile. It shows electron donation from the pi system to the electrophile.
In the Friedel-Crafts acylation mechanism, the electrophile generated is the .......... ion, formed by reacting the acyl chloride with .......... .
In the Friedel-Crafts acylation mechanism, the electrophile generated is the acylium ion (e.g. CH₃CO⁺), formed by reacting the acyl chloride with AlCl₃.
What intermediate structure is drawn in Step 2 of the electrophilic substitution mechanism, and what does it show?
A sigma complex (or carbocation intermediate) is drawn with a horseshoe covering at least 3 carbon atoms and a positive charge inside. It shows that aromaticity has been disrupted and only four pi electrons remain.
What are the reagent and conditions for Friedel-Crafts acylation of benzene with ethanoyl chloride?
Ethanoyl chloride (CH₃COCl) with anhydrous AlCl₃ as catalyst, heated under reflux. The product is phenylethanone (C₆H₅COCH₃) and HCl.
In the halogenation of benzene, what reaction regenerates the halogen carrier catalyst in Step 4?
The H⁺ ion released in Step 3 reacts with the complex ion:
H⁺ + [AlBr₄]^- → HBr + AlBr₃
This regenerates the AlBr₃ catalyst and produces HBr as a by-product.
True or False?
In nitration, H₂SO₄ is consumed and not regenerated.
False.
H₂SO₄ acts as a catalyst. The H⁺ released in Step 3 reacts with HSO₄^- to regenerate H₂SO₄, so it is not consumed overall.
Why does phenol react with bromine water without a catalyst, whereas benzene requires a halogen carrier?
In phenol, a lone pair of electrons on the oxygen atom of the -OH group overlaps with the pi system, increasing the electron density of the ring. This makes phenol more susceptible to electrophilic attack without the need for an activated electrophile.
What are the product and observations when phenol reacts with bromine water?
The orange bromine water is decolourised and a white precipitate of 2,4,6-tribromophenol is formed.
What is 2,4,6-tribromophenol?
2,4,6-tribromophenol is the product of phenol reacting with bromine water, formed by electrophilic substitution at the 2, 4 and 6 positions of the ring, which are activated by the -OH group.
True or False?
The -OH group in phenol directs incoming electrophiles to the 2, 4 and 6 positions of the benzene ring.
True.
The -OH group is an activating group. It donates electron density into the ring via lone pair overlap, directing electrophilic attack to the 2, 4 and 6 positions.
Phenol reacts with bromine water at .......... temperature, producing a white precipitate of .......... and decolourising the orange solution.
Phenol reacts with bromine water at room temperature, producing a white precipitate of 2,4,6-tribromophenol and decolourising the orange solution.

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