Chemistry/Chapter 8: Hydrocarbons
8.3 Reactivity of Benzene
The main idea is that benzene has a highly stable delocalized pi (\(\pi\)) electron ring which remains intact in most reactions
Explanation level
In the previous section, we learned that benzene has a highly stable, delocalized pi electron system. This exceptional stability dictates its chemical behavior. Unlike alkenes, which readily undergo addition reactions, benzene is relatively unreactive towards addition. Instead, its characteristic reactions are electrophilic aromatic substitution reactions, where one hydrogen atom on the ring is replaced by an electrophile, preserving the stable aromatic system. This section focuses on the general mechanism of these substitution reactions.
Main Idea
The main idea is that benzene has a highly stable delocalized pi (π) electron ring which remains intact in most reactions. Benzene shows different types of reactions such as addition reactions, oxidation reactions, and electrophilic substitution reactions. However, the most characteristic and important reactions of benzene are electrophilic aromatic substitution reactions, where an electrophile attacks the high electron density of the delocalized pi electron ring of the benzene molecule.
Core Concept
The core concept is that the delocalized pi electron system in benzene is far more stable than the localized pi bonds in alkenes. Because of this stability, benzene does not readily undergo addition reactions that would break the aromatic ring. Instead, it undergoes substitution reactions in which a hydrogen atom is replaced, allowing the stable aromatic ring to be regenerated. This process is thermodynamically favorable because the aromatic stability is preserved.
Detailed Explanation
Electrophilic Aromatic Substitution Reactions
Electrophilic aromatic substitution reactions involve the attack of an electrophile on the high electron density of the delocalized pi (π) electron ring of the benzene molecule.
The general mechanism of electrophilic aromatic substitution reactions involves three steps:
1. Generation of the electrophile: An electrophile is generated first from a suitable reagent with the help of a Lewis acid catalyst, like FeX_3. E-B + FeX_3 \rightleftharpoons E^+ + FeX_3B^- - Example: For bromination, the bromine molecule (Br_2) acts as a Lewis base (donates a pair of electrons) and iron(III) bromide (FeBr_3) acts as a Lewis acid (accepts an electron pair) to generate the bromonium ion (Br^+). Br_2 + FeBr_3 \rightleftharpoons Br^+ + FeBr_4^-
2. Reaction of electrophile with benzene: The pi (π) electrons of the benzene molecule attack the electrophile to make a cation intermediate called the arenium ion (or sigma complex). In this intermediate, the aromatic ring is partially broken. Benzene + E^+ \rightarrow Arenium\ ion - The arenium ion is a resonance-stabilized cation with the positive charge delocalized over three carbon atoms.
3. Removal of hydrogen by a base: A Lewis base takes the hydrogen from the arenium ion, resulting in the formation of a mono-substituted benzene. This restores the delocalized pi electron ring of the benzene molecule. Arenium\ ion + FeX_3B^- \rightarrow Mono\text{-}substituted\ benzene + HB + FeX_3 - The base is the FeX_3B^- complex, which removes the proton (H^+) from the arenium ion, regenerating the aromatic ring and the catalyst.
Key Information from the Textbook
The delocalized pi (π) electron system in the benzene molecule is far more stable than the localized pi (π) bonds in alkenes. This is why the benzene molecule always requires a catalyst for electrophilic substitution reactions of benzene.
Comparison with Alkenes
- Alkenes undergo addition reactions because their localized pi bonds are relatively weak and can be easily broken. The addition of an electrophile (like Br_2) across the double bond is thermodynamically favorable.
- Benzene undergoes substitution reactions because its delocalized pi system is very stable. Breaking the aromatic ring would require a large amount of energy. Instead, benzene reacts with an electrophile to replace a hydrogen atom, regenerating the aromatic ring. This preserves the aromatic stability, making the reaction thermodynamically favorable.
Intermediate Level
The reactivity of benzene is dominated by its stable delocalized pi electron system. Unlike alkenes, which undergo addition reactions, benzene undergoes electrophilic aromatic substitution reactions. The mechanism involves three steps: (1) generation of a strong electrophile using a Lewis acid catalyst (e.g., FeBr_3), (2) attack of the electrophile on the benzene ring to form a resonance-stabilized arenium ion (sigma complex), and (3) removal of a proton by a base to restore the aromatic ring. This substitution reaction is preferred because it preserves the aromatic stability of the benzene ring, which would be lost in an addition reaction.
Important Points
- - Reactivity of Benzene: Dominated by electrophilic aromatic substitution.
- - Reason: The delocalized pi electron system is highly stable.
- - Mechanism: Three steps:
- 1. Generation of electrophile (using a Lewis acid catalyst).
- 2. Formation of the arenium ion (sigma complex).
- 3. Removal of a proton (deprotonation) to restore the aromatic ring.
- - Catalyst: Required for the reaction (e.g., FeBr_3, AlCl_3).
- - Arenium Ion: A resonance-stabilized carbocation intermediate.
- - Contrast with Alkenes: Alkenes undergo addition; benzene undergoes substitution.
Remember
- - Benzene: Highly stable due to delocalized pi electrons.
- - Reaction Type: Electrophilic aromatic substitution.
- - Catalyst: Required (e.g., FeX_3, AlX_3).
- - Three Steps:
- 1. Generate electrophile.
- 2. Form arenium ion.
- 3. Deprotonate to restore aromaticity.
- - Arenium ion: Resonance-stabilized cation.
- - Benzene undergoes substitution, not addition.
Common Mistakes
- - Thinking that benzene reacts like an alkene. Benzene is much less reactive towards addition than alkenes because of its aromatic stability.
- - Forgetting the role of the catalyst. The catalyst is needed to generate the strong electrophile and to help remove the proton.
- - Not recognizing the arenium ion as an intermediate. The arenium ion is a key part of the mechanism and is resonance-stabilized.
- - Assuming that substitution breaks the aromatic ring. Substitution preserves the aromatic ring; that's why it is favored.
- - Confusing the terms "addition" and "substitution." In addition, atoms are added across a double bond. In substitution, one atom is replaced by another.
Quick Revision
Benzene is characterized by a highly stable delocalized pi electron system, which makes it much less reactive towards addition reactions than alkenes. Instead, its characteristic reactions are electrophilic aromatic substitution reactions. These reactions involve three steps: (1) generation of an electrophile using a Lewis acid catalyst (like FeBr_3 or AlCl_3), (2) attack of the electrophile on the benzene ring to form a resonance-stabilized arenium ion (sigma complex), and (3) removal of a proton to restore the aromatic ring. This substitution mechanism is favored because it preserves the exceptional stability of the aromatic system. The requirement for a catalyst and the preference for substitution over addition are key features of benzene's reactivity.
60-Second Revision
Benzene's reactivity is dominated by electrophilic aromatic substitution, not addition, because its delocalized pi system is very stable. The mechanism has three steps: (1) generate an electrophile using a catalyst (like FeBr_3), (2) form a resonance-stabilized arenium ion, and (3) remove a proton to restore the aromatic ring. This preserves the aromatic stability, making substitution favorable over addition.
Frequently Asked Questions (FAQs)
Q1. Why does benzene undergo substitution rather than addition reactions? Answer: Benzene undergoes substitution because its delocalized pi electron system is highly stable. An addition reaction would break the aromatic ring and destroy this stability. A substitution reaction, on the other hand, preserves the aromatic ring, so it is thermodynamically favorable. This is why benzene reacts with an electrophile to replace a hydrogen atom rather than adding across a double bond.
Q2. What is the role of the catalyst in electrophilic aromatic substitution? Answer: The catalyst (e.g., FeBr_3 or AlCl_3) is a Lewis acid that helps generate a strong electrophile from the reagent. For example, it polarizes the bromine molecule to form the bromonium ion (Br^+). The catalyst also assists in the removal of the proton from the arenium ion in the final step, regenerating the aromatic ring and the catalyst itself.
Q3. What is the arenium ion? Answer: The arenium ion (also called the sigma complex) is the resonance-stabilized carbocation intermediate formed when the electrophile attacks the benzene ring. In the arenium ion, the positive charge is delocalized over three carbon atoms, and the aromatic ring is partially broken. It is a key intermediate in electrophilic aromatic substitution reactions.
- Generation of the electrophile: The reagent reacts with a Lewis acid catalyst to form a strong electrophile (e.g., Br^+).
- Formation of the arenium ion: The electrophile attacks the benzene ring, forming a resonance-stabilized arenium ion (sigma complex).
- Deprotonation: A base removes a proton from the arenium ion, restoring the aromatic ring and regenerating the catalyst.
Q5. Why is a catalyst required for the electrophilic substitution of benzene? Answer: A catalyst is required because benzene is relatively unreactive due to its stable delocalized pi system. The catalyst helps generate a sufficiently strong electrophile to attack the benzene ring. Without the catalyst, the reaction would be very slow or would not occur.
Q6. Compare the reactivity of benzene with that of alkenes. Answer: Alkenes are more reactive than benzene towards addition reactions because their localized pi bonds are weaker and can be easily broken. Benzene, in contrast, is less reactive towards addition because breaking its delocalized pi system would require a large amount of energy. Benzene prefers substitution reactions, which preserve its aromatic stability.
Q7. What is the difference between the arenium ion and the benzene molecule? Answer: The benzene molecule has a fully delocalized, aromatic pi system with all six carbon atoms sp² hybridized. The arenium ion is an intermediate in which the aromatic ring is partially broken; one carbon is sp³ hybridized, and the positive charge is delocalized over three carbon atoms. The arenium ion is less stable than benzene but is still resonance-stabilized.
Q8. What is a sigma complex? Answer: A sigma complex is another name for the arenium ion, the resonance-stabilized carbocation intermediate formed during electrophilic aromatic substitution. It is called a sigma complex because the electrophile forms a sigma bond to one of the carbon atoms of the ring, temporarily breaking the aromatic pi system.
Q9. Why does benzene require a catalyst for substitution? Answer: The catalyst is needed to generate a strong electrophile. The reagents used (like Br_2) are not strong enough electrophiles on their own to attack the stable benzene ring. The Lewis acid catalyst polarizes the reagent, creating a more reactive electrophile (e.g., Br^+) that can overcome the stability of the aromatic ring.
Multiple Choice Questions (MCQs)
Q1. Why does benzene undergo electrophilic substitution rather than addition? A. Because it has localized double bonds. B. Because its delocalized pi system is very stable. C. Because it is a non-polar molecule. D. Because it has a low boiling point. Correct Answer: B Explanation: Benzene undergoes substitution because its delocalized pi system is highly stable. Addition would break the aromatic ring, which is energetically unfavorable.
- Q2. What is the role of FeBr_3 in the bromination of benzene?
- A. It acts as a base.
- B. It acts as a Lewis acid catalyst.
- C. It acts as a nucleophile.
- D. It acts as a solvent.
- Correct Answer: B
- Explanation: FeBr_3 is a Lewis acid catalyst that generates the electrophile (Br^+) and assists in the removal of the proton.
- Q3. What is the intermediate formed during electrophilic aromatic substitution?
- A. Carbanion
- B. Arenium ion
- C. Free radical
- D. Carbene
- Correct Answer: B
- Explanation: The arenium ion (sigma complex) is the resonance-stabilized carbocation intermediate.
- Q4. What is the first step in the electrophilic aromatic substitution mechanism?
- A. Deprotonation
- B. Formation of the arenium ion
- C. Generation of the electrophile
- D. Attack of the nucleophile
- Correct Answer: C
- Explanation: The first step is the generation of the electrophile using a Lewis acid catalyst.
- Q5. Which of the following statements is true about benzene's reactivity?
- A. Benzene is more reactive than alkenes towards addition.
- B. Benzene is less reactive than alkenes towards addition.
- C. Benzene is equally reactive as alkenes.
- D. Benzene does not react at all.
- Correct Answer: B
- Explanation: Benzene is less reactive than alkenes towards addition because its delocalized pi system is very stable.
- Q6. What is the driving force for the substitution reaction of benzene?
- A. The formation of a more stable arenium ion.
- B. The preservation of the aromatic ring.
- C. The formation of a new double bond.
- D. The release of heat.
- Correct Answer: B
- Explanation: The substitution reaction is favored because it preserves the stable aromatic ring.
- Q7. What is the role of the Lewis base in the final step of electrophilic aromatic substitution?
- A. It generates the electrophile.
- B. It removes a proton from the arenium ion.
- C. It attacks the benzene ring.
- D. It acts as a catalyst.
- Correct Answer: B
- Explanation: The Lewis base (e.g., FeBr_4^-) removes a proton from the arenium ion, restoring the aromatic ring.
- Q8. Which of the following is a Lewis acid catalyst used in electrophilic aromatic substitution?
- A. NaOH
- B. FeBr_3
- C. H_2SO_4
- D. NaCl
- Correct Answer: B
- Explanation: FeBr_3 is a common Lewis acid catalyst used in electrophilic aromatic substitution.
- Q9. Why is the arenium ion resonance-stabilized?
- A. Because it has a full octet.
- B. Because the positive charge is delocalized over three carbon atoms.
- C. Because it has a double bond.
- D. Because it has a lone pair of electrons.
- Correct Answer: B
- Explanation: The positive charge in the arenium ion is delocalized over three carbon atoms through resonance, providing stability.
- Q10. What is the difference between the intermediate in electrophilic addition to alkenes and electrophilic substitution in benzene?
- A. The intermediate in alkenes is a carbocation; the intermediate in benzene is an arenium ion.
- B. The intermediate in alkenes is an arenium ion; the intermediate in benzene is a carbocation.
- C. Both intermediates are the same.
- D. There is no intermediate in benzene substitution.
- Correct Answer: A
- Explanation: In alkenes, the intermediate is a carbocation. In benzene, the intermediate is an arenium ion (sigma complex).
Multiple Choice Questions (MCQs)
- Q1. Why does benzene undergo electrophilic substitution rather than addition?
- A. Because it has localized double bonds.
- B. Because its delocalized pi system is very stable.
- C. Because it is a non-polar molecule.
- D. Because it has a low boiling point.
- Correct Answer: B
- Explanation: Benzene undergoes substitution because its delocalized pi system is highly stable.
- Q2. What is the role of FeBr_3 in the bromination of benzene?
- A. It acts as a base.
- B. It acts as a Lewis acid catalyst.
- C. It acts as a nucleophile.
- D. It acts as a solvent.
- Correct Answer: B
- Explanation: FeBr_3 is a Lewis acid catalyst.
- Q3. What is the intermediate formed during electrophilic aromatic substitution?
- A. Carbanion
- B. Arenium ion
- C. Free radical
- D. Carbene
- Correct Answer: B
- Explanation: The arenium ion is the intermediate.
- Q4. What is the first step in the electrophilic aromatic substitution mechanism?
- A. Deprotonation
- B. Formation of the arenium ion
- C. Generation of the electrophile
- D. Attack of the nucleophile
- Correct Answer: C
- Explanation: The first step is the generation of the electrophile.
- Q5. Which of the following statements is true about benzene's reactivity?
- A. Benzene is more reactive than alkenes towards addition.
- B. Benzene is less reactive than alkenes towards addition.
- C. Benzene is equally reactive as alkenes.
- D. Benzene does not react at all.
- Correct Answer: B
- Explanation: Benzene is less reactive than alkenes towards addition.
- Q6. What is the driving force for the substitution reaction of benzene?
- A. The formation of a more stable arenium ion.
- B. The preservation of the aromatic ring.
- C. The formation of a new double bond.
- D. The release of heat.
- Correct Answer: B
- Explanation: The substitution reaction is favored because it preserves the aromatic ring.
- Q7. What is the role of the Lewis base in the final step of electrophilic aromatic substitution?
- A. It generates the electrophile.
- B. It removes a proton from the arenium ion.
- C. It attacks the benzene ring.
- D. It acts as a catalyst.
- Correct Answer: B
- Explanation: The Lewis base removes a proton from the arenium ion.
- Q8. Which of the following is a Lewis acid catalyst used in electrophilic aromatic substitution?
- A. NaOH
- B. FeBr_3
- C. H_2SO_4
- D. NaCl
- Correct Answer: B
- Explanation: FeBr_3 is a common Lewis acid catalyst.
- Q9. Why is the arenium ion resonance-stabilized?
- A. Because it has a full octet.
- B. Because the positive charge is delocalized over three carbon atoms.
- C. Because it has a double bond.
- D. Because it has a lone pair of electrons.
- Correct Answer: B
- Explanation: The positive charge in the arenium ion is delocalized over three carbon atoms.
- Q10. What is the difference between the intermediate in electrophilic addition to alkenes and electrophilic substitution in benzene?
- A. The intermediate in alkenes is a carbocation; the intermediate in benzene is an arenium ion.
- B. The intermediate in alkenes is an arenium ion; the intermediate in benzene is a carbocation.
- C. Both intermediates are the same.
- D. There is no intermediate in benzene substitution.
- Correct Answer: A
- Explanation: In alkenes, the intermediate is a carbocation; in benzene, it is an arenium ion.
Multiple Choice Questions (MCQs)
- Q1. What type of reaction is characteristic of benzene?
- A. Nucleophilic addition
- B. Electrophilic addition
- C. Electrophilic substitution
- D. Free radical substitution
- Correct Answer: C
- Explanation: Benzene is characterized by electrophilic substitution reactions.
- Q2. What is the first step in the mechanism of electrophilic substitution of benzene?
- A. Attack of the electrophile on the benzene ring.
- B. Generation of the electrophile.
- C. Removal of a proton.
- D. Formation of a sigma complex.
- Correct Answer: B
- Explanation: The first step is the generation of the electrophile using a catalyst.
- Q3. What is the role of a Lewis acid in the electrophilic substitution of benzene?
- A. It acts as a nucleophile.
- B. It generates the electrophile.
- C. It removes a proton.
- D. It stabilizes the benzene ring.
- Correct Answer: B
- Explanation: The Lewis acid catalyst generates the electrophile (e.g., Br^+).
- Q4. What is the name of the intermediate formed when an electrophile attacks benzene?
- A. Carbocation
- B. Carbanion
- C. Arenium ion
- D. Radical
- Correct Answer: C
- Explanation: The intermediate is the arenium ion (sigma complex).
- Q5. Which of the following is NOT a step in the electrophilic aromatic substitution mechanism?
- A. Generation of the electrophile.
- B. Formation of the arenium ion.
- C. Deprotonation to restore the aromatic ring.
- D. Addition of a nucleophile to the ring.
- Correct Answer: D
- Explanation: Addition of a nucleophile is not part of the mechanism. The mechanism involves electrophilic attack and deprotonation.
- Q6. Why is benzene less reactive than alkenes towards addition reactions?
- A. Because benzene is a solid.
- B. Because benzene has a stable delocalized pi system.
- C. Because benzene has no double bonds.
- D. Because benzene is non-polar.
- Correct Answer: B
- Explanation: The stability of the delocalized pi system makes benzene less reactive towards addition.
- Q7. What is the role of the base in the final step of electrophilic aromatic substitution?
- A. It generates the electrophile.
- B. It removes a proton from the arenium ion.
- C. It attacks the benzene ring.
- D. It acts as a catalyst.
- Correct Answer: B
- Explanation: The base removes a proton from the arenium ion.
- Q8. Which of the following is a correct statement about the reactivity of benzene?
- A. Benzene undergoes addition reactions easily.
- B. Benzene undergoes substitution reactions to preserve aromaticity.
- C. Benzene does not react with electrophiles.
- D. Benzene is more reactive than alkenes.
- Correct Answer: B
- Explanation: Benzene undergoes substitution reactions to preserve aromaticity.
- Q9. What is the driving force for the substitution reaction of benzene?
- A. The formation of a more stable arenium ion.
- B. The preservation of the aromatic ring.
- C. The formation of a new double bond.
- D. The release of heat.
- Correct Answer: B
- Explanation: The substitution reaction is favored because it preserves the aromatic ring.
- Q10. What is the difference between the intermediate in electrophilic addition to alkenes and electrophilic substitution in benzene?
- A. The intermediate in alkenes is a carbocation; the intermediate in benzene is an arenium ion.
- B. The intermediate in alkenes is an arenium ion; the intermediate in benzene is a carbocation.
- C. Both intermediates are the same.
- D. There is no intermediate in benzene substitution.
- Correct Answer: A
- Explanation: In alkenes, the intermediate is a carbocation; in benzene, it is an arenium ion.
Multiple Choice Questions (MCQs)
- Q1. What type of reaction is characteristic of benzene?
- A. Nucleophilic addition
- B. Electrophilic addition
- C. Electrophilic substitution
- D. Free radical substitution
- Correct Answer: C
- Explanation: Benzene is characterized by electrophilic substitution reactions.
- Q2. What is the first step in the mechanism of electrophilic substitution of benzene?
- A. Attack of the electrophile on the benzene ring.
- B. Generation of the electrophile.
- C. Removal of a proton.
- D. Formation of a sigma complex.
- Correct Answer: B
- Explanation: The first step is the generation of the electrophile.
- Q3. What is the role of a Lewis acid in the electrophilic substitution of benzene?
- A. It acts as a nucleophile.
- B. It generates the electrophile.
- C. It removes a proton.
- D. It stabilizes the benzene ring.
- Correct Answer: B
- Explanation: The Lewis acid catalyst generates the electrophile.
- Q4. What is the name of the intermediate formed when an electrophile attacks benzene?
- A. Carbocation
- B. Carbanion
- C. Arenium ion
- D. Radical
- Correct Answer: C
- Explanation: The intermediate is the arenium ion.
- Q5. Which of the following is NOT a step in the electrophilic aromatic substitution mechanism?
- A. Generation of the electrophile.
- B. Formation of the arenium ion.
- C. Deprotonation to restore the aromatic ring.
- D. Addition of a nucleophile to the ring.
- Correct Answer: D
- Explanation: Addition of a nucleophile is not part of the mechanism.
- Q6. Why is benzene less reactive than alkenes towards addition reactions?
- A. Because benzene is a solid.
- B. Because benzene has a stable delocalized pi system.
- C. Because benzene has no double bonds.
- D. Because benzene is non-polar.
- Correct Answer: B
- Explanation: The stability of the delocalized pi system makes benzene less reactive.
- Q7. What is the role of the base in the final step of electrophilic aromatic substitution?
- A. It generates the electrophile.
- B. It removes a proton from the arenium ion.
- C. It attacks the benzene ring.
- D. It acts as a catalyst.
- Correct Answer: B
- Explanation: The base removes a proton from the arenium ion.
- Q8. Which of the following is a correct statement about the reactivity of benzene?
- A. Benzene undergoes addition reactions easily.
- B. Benzene undergoes substitution reactions to preserve aromaticity.
- C. Benzene does not react with electrophiles.
- D. Benzene is more reactive than alkenes.
- Correct Answer: B
- Explanation: Benzene undergoes substitution reactions to preserve aromaticity.
- Q9. What is the driving force for the substitution reaction of benzene?
- A. The formation of a more stable arenium ion.
- B. The preservation of the aromatic ring.
- C. The formation of a new double bond.
- D. The release of heat.
- Correct Answer: B
- Explanation: The substitution reaction is favored because it preserves the aromatic ring.
- Q10. What is the difference between the intermediate in electrophilic addition to alkenes and electrophilic substitution in benzene?
- A. The intermediate in alkenes is a carbocation; the intermediate in benzene is an arenium ion.
- B. The intermediate in alkenes is an arenium ion; the intermediate in benzene is a carbocation.
- C. Both intermediates are the same.
- D. There is no intermediate in benzene substitution.
- Correct Answer: A
- Explanation: In alkenes, the intermediate is a carbocation; in benzene, it is an arenium ion.
Multiple Choice Questions (MCQs)
- Q1. What type of reaction is characteristic of benzene?
- A. Nucleophilic addition
- B. Electrophilic addition
- C. Electrophilic substitution
- D. Free radical substitution
- Correct Answer: C
- Explanation: Benzene is characterized by electrophilic substitution reactions.
- Q2. What is the first step in the mechanism of electrophilic substitution of benzene?
- A. Attack of the electrophile on the benzene ring.
- B. Generation of the electrophile.
- C. Removal of a proton.
- D. Formation of a sigma complex.
- Correct Answer: B
- Explanation: The first step is the generation of the electrophile.
- Q3. What is the role of a Lewis acid in the electrophilic substitution of benzene?
- A. It acts as a nucleophile.
- B. It generates the electrophile.
- C. It removes a proton.
- D. It stabilizes the benzene ring.
- Correct Answer: B
- Explanation: The Lewis acid catalyst generates the electrophile.
- Q4. What is the name of the intermediate formed when an electrophile attacks benzene?
- A. Carbocation
- B. Carbanion
- C. Arenium ion
- D. Radical
- Correct Answer: C
- Explanation: The intermediate is the arenium ion.
- Q5. Which of the following is NOT a step in the electrophilic aromatic substitution mechanism?
- A. Generation of the electrophile.
- B. Formation of the arenium ion.
- C. Deprotonation to restore the aromatic ring.
- D. Addition of a nucleophile to the ring.
- Correct Answer: D
- Explanation: Addition of a nucleophile is not part of the mechanism.
- Q6. Why is benzene less reactive than alkenes towards addition reactions?
- A. Because benzene is a solid.
- B. Because benzene has a stable delocalized pi system.
- C. Because benzene has no double bonds.
- D. Because benzene is non-polar.
- Correct Answer: B
- Explanation: The stability of the delocalized pi system makes benzene less reactive.
- Q7. What is the role of the base in the final step of electrophilic aromatic substitution?
- A. It generates the electrophile.
- B. It removes a proton from the arenium ion.
- C. It attacks the benzene ring.
- D. It acts as a catalyst.
- Correct Answer: B
- Explanation: The base removes a proton from the arenium ion.
- Q8. Which of the following is a correct statement about the reactivity of benzene?
- A. Benzene undergoes addition reactions easily.
- B. Benzene undergoes substitution reactions to preserve aromaticity.
- C. Benzene does not react with electrophiles.
- D. Benzene is more reactive than alkenes.
- Correct Answer: B
- Explanation: Benzene undergoes substitution reactions to preserve aromaticity.
- Q9. What is the driving force for the substitution reaction of benzene?
- A. The formation of a more stable arenium ion.
- B. The preservation of the aromatic ring.
- C. The formation of a new double bond.
- D. The release of heat.
- Correct Answer: B
- Explanation: The substitution reaction is favored because it preserves the aromatic ring.
- Q10. What is the difference between the intermediate in electrophilic addition to alkenes and electrophilic substitution in benzene?
- A. The intermediate in alkenes is a carbocation; the intermediate in benzene is an arenium ion.
- B. The intermediate in alkenes is an arenium ion; the intermediate in benzene is a carbocation.
- C. Both intermediates are the same.
- D. There is no intermediate in benzene substitution.
- Correct Answer: A
- Explanation: In alkenes, the intermediate is a carbocation; in benzene, it is an arenium ion.
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8.3 Reactivity of Benzene
Sticky Notes · Quick Revision
Key Point 1
- Benzene reactivity: Electrophilic aromatic substitution.
Key Point 2
- Reason: Stable delocalized pi system.
Key Point 3
- Catalyst: Required (e.g., FeBr_3).
Key Point 4
- Three steps: Generate electrophile → form arenium ion → deprotonate.
Key Point 5
- Arenium ion: Resonance-stabilized cation.
Key Point 6
- Substitution preserves aromaticity.
Key Point 7
- Alkenes undergo addition; benzene undergoes substitution.
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