Electrophilic Aromatic Substitution Reactions

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Questions and Answers

Which statement accurately describes the stability of aromatic compounds in chemical reactions?

  • Aromatic compounds readily undergo addition reactions due to their unstable structure.
  • Aromatic compounds easily lose their aromatic character and electronic structure upon entering chemical reactions.
  • Aromatic compounds are unstable and readily undergo chemical changes.
  • Aromatic compounds are exceptionally stable and tend to retain their aromatic character during chemical reactions. (correct)

What is the primary reason benzene and its derivatives undergo electrophilic aromatic substitution?

  • Due to the presence of electron-deficient clouds of π electrons.
  • Due to the lack of sigma bonds in the ring structure.
  • Due to the presence of electron-rich clouds of π electrons. (correct)
  • Due to the high reactivity of the carbon atoms.

In electrophilic aromatic substitution, what role does the benzene ring play?

  • It donates electron-deficient species to initiate the reaction.
  • It attracts electron-deficient species (electrophiles). (correct)
  • It acts as a catalyst to speed up the reaction.
  • It neutralizes any charged species in the reaction mixture.

What best describes the role of substitution reactions with respect to the benzene ring?

<p>Substitution reactions preserve the integrity of the benzene ring. (B)</p> Signup and view all the answers

What occurs during electrophilic aromatic substitution?

<p>An electrophile replaces a hydrogen on the ring. (B)</p> Signup and view all the answers

Which of the following is a key characteristic of the reaction between benzene and alkenes?

<p>Benzene undergoes substitution reactions in preference to addition reactions, unlike alkenes. (C)</p> Signup and view all the answers

What type of reaction is characteristic of benzene due to its stability and electronic structure?

<p>Substitution (B)</p> Signup and view all the answers

Which of the following is a step in the mechanism of aromatic electrophilic substitution?

<p>Formation of an intermediate carbocation by electrophilic attack. (C)</p> Signup and view all the answers

What role does the anion (Z-) play in the final step of electrophilic aromatic substitution?

<p>It removes a proton to regenerate aromaticity. (B)</p> Signup and view all the answers

What is the rate-determining step in the mechanism of electrophilic aromatic substitution?

<p>Formation of the intermediate carbocation. (D)</p> Signup and view all the answers

What is the first step in the nitration of benzene?

<p>Generation of the nitronium ion. (C)</p> Signup and view all the answers

In the sulfonation of benzene, what is the electrophile that attacks the benzene ring?

<p>Sulfur trioxide ($SO_3H$) (C)</p> Signup and view all the answers

What best describes the role of Lewis acids like $AlCl_3$ in halogenation reactions?

<p>They act as catalysts forming the electrophile. (C)</p> Signup and view all the answers

Why is fluorine generally not used in direct halogenation of benzene?

<p>It leads to uncontrollable reactions and possible explosions. (D)</p> Signup and view all the answers

What is the role of the alkyl halide and $AlCl_3$ in Friedel-Crafts alkylation?

<p>The alkyl halide acts as an electrophile, and $AlCl_3$ generates the electrophile. (A)</p> Signup and view all the answers

What type of intermediate is formed during the Friedel-Crafts acylation?

<p>Acylium ion. (C)</p> Signup and view all the answers

What is the role of the catalyst in the generation of the electrophile during aromatic electrophilic substitution?

<p>The catalyst facilitates the formation of a more reactive electrophile. (D)</p> Signup and view all the answers

In the two-step substitution process of electrophilic aromatic substitution, what type of intermediate is formed when the electrophile attacks?

<p>Carbocation (C)</p> Signup and view all the answers

What best describes the removal of a proton by an anion (Z-) during the deprotonation step?

<p>It regenerates the aromaticity by forming the substituted product (B)</p> Signup and view all the answers

What role does sulfuric acid ($H_2SO_4$) play in the nitration of benzene?

<p>It acts as a catalyst. (B)</p> Signup and view all the answers

In the sulfonation of benzene, what happens after the generation of the electrophile ($SO_3$)?

<p>Formation of a carbocation intermediate where $SO_3$ attaches to the benzene ring. (B)</p> Signup and view all the answers

Which step is essential for regenerating the aromaticity of the benzene ring during sulfonation?

<p>Deprotonation of the carbocation intermediate. (C)</p> Signup and view all the answers

What is the function of a Lewis acid ($FeCl_3$ or $AlCl_3$) in the halogenation of benzene?

<p>To act as a catalyst, activating the halogen for electrophilic attack. (B)</p> Signup and view all the answers

What products will form after the carbocation intermediate is deprotonated?

<p>A halogenated benzene, hydrochloric acid (HCl) and the Lewis acid. (C)</p> Signup and view all the answers

In Friedel-Crafts alkylation, what is the role of aluminum chloride ($AlCl_3$)?

<p>Acts as a catalyst, helping to generate a carbocation. (A)</p> Signup and view all the answers

What occurs when the carbocation intermediate is deprotonated?

<p>Forms the alkyl benzene and regenerates the $AlCl_3$ catalyst (C)</p> Signup and view all the answers

In Friedel-Crafts acylation, what electrophile attacks the benzene ring?

<p>Acylium ion (D)</p> Signup and view all the answers

Which step regenerates the aromatic ring in the Friedel-Crafts acylation mechanism?

<p>Deprotonation (A)</p> Signup and view all the answers

Which compound is used with nitric acid to generate the electrophile in the nitration of benzene?

<p>Sulfuric acid ($H_2SO_4$). (A)</p> Signup and view all the answers

Why do substitution reactions of aromatic compounds typically retain their aromatic structure?

<p>Because maintaining the aromatic structure provides increased stability. (C)</p> Signup and view all the answers

Which of the following statements is true regarding the relative reactivity of benzene and alkenes?

<p>Benzene preferentially undergoes substitution reactions over addition reactions. (D)</p> Signup and view all the answers

What type of mechanism is typical for electrophilic aromatic substitution?

<p>A two-step mechanism involving a carbocation intermediate. (C)</p> Signup and view all the answers

What is the immediate result of the generation of the electrophile in electrophilic aromatic substitution?

<p>Removal of leaving group. (B)</p> Signup and view all the answers

Which of the following best describes the role of the arenium ion in electrophilic aromatic substitution?

<p>It is a stable intermediate that leads to the final product. (A)</p> Signup and view all the answers

What is often required to make the halogen more reactive in a halogenation reaction?

<p>A Lewis acid catalyst. (A)</p> Signup and view all the answers

How does branching at the alpha-carbon of an alkyl halide affect the formation of the carbocation intermediate in a Friedel-Crafts alkylation?

<p>It favors the formation of a more stable carbocation. (D)</p> Signup and view all the answers

Which of the following is TRUE regarding the Friedel-Crafts acylation?

<p>It has the same limitations as a traditional reaction involving an acyl halide. (C)</p> Signup and view all the answers

What type of catalyst is typically used in sulphonation reactions?

<p>Sulfuric acid. (C)</p> Signup and view all the answers

Flashcards

Electrophilic Aromatic Substitution

A reaction where an electrophile replaces a hydrogen on an aromatic ring.

Aromatic stability

Aromatic compounds resist chemical change, retaining their structure.

Benzene Substitution

Reactions that preserve benzene ring integrity, maintaining the bonding pattern.

Benzene Ring Attraction

The positive species attracts electron-deficient species (electrophiles).

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Nitration

A multi-step reaction to attach NO2 to a benzene ring using HNO3 and H2SO4.

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Nitronium Ion

The positively charged ion formed during nitration, attacking the benzene ring.

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Sulfonation

A reaction to add SO3H to a benzene ring using concentrated H2SO4.

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Lewis Acids

Electron acceptors in halogenation processes, like AlCl3 or FeCl3.

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Halogenation

A reaction to add a halogen to a benzene ring using a halogen and a Lewis acid.

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Friedel-Crafts Alkylation

Attaching an alkyl group to benzene using an alkyl halide and a Lewis acid.

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Friedel-Crafts Acylation

Attaching an acyl group to benzene using an acyl halide and a Lewis acid.

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Generation of the electrophile

The first step in electrophilic aromatic substitution; generates E+.

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Two-Steps substitution

A two-step process for aromatic electrophilic substitution.

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Intermediate carbocation formation

Carbocation formed upon electrophilic attack, where electrophile accepts electrons.

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Rate determining step

The rate determining step of aromatic electrophilic.

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Deprotonation

The removal of a proton; regenerating the ring.

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Study Notes

  • Electrophilic Aromatic Substitution (SE)

Stability and Reactions

  • Aromatic compounds exhibit exceptional stability, resisting chemical changes.
  • When aromatic compounds undergo chemical reactions, they tend to preserve their aromatic character and benzene ring's electronic structure.
  • Addition reactions disrupt the electronic structure by breaking the ∏ bond.
  • Substitution reactions maintain the integrity of the benzene ring, keeping the ∏-bonding pattern intact.
  • Electrophilic aromatic substitution (SE) is characteristic of benzene and its derivatives due to their electron rich ∏ electrons.
  • The benzene ring attracts electron-deficient species (electrophiles).
  • During electrophilic aromatic substitution, the electrophile replaces hydrogen on the ring, maintaining the ∏-bonding pattern.
  • Substitution reactions are favored over addition reactions in benzene.

Electrophilic Reactions

  • Electrophilic reactions include:
    • Nitration
  • Chlorination
  • Bromination
  • Friedel-Crafts alkylation
  • Friedel-Crafts acylation

Mechanism of Electrophilic Aromatic Substitution

  • Aromatic electrophilic substitution follows an SE mechanism.

Generation of the Electrophile

  • The process catalyst E-Z leading ot E+ + Z-
  • Electrophilic aromatic substitution occurs in two steps.

Formation of Intermediate Carbocation

  • The electrophile accepts two electrons from the π system through electrophilic attack, forming a sigma bond with a ring carbon
  • An intermediate carbocation, either a σ complex or an arene, is formed during the first step.
  • This step is the slowest and is a rate determining step

Deprotonation

  • The anion Z- removes a proton to regenerate aromaticity, resulting in the substituted product in a fast step.

Nitration

  • Benzene interacts with HNO3 in the presence of H2SO4 to form NO2 and H2O

Generation of Electrophile in Nitration

  • H combines with O of HNO3, and breaks off to form water H2O. A nitronium ion then forms when NO2+ combines with HSO4-.

Two Steps Substitution Reaction in Nitration

  • Formation of carbocation by electrophilic attack.
  • Deprotonation

Sulphonation

  • Benzene interacts with H2SO4 in the presence of heat to form SO3H and H2O

Generation of Electrophile in Sulphonation

  • 2H2SO4 forms H-O--S-OH + H-O-S-OH
  • Becomes HO-S-O-H+ HSO4, then H2O + SO3H+ HSO4

Two Steps Substitution Reaction in Sulphonation

  • Formation of carbocation by electrophilic attack.
  • Deprotonation.

Halogenation

  • Benzene reacts with X2 in the presence of FeX3 or AlX3 to form an X molecule and HX.
  • X = Cl or Br.
  • F is too reactive and can cause explosions
  • I is unreactive and needs special conditions

Generation of Electrophile in Halogenation

  • Cl combines with AlCl3 to form Cl + AlCl3

Two Step Substitution in Halogenation

  • Formation of carbocation by electrophilic attach
  • Deprotonation

Friedel-Crafts Alkylation

  • Benzene reacts with alkyl halide in the presence of AlCl3(Lewis Acid) to form an alkyl group and HCl

Generation of Electrophile in Friedel-Crafts Alkylation

  • R-Cl combines with AlCl3 to form an a new molecule R + : Cl-AlCl3

Two Step Substitution in Friedel-Crafts Alkylation

  • Formation of carbocation by electrophilic attach.
  • Deprotonation

Friedel-Crafts Acylation

  • Benzene reacts with acid chloride in the presence of AlCl3 to form an acyl group and HCl

Generation of Electrophile in Friedel-Crafts Acylation

  • O combined with R-C-Cl, then combines with AlCl3 to form + R-C=O + : Cl-AlCl3, and acylium cation

Two Step Substitution in Friedel-Crafts Acylation

  • Formation of carbocation by electrophilic attach
  • Deprotonation

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