Aromatic Compounds 2

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Questions and Answers

What is the primary electrophile in Friedel-Crafts alkylation?

  • Halide ion
  • Carbocation (R+) (correct)
  • Benzene derivative
  • Acylium ion

Which reagent is necessary for the nitration of benzene?

  • SO3
  • RCl
  • HNO3 (correct)
  • FeBr3

What is the main characteristic feature of the arenium ion during electrophilic substitution?

  • It directly converts to the final product without any further loss.
  • It has an increased positive charge on the benzene ring. (correct)
  • It forms after the loss of a proton.
  • It retains a full aromatic structure.

Which reaction represents Friedel-Crafts acylation?

<p>RCOCl + AlCl3 → RCO+ + HCl (B)</p> Signup and view all the answers

What is the role of AlCl3 in Friedel-Crafts reactions?

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

In what type of substitution reaction is the formation of a σ-complex critical?

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

What is the purpose of using H2SO4 in the nitration reaction of benzene?

<p>To increase the yield of nitronium ion (A)</p> Signup and view all the answers

What type of compound is typically produced in a Friedel-Crafts alkylation?

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

What is the initial step in the Friedel-Crafts alkylation mechanism for tertiary alkyl halides?

<p>Formation of carbocation (C)</p> Signup and view all the answers

What is the primary product of the Friedel-Crafts acylation reaction?

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

In the context of Friedel-Crafts alkylation, how does the mechanism differ for primary alkyl halides compared to tertiary ones?

<p>An electrophilic complex is formed. (C)</p> Signup and view all the answers

Why is the acylium ion more stable than a primary carbocation during a Friedel-Crafts acylation?

<p>It has resonance stabilization. (A)</p> Signup and view all the answers

Which ion is formed during the Friedel-Crafts alkylation process after the loss of a proton from the arenium ion?

<p>AlCl4- (B)</p> Signup and view all the answers

What is the main electrophile in the nitration of benzene?

<p>Nitronium ion (NO2+) (A)</p> Signup and view all the answers

Which component acts as the electrophile in the Friedel-Crafts alkylation mechanism when a primary alkyl halide is used?

<p>Electrophilic complex of alkyl halide and AlCl3 (B)</p> Signup and view all the answers

In Friedel-Crafts reactions, what is a potential issue when using overly reactive alkyl halides?

<p>The reaction may yield multiple products (A)</p> Signup and view all the answers

What role does the π electron cloud play in electrophilic aromatic substitution?

<p>It actively participates in the formation of the σ-bond with the electrophile. (A)</p> Signup and view all the answers

Which of the following statements about acylium ions is true?

<p>They serve as strong electrophiles in acylation. (B)</p> Signup and view all the answers

Which substituent would most likely activate a benzene ring toward electrophilic substitution reactions?

<p>Alkyl group (-R) (C)</p> Signup and view all the answers

What type of reaction mechanism is involved in the halogenation of benzene?

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

Which of the following is a necessary condition for the halogenation of benzene?

<p>An acid catalyst (D)</p> Signup and view all the answers

What is a common characteristic of electrophilic substitution reactions in aromatic compounds?

<p>Regeneration of the aromaticity after the reaction (D)</p> Signup and view all the answers

During the electrophilic aromatic substitution mechanism, what intermediate is formed prior to regeneration of the aromatic π system?

<p>Non-aromatic carbocation (arenium ion) (C)</p> Signup and view all the answers

What is the effect of strongly deactivating groups on the rate of electrophilic substitution of benzene?

<p>They slow down or inhibit the reaction. (D)</p> Signup and view all the answers

What types of reagents can be substituted for alkyl halides in Friedel-Crafts alkylation?

<p>Reagents such as alkenes mixed with Brønsted acids or alcohols with Lewis acids can be used.</p> Signup and view all the answers

What factors limit the effectiveness of Friedel-Crafts alkylation reactions?

<p>Aryl and vinyl halides, strong electron-withdrawing substituents, and the presence of amino groups limit the reactions.</p> Signup and view all the answers

What is a potential outcome of using a primary alkyl halide in Friedel-Crafts alkylation?

<p>The primary alkyl halide-AlCl3 complex may undergo rearrangement to form a more stable secondary carbocation.</p> Signup and view all the answers

Can Friedel-Crafts alkylation produce multiple alkylation products? If so, explain why.

<p>Yes, multiple substitutions can occur due to the reactivity of carbocations formed during the process.</p> Signup and view all the answers

How does the use of different acids affect the outcomes of Friedel-Crafts alkylation?

<p>Using different acids can generate varied carbocation species, influencing the selectivity and efficiency of the alkylation.</p> Signup and view all the answers

What stabilizes benzene's special stability during electrophilic substitution reactions?

<p>The closed shell molecular orbital configuration of six π electrons stabilizes benzene's structure during electrophilic substitution.</p> Signup and view all the answers

Explain the role of the π electron cloud in the electrophilic substitution of benzene.

<p>The π electron cloud acts as a source of electrons that can be donated to electrophiles during the substitution reaction.</p> Signup and view all the answers

Describe the nature of the intermediate formed during electrophilic substitution of benzene.

<p>The intermediate formed is a delocalized, non-aromatic carbocation known as an arenium ion or σ-complex.</p> Signup and view all the answers

What distinguishes an arene from a phenyl group?

<p>An arene is an aromatic hydrocarbon containing a benzene ring, while a phenyl group is a benzene ring with one hydrogen atom removed.</p> Signup and view all the answers

Why do electrophilic substitutions generally occur instead of addition reactions in benzene?

<p>Electrophilic substitutions occur instead of addition reactions because they allow the aromatic sextet of π electrons to be regenerated.</p> Signup and view all the answers

What occurs to the hydrogen atom during electrophilic substitution of benzene?

<p>The electrophile replaces one of the hydrogen atoms on the benzene ring during the substitution reaction.</p> Signup and view all the answers

What happens to the stability of the arenium ion compared to benzene?

<p>The arenium ion is less stable than benzene due to the loss of aromaticity.</p> Signup and view all the answers

What characteristic feature of electrophiles allows them to react with benzene?

<p>Electrophiles are electron-loving species, making them attracted to the electron-rich π system of benzene.</p> Signup and view all the answers

Explain the role of AlCl3 in Friedel-Crafts acylation.

<p>AlCl3 acts as a Lewis acid that facilitates the generation of the acylium ion from acyl chloride, allowing it to react with benzene.</p> Signup and view all the answers

Why does poly-acylation not occur in Friedel-Crafts acylation?

<p>The electron-withdrawing properties of the acyl group deactivate the substituted benzene ring, preventing further electrophilic substitution.</p> Signup and view all the answers

Describe the first step of the Friedel-Crafts acylation mechanism.

<p>The first step involves the generation of the acylium ion, where the acyl chloride reacts with AlCl3 to form an acylium cation and AlCl4-.</p> Signup and view all the answers

How does the introduction of an acylium ion affect a benzene ring's reactivity?

<p>The introduction of an acylium ion decreases the reactivity of the benzene ring towards further electrophilic substitutions due to the electron-withdrawing nature of the acyl group.</p> Signup and view all the answers

What are the products formed when benzene reacts with acetyl chloride in the presence of AlCl3?

<p>The reaction produces acetophenone and HCl as byproducts.</p> Signup and view all the answers

Explain why rearrangements of the acylium ion do not occur during Friedel-Crafts acylation.

<p>Rearrangements do not occur because the acylium ion is resonance stabilized, which makes it less prone to structural changes.</p> Signup and view all the answers

What is the significance of the reaction temperature in Friedel-Crafts acylation?

<p>The reaction temperature, typically around 80 °C, aids in promoting the reaction rate and facilitating the reaction between benzene and the acylium cation.</p> Signup and view all the answers

Discuss the protonation step that follows the formation of the acyl group during Friedel-Crafts acylation.

<p>After the electrophilic substitution, a protonation step occurs where the arenium ion loses a proton, regenerating the aromatic ring and completing the reaction.</p> Signup and view all the answers

What is the first step in the Friedel-Crafts alkylation mechanism for tertiary alkyl halides?

<p>The first step involves the formation of a carbocation when the alkyl halide reacts with AlCl3.</p> Signup and view all the answers

How does the formation of a complex differ in the Friedel-Crafts alkylation mechanism for primary alkyl halides?

<p>For primary alkyl halides, AlCl3 forms a complex with the alkyl halide, acting as the electrophile instead of forming a carbocation.</p> Signup and view all the answers

What happens during the second step of Friedel-Crafts alkylation involving the arenium ion?

<p>An arenium ion is formed after the electrophile attaches to the aromatic ring, creating a positively charged intermediate.</p> Signup and view all the answers

What is the outcome of the third step in the Friedel-Crafts alkylation process?

<p>The loss of a proton from the arenium ion results in the regeneration of the aromatic system and the formation of the product.</p> Signup and view all the answers

In Friedel-Crafts alkylation, what is a potential drawback when using very reactive alkyl halides?

<p>Using overly reactive alkyl halides can lead to polysubstitution, where multiple alkyl groups may attach to the aromatic ring.</p> Signup and view all the answers

Why is the carbocation generated in Friedel-Crafts alkylation important for the reaction?

<p>The carbocation acts as the electrophile that attacks the aromatic ring, initiating the electrophilic substitution process.</p> Signup and view all the answers

What type of ion is formed after the loss of a proton from the arenium ion during Friedel-Crafts alkylation?

<p>A neutral aromatic product is formed after the arenium ion loses a proton.</p> Signup and view all the answers

Explain how the role of AlCl3 in Friedel-Crafts reactions impacts the formation of the electrophile.

<p>AlCl3 facilitates the ionization of alkyl halides, generating a carbocation that serves as the electrophile for the reaction.</p> Signup and view all the answers

Why is the AlCl3-Friedel Crafts complex for primary alkyl halides not a simple carbocation?

<p>The complex formed is not a simple carbocation because it involves a bonding interaction between the alkyl halide and AlCl3.</p> Signup and view all the answers

What role does the arenium ion play in the Friedel-Crafts alkylation mechanism?

<p>The arenium ion acts as a key intermediate that undergoes deprotonation to yield the final alkylated aromatic product.</p> Signup and view all the answers

What potential issues can arise from using highly reactive alkyl halides in Friedel-Crafts alkylation?

<p>Using highly reactive alkyl halides can lead to polysubstitution, where multiple alkyl groups attach to the aromatic ring, reducing selectivity.</p> Signup and view all the answers

Describe the significance of the arenium ion in the context of electrophilic substitution reactions.

<p>The arenium ion is a critical intermediate that temporarily stabilizes the system before regeneration of the aromaticity of the benzene ring.</p> Signup and view all the answers

How do the mechanisms of Friedel-Crafts alkylation differ for primary and tertiary alkyl halides?

<p>Tertiary alkyl halides form more stable carbocations and react faster, while primary alkyl halides can lead to carbocation rearrangements or slower reactions.</p> Signup and view all the answers

In the context of electrophilic aromatic substitution, what is the importance of the σ-complex?

<p>The σ-complex is an essential intermediate that illustrates the temporary loss of aromatic character and the formation of a cyclohexadiene-like structure.</p> Signup and view all the answers

What role does FeCl3 play in the halogenation of benzene?

<p>FeCl3 acts as a Lewis acid catalyst that facilitates the generation of the electrophile (halo cation) during the halogenation process.</p> Signup and view all the answers

What is the effect of substituents that deactivate a benzene ring on electrophilic substitution reactions?

<p>Deactivating substituents slow down the rate of electrophilic substitution, making the ring less reactive toward electrophiles.</p> Signup and view all the answers

Explain why I2 requires an oxidizing agent for the iodination of benzene.

<p>I2 is unreactive with benzene under normal conditions, so it needs an oxidizing agent like HNO3 to generate an effective electrophile for the reaction.</p> Signup and view all the answers

What distinguishes Friedel-Crafts acylation from alkylation in terms of the generated electrophile?

<p>Friedel-Crafts acylation utilizes an acylium ion as the electrophile, which is generally more stable than the carbocation used in alkylation.</p> Signup and view all the answers

Describe the role of H2SO4 in the nitration of benzene.

<p>H2SO4 increases the concentration of the nitronium ion (NO2+), which is the electrophile in the nitration reaction, thereby speeding up the reaction.</p> Signup and view all the answers

What mechanism is involved in the formation of the arenium ion during electrophilic aromatic substitution?

<p>The arenium ion is formed when the benzene ring attacks the electrophile, leading to a temporary loss of aromaticity.</p> Signup and view all the answers

Identify the key steps involved in the electrophilic attack during the halogenation of benzene.

<p>The key steps involve the formation of the halo cation and the subsequent electrophilic attack on the benzene ring, resulting in the arenium ion.</p> Signup and view all the answers

What is the final product when benzene undergoes nitration with concentrated HNO3 and H2SO4?

<p>The final product is nitrobenzene, which results from the electrophilic substitution of a nitro group onto the benzene ring.</p> Signup and view all the answers

Why does fluorination of benzene require special conditions?

<p>Fluorination is highly reactive due to the strength of the F-F bond, necessitating controlled conditions to manage the reaction.</p> Signup and view all the answers

What illustrates the difference in reactivity between bromination and iodination of benzene?

<p>Bromination occurs readily with a Lewis acid, while iodination requires an oxidizing agent due to the low reactivity of I2.</p> Signup and view all the answers

Flashcards

Electrophilic Aromatic Substitution

A reaction where an electrophile attacks an aromatic ring, replacing a hydrogen atom.

Arenium Ion

A temporary intermediate formed during electrophilic aromatic substitution, characterized by a positively charged carbon.

Halogenation

Electrophilic aromatic substitution reaction, introducing halogen atoms (Cl, Br) to benzene rings, using FeX3 as a catalyst.

Nitration

Electrophilic aromatic substitution, replacing a hydrogen atom with a nitro group (NO2), using HNO3(nitric acid) and H2SO4(sulfuric acid).

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Sulfonation

Electrophilic aromatic substitution that introduces a sulfonic acid group (SO3H) to the aromatic ring, using SO3 + H2SO4.

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Friedel-Crafts Alkylation

Electrophilic aromatic substitution, adding an alkyl group (R) to an aromatic ring using RCl (alkyl halide) and AlCl3 (catalyst).

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Friedel-Crafts Acylation

Adding an acyl group (COR) to an aromatic ring (using RCOCl and AlCl3).

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Carbocation

Positively charged ion with an alkyl group, used as an electrophile in Friedel-Crafts Alkylation.

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Arenes

Aromatic hydrocarbons.

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Phenyl group

Benzene ring with one hydrogen removed (C6H5-).

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Aryl group

Aromatic hydrocarbon with one hydrogen removed (Ar-).

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Electrophile

Electron-loving, electron-deficient species.

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Electrophilic substitution

Electrophile replaces a hydrogen in benzene ring.

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Benzene's stability

Benzene's closed-shell π electron configuration makes it stable.

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Mechanism of Electrophilic substitution (Step 1)

Electrophile accepts π electrons forming a σ bond.

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Arenium ion (σ-complex)

Delocalized carbocation formed during electrophilic substitution, temporarily non-aromatic.

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Tertiary alkyl halide FC alkylation

A reaction where a tertiary alkyl halide reacts with an aromatic compound (e.g., benzene), assisted by a Lewis acid catalyst (like AlCl3), to form a substituted aromatic compound. The process involves carbocation formation.

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Carbocation formation (Step 1)

The initial step in the reaction, where the Lewis acid (AlCl3) removes a chloride ion from the alkyl halide, forming a carbocation intermediate that is more stable if the alkyl halide is tertiary in nature.

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Arenium ion formation (Step 2)

The carbocation (or the complex resembling one) attacks the aromatic ring, temporarily destabilizing and forming a positive charge on the ring. This forms an arenium ion, a cyclic intermediate.

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Proton loss (Step 3)

The arenium ion loses a proton to regenerate a stable aromatic ring and forms products.

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Primary alkyl halide FC alkylation

A reaction mechanism where an alkyl halide with primary carbon atoms reacts with an aromatic ring in presence of AlCl3. The AlCl3 forms a complex with the alkyl halide acting as electrophile.

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Lewis acid

A Lewis acid is a substance that can accept a pair of electrons to form a covalent bond.

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Alkylation reaction

In chemistry, an alkylation reaction is a type of substitution reaction that functions to add an alkyl group to a molecule.

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Why does benzene undergo substitution instead of addition?

Benzene's stable closed-shell π electron configuration makes it resistant to addition reactions. Substitution reactions allow the aromatic sextet of π electrons to be regenerated, retaining its stability.

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Mechanism Step 1

The electrophile accepts two electrons from the π system of benzene to form a σ bond with one of the carbon atoms.

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Carbocation Formation

The initial step in Friedel-Crafts alkylation where the Lewis acid (AlCl3) removes a chloride ion from the alkyl halide, forming a positively charged carbocation intermediate.

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Tertiary Alkyl Halide

An alkyl halide where the carbon atom bonded to the halogen is attached to three other carbon atoms.

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Primary Alkyl Halide

An alkyl halide where the carbon atom bonded to the halogen is attached to only one other carbon atom.

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Complex formation (Step 1)

The first step in primary alkylation involves the AlCl3 forming a complex with the primary alkyl halide, which acts as the electrophile.

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Electrophile in primary alkylation

The complex formed between the AlCl3 and the primary alkyl halide acts as the electrophile, attacking the aromatic ring.

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Carbocation Rearrangements

In Friedel-Crafts alkylation, unstable primary carbocations can rearrange to form more stable secondary or tertiary carbocations, impacting the final product formed.

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Limitations of Friedel-Crafts Alkylation

This reaction doesn't work with aromatic halides, vinyl halides, or aromatic rings containing electron-withdrawing groups like NO2 or CN. Multiple substitutions can also occur, making it less selective.

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Why use an alkene and an acid?

In Friedel-Crafts alkylation, a mixture of an alkene and a Brønsted acid can be used to generate a carbocation intermediate, which then reacts with the aromatic ring.

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Why use an alcohol and a Lewis acid?

A mixture of an alcohol and a Lewis acid can also generate a carbocation intermediate for the Friedel-Crafts alkylation reaction.

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Acyl Cation Generation

The first step in Friedel-Crafts acylation involves the generation of an acyl cation by reacting an acid chloride with AlCl3. The AlCl3 removes a chloride ion, leaving a positively charged acyl group.

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Benzene's Reaction with Acyl Cation

The acyl cation, generated in the first step, attacks the benzene ring, forming a temporary intermediate called an arenium ion, where the positively charged carbon is delocalized across the aromatic ring.

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Why is poly-acylation not a problem?

The electron-withdrawing nature of the acyl group deactivates the substituted benzene ring, making further reaction less likely. This prevents multiple acylation reactions from occurring.

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Why don't acylium ion rearrangements occur?

The acylium ion is resonance stabilized, meaning the positive charge is distributed over multiple atoms, preventing rearrangements.

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What are acid chlorides?

Acid chlorides are organic compounds with the general formula RCOCl. They are important precursors for Friedel-Crafts acylation reactions.

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How are acid chlorides prepared?

Acid chlorides can be prepared by reacting carboxylic acids with thionyl chloride (SOCl2) or phosphorus pentachloride (PCl5).

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Halogenation of Benzene

A substitution reaction where a halogen atom (like Br or Cl) replaces a hydrogen atom on a benzene ring. This reaction requires a catalyst, such as FeCl3.

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Electrophilic Attack

In halogenation, the positively charged halogen cation (Br+ or Cl+) acts as an electrophile, attracted to the electron-rich benzene ring.

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Proton Abstraction

The final step where the arenium ion loses a proton (H+) to restore the aromatic ring and form the final product.

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Nitration of Benzene

A substitution reaction where a nitro group (NO2) replaces a hydrogen atom on a benzene ring. This requires a mixture of concentrated nitric and sulfuric acids.

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Nitronium Ion

A positively charged, reactive species (NO2+) that acts as the electrophile in nitration. It is generated by reacting nitric acid with sulfuric acid.

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Carbocation Intermediate

A positively charged species formed from the alkyl halide in Friedel-Crafts Alkylation. It acts as the electrophile in this reaction.

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Study Notes

Pharmaceutical Chemistry - Aromatic Compounds (Lecture 2)

  • Arenes: Aromatic hydrocarbons are called arenes.
  • Phenyl Group: A benzene ring with one hydrogen atom removed (C6H5−) is the phenyl group, denoted by Ph−.
  • Aryl Group: Aromatic hydrocarbons with one removed hydrogen atom are aryl groups, designated by Ar−. These are substituted benzene rings.
  • Electrophile: Electron-loving or electron-deficient species. Example: +C(CH3)3 (tertiary carbocation)
  • Electrophilic Substitution of Benzene: An electrophile (E+) reacts with a benzene ring and replaces a hydrogen atom. Benzene's stable closed-shell MO configuration of six Ï€ electrons allows substitution reactions over addition reactions.
  • Mechanism of Electrophilic Substitution:
    • Step 1: Electrophile takes two electrons from the six Ï€ electron system forming a σ bond with a carbon atom. This forms an arenium ion (σ-complex) that is non-aromatic.
    • Step 2: The arenium ion loses a proton from the carbon atom with the electrophile. This regenerates the aromatic ring's six Ï€ electrons.

Friedel-Crafts Alkylation

  • Development: Developed in 1877 by Charles Friedel and James Crafts.
  • Electrophile: The carbocation, R+. This carbocation forms from the ionization of RX with an aluminum chloride catalyst.
  • Reaction: An electrophilic aromatic substitution reaction where the electrophile is a carbocation. Example RX = isopropyl chloride.
  • Mechanism Steps:
    • Step 1: Formation of the carbocation: Example: AICI3 reacting with tert-butyl chloride to form tert-butyl-cations.
    • Step 2: Formation of the arenium ion: The carbocation attacks the benzene ring and forms a temporary non-aromatic structure.
    • Step 3: Loss of proton and regneration of the aromatic ring: The arenium ion loses proton and the ring is reformed.

Friedel-Crafts Alkylation - Mechanism for Primary Alkyl Halides

  • Difference to tertiary: In primary alkyl halide reactions, the AICI3 forms a complex with the alkyl halide. This complex acts as the electrophile.
  • Electrophilic attack: The complex transfers a positive alkyl group to the aromatic ring.

Examples of Friedel-Crafts Alkylation

  • Alkenes and Broonsted acids: Alkene mixed with Brønsted Acid forms the electrophile needed for substitution. Example: Propane (CH3CH=CH2) with Hydrogen Fluoride (HF).
  • Aromatics and Lewis acids: An alcohol (for example: cyclohexanol) reacts with a Lewis acid (for example: BF3) to form an electrophile, enabling substitution.

Limitations to Friedel-Crafts Alkylation

  • Aromatic/Vinyl/Alkyl halides: Aromatic, alkyl, and vinyl halides do not work for the reaction.
  • Electron-Withdrawing Substituents: Reactions will not occur with strong electron-withdrawing groups (e.g. -NO2, -CN, -CHO, -COR,...). or if the ring has -NH2, -NHR, or -NR2 groups
  • Multiple Substitutions: Multiple substitutions on the ring can occur.
  • Carbocation Rearrangements: Carbocation rearrangements may occur.
  • Primary alkyl halide mechanism: Primary alkyl halides form complex with the AICI3, which subsequently attacks the benzene ring and substitutes a positive alkyl group to the ring.

Friedel–Crafts Acylation

  • Acyl Group Introduction: Reacting benzene with an acid chloride to introduce an acyl group (COR). Example: CH3COCl.
  • Reaction Conditions: AICI3 as a catalyst and excess benzene, 80°C.
  • Preparation of Acid Chlorides: Preparing acid chlorides can be done by treating carboxylic acids with thionyl chloride or phosphorus pentachloride.

Friedel-Crafts Acylation - Mechanism

  • Step 1: Generation of acyl cation.: The acid chloride reacts with AICI3 to form an acyl cation.
  • Step 2: Attack by acyl cation on benzene ring.: The acyl cation attacks the benzene ring, forming a temporary non-aromatic structure.

Halogenation of Benzene

  • Reaction with Lewis Acid: In the presence of anhydrous Lewis acids (e.g., FeCl3), benzene readily reacts with either bromine or chlorine to form halogenated benzenes. Example: chlorobenzene.
  • Reactivity: F2 reacts very quickly, requiring more extensive apparatus. I2 is not reactive and requires oxidising agents (e.g., HNO3) to perform iodination.

Nitration of Benzene

  • Reaction with Nitric Acid: Benzene reacts slowly with hot concentrated nitric acid (HNO3) to form nitrobenzene.
  • Using Sulfuric Acid: Using concentrated sulfuric acid (H2SO4) alongside nitric acid increases the rate.
  • Electrophile: The nitronium ion (NO2+) acts as the electrophile.

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