Electrophilic Aromatic Substitution (EAS) Quiz

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Questions and Answers

Which of the following statements accurately describes the role of π-bonds in electrophilic aromatic substitution (EAS)?

  • π-bonds provide stability to the aromatic ring, preventing EAS reactions.
  • π-bonds act as electrophiles, initiating the reaction.
  • π-bonds act as nucleophiles, attacking the electrophile. (correct)
  • π-bonds remain unchanged throughout the EAS mechanism.

Benzene reacts readily with bromine in the absence of a catalyst.

False (B)

What is the general two-step mechanism involved in all electrophilic aromatic substitution (EAS) reactions?

The two-step mechanism of EAS involves the formation of a sigma complex (intermediate) followed by the loss of a proton to regenerate the aromatic ring.

The reaction of benzene with bromine in the presence of a catalyst is an example of ______ aromatic substitution.

<p>electrophilic</p> Signup and view all the answers

Match the following electrophilic aromatic substitution (EAS) reactions with their respective electrophiles:

<p>Halogenation = Br+ Nitration = Br+ Sulfonation = Br+ Friedel-Crafts Alkylation = Br+ Friedel-Crafts Acylation = Br+</p> Signup and view all the answers

Which type of group is characterized by accelerating reaction rates in electrophilic aromatic substitution?

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

Meta directors favor the formation of ortho and para regioisomers.

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

What is the primary effect of electron-donating groups in electrophilic aromatic substitution?

<p>They stabilize the arenium ion.</p> Signup and view all the answers

Electron-withdrawing groups are considered __________ groups in electrophilic aromatic substitution.

<p>deactivating</p> Signup and view all the answers

Match the following groups with their effects on reaction rate:

<p>Activating Groups = Faster reaction than benzene Deactivating Groups = Slower reaction than benzene Ortho/Para Directors = Favor both ortho and para products Meta Directors = Favor the meta regioisomer</p> Signup and view all the answers

What is a likely outcome when phenol reacts with bromine?

<p>2,4,6-Tribromophenol (C)</p> Signup and view all the answers

Strongly activating groups can lead to mono-substituted products during electrophilic aromatic substitution.

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

What should anilines be converted to in order to conduct electrophilic aromatic substitution reactions effectively?

<p>Amides</p> Signup and view all the answers

A Friedel-Crafts reaction will not occur if a ______-directing deactivator is present on the ring.

<p>meta</p> Signup and view all the answers

Match the following groups with their role in electrophilic aromatic substitution:

<p>-OH = Strongly activating group -NH2 = Strongly activating group NO2 = Meta-directing deactivator -COOH = Deactivating group</p> Signup and view all the answers

What is the active electrophile in the nitration reaction?

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

Sulfonation of aromatics can be reversed using water.

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

Which group is replaced by a sulfonic acid group in sulfonation?

<p>Hydrogen</p> Signup and view all the answers

The combination of SO3 and H2SO4 is called __________.

<p>fuming sulfuric acid</p> Signup and view all the answers

What type of catalyst is used in Friedel-Crafts alkylation?

<p>AlCl3 (C)</p> Signup and view all the answers

Match the following terms with their definitions:

<p>Nitronium ion = Electrophile in nitration Fuming sulfuric acid = Combination of SO3 and H2SO4 Friedel-Crafts alkylation = Process to add alkyl groups to aromatic rings De-sulfonation = Reversal of sulfonation using strong acid in water</p> Signup and view all the answers

What type of hydrocarbons would be most suitable for Friedel-Crafts alkylations?

<p>Tertiary alkyl halides</p> Signup and view all the answers

Which of the following is a reaction that results in the formation of aromatic amines?

<p>Reduction of nitro-substituted aromatics (D)</p> Signup and view all the answers

Which reaction mechanism involves the creation of a carbocation as an active electrophile?

<p>Friedel-Crafts alkylation (A)</p> Signup and view all the answers

Friedel-Crafts acylation can produce aromatic aldehydes.

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

What is the result of carbocation rearrangements during Friedel-Crafts reactions?

<p>Product mixtures</p> Signup and view all the answers

Friedel-Crafts reactions use an __________ to form an acylium ion.

<p>acyl chloride</p> Signup and view all the answers

Match the following types of alkyl halides with their classification:

<p>1° alkyl halides = Reactive to Friedel-Crafts alkylation 2° alkyl halides = Moderately reactive to Friedel-Crafts alkylation 3° alkyl halides = Most reactive to Friedel-Crafts alkylation D = Not applicable</p> Signup and view all the answers

Which of the following affects the rate and regioselectivity of SEAr reactions?

<p>Existing substituents on the aromatic ring (B)</p> Signup and view all the answers

Acid protonation of an alkene results in the formation of a stable carbocation.

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

What type of shift do carbocations undergo to achieve more stability?

<p>Alkyl or hydride shift</p> Signup and view all the answers

Which of the following groups is considered a strong activating ortho/para director?

<p>Heteroatoms with lone pairs (C)</p> Signup and view all the answers

Meta substitution in electrophilic aromatic substitution is favored due to resonance stabilization.

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

What type of directing groups contain electron-rich groups without a lone pair capable of delocalizing into the benzene ring?

<p>Weakly activating ortho/para directors</p> Signup and view all the answers

Halogens are considered __________ directors in electrophilic aromatic substitution despite being deactivating.

<p>ortho/para</p> Signup and view all the answers

Match the activating groups with their characteristics:

<p>Strongly activating = Contain heteroatoms with lone pairs Moderately activating = Lone pair delocalized into a different π-system Weakly activating = Electron-rich groups without lone pairs Deactivating = Electron-withdrawing groups favoring meta substitution</p> Signup and view all the answers

Which statement about arenium ion stability is correct?

<p>Ortho/para substituted arenium ions benefit from lone pair stabilization (A)</p> Signup and view all the answers

Moderately activating ortho/para directors do not contain lone pairs that can delocalize.

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

Which type of directing group is characterized by having polar π bonds and an electronegative atom?

<p>Deactivating meta directing groups</p> Signup and view all the answers

The _______ position is less stable due to resonance contributors that yield a positive charge next to an electron-withdrawing group.

<p>ortho</p> Signup and view all the answers

Which of these factors does NOT contribute to the regioselectivity of electrophilic aromatic substitution reactions?

<p>Electronegativity of substituent (D)</p> Signup and view all the answers

Flashcards

Aromatic Compounds

Compounds containing one or more aromatic rings, known for stability and unique reactivity.

Electrophilic Aromatic Substitution (EAS)

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

Halogenation

A type of EAS where a halogen replaces a hydrogen atom in an aromatic ring.

Directing Groups

Substituents on an aromatic ring that influence the position of new substituents during EAS reactions.

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Ortho/Para-Directing

Group that directs new substituents to the ortho or para position relative to itself on an aromatic ring.

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Meta-Directing

Group that directs new substituents to the meta position, away from itself on the ring.

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Friedel-Crafts Alkylation

A type of EAS that introduces an alkyl group to an aromatic ring using a carbocation.

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Retrosynthetic Analysis

A method to break down complex molecules into simpler starting materials for synthesis.

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Nitronium ion

The active electrophile in nitration, formed by dehydration of HNO3 with H2SO4.

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Nitration of aromatics

A reaction where a nitro group (–NO2) replaces a hydrogen on an aromatic ring.

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Anilines

Amines derived from nitro-substituted aromatics through reduction.

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Fuming sulfuric acid

A mixture of SO3 and H2SO4 used in the sulfonation of aromatics.

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Sulfonation of aromatics

The process of adding a sulfonic acid group (–SO3H) to an aromatic ring.

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SO3H+

The active electrophile in the sulfonation process of aromatics.

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Carbocation stability

The tendency of a carbocation to be stable based on its structure and substituents.

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Carbocation Rearrangement

A process where carbocations shift to form more stable structures via alkyl or hydride shifts.

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Types of Alkyl Halides

3°, 2°, and 1° alkyl halides differ in stability, with 3° being most stable.

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Acylium Ion

The active electrophilic species formed during Friedel-Crafts acylation.

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Directing Groups in EAS

Substituents that influence the rate and position of new substituent addition in electrophilic aromatic substitution.

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Reaction Rate

Speed of reaction influenced by substituents compared to unsubstituted aromatic compounds.

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Regioselectivity

The preference for a substituent to be added in ortho, meta, or para positions relative to existing groups.

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Activating Groups

Substituents that speed up the EAS reaction rate compared to benzene.

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Deactivating Groups

Substituents that slow down the EAS reaction rate compared to benzene.

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Ortho/Para Directors

Groups that favor the formation of ortho and para regioisomers in EAS.

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Meta Directors

Groups that favor the formation of the meta regioisomer in EAS.

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Arenium Ion Stability

Electron-donating groups stabilize the arenium ion, making the reaction faster.

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Strongly Activating Groups

Substituents like -OH and -NH2 that increase electron density in EAS reactions, often leading to poly-substitution.

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Protonation of NH2

Under acidic conditions, -NH2 can become -NH3+, acting as an electron-withdrawing group (EWG) instead of an electron-donating group (EDG).

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Conversion to Amides

Temporarily converting aniline to amides helps prevent overreaction and encourages ortho/para selectivity in EAS reactions.

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Friedel-Crafts Reaction Limits

Certain deactivating groups (like NO2) prevent Friedel-Crafts reactions as carbocations fail to react with deactivated rings.

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Rearrangements in Carbocations

During reactions, carbocations can rearrange to form more stable structures, impacting EAS outcomes.

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Arenium Ion

An intermediate formed during electrophilic aromatic substitution that can be stabilized by resonance.

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Resonance Stabilization

The delocalization of electrons that stabilizes intermediates like the arenium ion.

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Meta Substitution

Electrophilic substitution occurring at the meta position, leading to a less stable arenium ion.

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Moderately Activating Groups

Directing groups that can stabilize the arenium ion through cross-conjugation.

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Weakly Activating Groups

Groups that increase electron density through σ-bonds but lack lone pairs for resonance.

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Halogens as Directors

Deactivating groups that are ortho/para directors but have weak resonance effects.

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Electronic Effects

The impact of substituents on the electronic properties of the aromatic ring.

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Study Notes

Introduction to Organic Chemistry II - CHEM 2021

  • Course Information:
    • Introductory Organic Chemistry II, Winter 2024, York University
    • Instructor: Dr. Lana Hébert

Chapter 10: Synthesis Using Aromatic Materials

  • Focuses on reactions involving aromatic compounds, specifically using aromatic materials in synthesis.

Aromatic Compounds Are Everywhere!

  • Many biomolecules contain aromatic rings.
  • Aromatic rings are important in active research areas.
  • Examples of compounds containing aromatic rings include:
    • chloramphenicol (antibiotic)
    • mescaline (peyote cactus active agent)
    • adrenaline (hormone)
    • ephedrine (bronchodilator)
    • epinephrine (stress response hormone)
    • amphetamine (appetite suppressant)
    • methamphetamine ("speed")
    • saccharin (artificial sweetener)
    • p-dichlorobenzene (mothballs, air fresheners)
    • coronene
    • hexa-peri-hexabenzocoronene

Electrophilic Aromatic Substitution (EAS)

  • General two-step mechanism:
      1. Addition of an electrophile
      1. Elimination of a leaving group to restore aromaticity
  • Substituents can affect the rate and position of substitution.
  • Different substituents can either activate or deactivate the ring for subsequent EAS reactions.

Halogenation of Aromatics: X₂ + Lewis Acid

  • Aromatic rings can be halogenated using halogens (Br₂ or Cl₂) and a Lewis acid catalyst.
  • Lewis acid catalysis increases the electrophilicity of a halogen.
  • The halogen reacts with the aromatic ring to form an arenium ion intermediate.
  • The specific halogen in the Lewis acid often corresponds to the substituted halogen.
  • Fluorine does not require a catalyst.
  • Iodine requires an oxidant to create I+, e.g. HNO3 or CuCl2.

Nitration of Aromatics: HNO₃ + H₂SO₄

  • Nitration involves replacing a hydrogen atom with a nitro group.
  • Nitric acid (HNO3) and sulfuric acid (H2SO4) are components in this reaction.
  • The active electrophile in this reaction is the nitronium ion (NO2+). The nitronium ion is generated from the dehydration of HNO3 using H2SO4.

Friedel-Crafts Alkylation: Alkyl Halide + AlCl₃

  • Friedel-Crafts alkylation adds alkyl groups to aromatic rings.
  • The Lewis acid AlCl₃ creates a carbocation as the electrophile. A carbocation intermediate is formed when the halogen group is lost.

Friedel-Crafts Acylation: Acyl Chloride + AlCl₃

  • Friedel-Crafts acylation adds acyl groups to aromatic compounds.
  • Acylium ion is the electrophile.
  • Acylium ion is formed when the activated leaving group is lost.

Friedel-Crafts Acylation: Variations

  • Friedel-Crafts acylation can use anhydrides instead of acid chlorides.
  • Carbon monoxide can be used in synthesis of aromatic aldehydes, using a Gatterman-Koch reaction.

Directing groups in EAS

  • Different functional groups can either activate or deactivate the aromatic ring.
  • Activating groups increase the rate of electrophilic aromatic substitution.
  • Deactivating groups decrease the rate of electrophilic aromatic substitution
  • Ortho-para vs. meta directing

Strongly Activating Ortho/Para Directors

  • Contain heteroatoms with lone pairs.
  • Lone pairs can delocalize into the ring via resonance.

Moderately Activating Ortho/Para Directors

  • Contain a heteroatom involved in cross-conjugation.
  • Lone pairs are delocalized into a different 𝜋-system.

Weakly Activating Ortho/Para Directors

  • Mostly alkyl groups.
  • Electron donating ability is predominantly through sigma bonds (inductive).
  • These groups are considered comparatively weak electron donors.

Activating Ortho/Para Directing Groups: Aromatic Rings

  • Aromatic rings can stabilize the arenium ion intermediate.
  • Resonance is not favorable due to loss of aromaticity and increased steric strain.

Deactivating Ortho/Para Directing Groups: Halogens

  • Halogens are deactivating via induction (electron withdrawing since they are electronegative)
  • Show ortho/para directing behavior since they possess lone pairs.
  • Resonance effect is weak.
  • Orbital overlap between 2p ring orbitals and lone pair p orbitals for Cl and Br is also poor.

Deactivating Meta Directing Groups

  • Electron-withdrawing groups (EWGs) typically deactivate an aromatic ring towards EAS.
  • EWGs typically favor meta-substitution because one resonance form having the positive charge next to an EWG is less stable.
  • These groups often contain, but aren't limited to polar bonds attached to atoms.

Limitations of EAS Reaction

  • Strongly activating groups can lead to over substitution.
  • Deactivating groups may decrease reaction rate or prevent the reaction entirely.

Retrosynthetic Analysis in Aromatic Synthesis

  • Method for planning synthesis by reversing the reaction sequence.
  • Involves identifying the fragments where the targeted molecule originates
  • Identifies reagents to form the targeted substituents

Order of Synthetic Operations

  • Order of reactions is critical to target product success.
  • Often one must separate o/p reaction products.

Worked Examples

  • Various examples are presented showing reactions using different reagents and the mechanisms underlying different reactions in the synthesis of various compounds.

Class Questions

  • Different class questions are included throughout to evaluate understanding of the concepts.

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Related Documents

CHEM 2021: Introductory Organic Chemistry II Chapter 10 Synthesis Using Aromatic Materials PDF

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