Organic Chemistry: Aromatic Reactions Quiz
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Questions and Answers

What is the name of the individual who developed the Wheland intermediate?

  • George Willard Wheland (correct)
  • W. Wheland
  • University of Chicago
  • None of the above

    • Which of the following elements, when used directly for fluorination, results in poor yields of mono-fluoroaromatic products?

  • Fluorine (correct)
  • Bromine
  • Iodine
  • Chlorine

    • Which of these elements, when used for aromatic substitution, require the addition of other reagents to promote the reaction?

  • Both A and B (correct)
  • Iodine
  • Fluorine
  • Chlorine

    • What is the name of the intermediate that is often attributed to W. Wheland?

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

    Which type of chemical reaction is mentioned in the provided text for producing aromatic substitutions?

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

    What is the role of the catalyst in this reaction?

    <p>It makes the Br2 molecule more electrophilic. (B)</p> Signup and view all the answers

    How does the catalyst polarize the Br2 molecule?

    <p>It forms a bond with one of the bromine atoms. (D)</p> Signup and view all the answers

    What is the electrophilic species that reacts in this reaction?

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

    Why is the Br+ species considered to be electrophilic?

    <p>It is attracted to electron-rich species. (A)</p> Signup and view all the answers

    Which of these options is a plausible explanation for why the catalyst makes the Br2 molecule more electrophilic?

    <p>The catalyst forms a bond with the Br2 molecule. (C)</p> Signup and view all the answers

    What is required for Br2 to react with benzene at room temperature?

    <p>A catalyst such as FeBr3 (A)</p> Signup and view all the answers

    Which compound does Br2 in CH2Cl2 solution NOT react with at room temperature?

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

    What type of solvent is used for the reaction of Br2 with most alkenes?

    <p>CH2Cl2 (D)</p> Signup and view all the answers

    What characteristic of Br2's reaction with alkenes distinguishes it from its reaction with benzene?

    <p>Speed of reaction (C)</p> Signup and view all the answers

    In the reaction of Br2 with alkenes, what is the role of FeBr3 when benzene is present?

    <p>It allows the reaction to occur at room temperature (C)</p> Signup and view all the answers

    What do halogens, C=O, CN, and NO2 have in common regarding their interaction with electrons?

    <p>They withdraw electrons through sigma bonds connected to the ring. (C)</p> Signup and view all the answers

    What factor primarily influences the electron withdrawal ability of the functional groups mentioned?

    <p>The electronegativity and polarity of the bonds in the functional groups. (D)</p> Signup and view all the answers

    How do alkyl groups affect electron density in a molecular structure?

    <p>They donate electrons to the molecular structure. (C)</p> Signup and view all the answers

    Which of the following correctly describes the resonance effects related to the functional groups mentioned?

    <p>Resonance effects can enhance the electron-withdrawing ability of functional groups. (C)</p> Signup and view all the answers

    Which statement is true regarding the behavior of electrons in relation to the specified functional groups?

    <p>Functional groups withdrawing electrons can stabilize cations. (D)</p> Signup and view all the answers

    What is the reactive electrophile in Friedel-Crafts acylation?

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

    Why does an acyl cation not rearrange?

    <p>Rearrangement would disrupt the resonance stabilization. (D)</p> Signup and view all the answers

    What is the similarity between Friedel-Crafts acylation and alkylation?

    <p>Both reactions involve the formation of a carbocation as the electrophile. (A)</p> Signup and view all the answers

    Which of the following statements about Friedel-Crafts acylation is FALSE?

    <p>Acyl cation rearrangements are common in this reaction. (D)</p> Signup and view all the answers

    What is the key difference between Friedel-Crafts acylation and alkylation?

    <p>The product of acylation is a ketone, while the product of alkylation is an alkane. (C)</p> Signup and view all the answers

    Which of the following is the best precursor for synthesizing 4-chloro-2-propylbenzenesulfonic acid from benzene?

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

    Why is the synthesis of 4-chloro-2-propylbenzenesulfonic acid from benzene likely to result in a product mixture?

    <p>Multiple reaction pathways are possible. (B)</p> Signup and view all the answers

    Which of the following is the most likely product of the reaction described in the content?

    <p>4-chloro-2-propylbenzenesulfonic acid (C)</p> Signup and view all the answers

    What is the primary challenge associated with synthesizing 4-chloro-2-propylbenzenesulfonic acid from benzene?

    <p>Separating the desired product from the product mixture. (B)</p> Signup and view all the answers

    What functional group is introduced to benzene in the first step of this synthesis?

    <p>Methyl group ($CH_3$) (A)</p> Signup and view all the answers

    Flashcards

    Br2 in CH2Cl2

    A bromine reagent dissolved in dichloromethane, used to react with alkenes.

    Reactivity with alkenes

    Br2 in CH2Cl2 reacts instantly with most alkenes due to double bonds.

    Non-reaction with benzene

    Benzene does not react with Br2 in CH2Cl2 at room temperature without a catalyst.

    Role of FeBr3

    Iron(III) bromide catalyst that facilitates the reaction of Br2 with benzene.

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    Temperature effect on reactions

    Room temperature conditions affect reactivity of compounds like benzene.

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    Wheland Intermediate

    A specific reaction intermediate in aromatic halogenation named after George W. Wheland.

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    Chlorination

    A chemical reaction where chlorine is added to an aromatic compound, leading to substitution.

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    Iodination

    A process of adding iodine to an aromatic compound through a substitution reaction.

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    Fluorination

    The addition of fluorine to an aromatic compound but yields poor results due to high reactivity.

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    Aromatic Substitution

    A reaction where a hydrogen atom in an aromatic compound is replaced by another atom or group.

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    p-bromotoluene

    A type of bromobenzene with a methyl group, used as a precursor.

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    Product mixture

    A mixture of different products resulting from a chemical reaction.

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    4-chloro-2-propylbenzenesulfonic acid

    A sulfonic acid compound derived from benzene, with chloride and propyl groups.

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    Benzene synthesis reaction

    A reaction to create derivatives from the benzene structure.

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    Separation techniques

    Methods used to separate product mixtures into individual components.

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    Halogens

    Elements in Group 17 that pull electrons through sigma bonds.

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    Electron Withdrawal

    The process of attracting electrons away from a molecule or group.

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    Alkyl Groups

    Hydrocarbon groups that donate electrons to other groups in a molecule.

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    Polarity of Bonds

    The distribution of electrical charge across a bond due to electronegativity differences.

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    Resonance Effects

    The influence of electron distribution through structures that can have multiple forms.

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    Friedel-Crafts Acylation

    A reaction that introduces acyl groups into aromatic compounds.

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    Reactive Electrophile

    A species that accepts electrons; in acylation, it's a resonance-stabilized acyl cation.

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    Acyl Cation

    A positively charged species derived from an acyl group with resonance stabilization.

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    Resonance Stabilization

    The delocalization of electrons within a molecule, increasing stability.

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    Charge Rearrangement

    The process where a charge moves within a molecule; acyl cations do not rearrange.

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    Catalyst

    A substance that increases the rate of a chemical reaction without being consumed.

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    Electrophilic

    A species that attracts electrons in a chemical reaction, often positively charged.

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    Br2 molecule polarization

    The process by which the Br2 molecule becomes polarized, enhancing its reactivity.

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    FeBr4-Br+

    A species formed when the Br2 is polarized, behaving as if it were a Br+ ion.

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    Br+ species

    A highly reactive species that acts as a strong electrophile in reactions.

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    Study Notes

    Chemistry of Benzene: Electrophilic Aromatic Substitution

    • Benzene is an aromatic compound, a cyclic conjugated compound with 6 π electrons
    • Reactions of benzene retain the aromatic core
    • Electrophilic aromatic substitution: an electrophile (E⁺) reacts with an aromatic ring, substituting for a hydrogen
    • Replaces a proton on benzene with another electrophile

    Bromination of Aromatic Rings

    • Benzene's π electrons act as a Lewis base in reactions with Lewis acids
    • Bromination product is formed by losing a proton, replaced by bromine
    • A catalyst, such as FeBr₃, polarizes Br₂ to FeBr₄-Br⁺, which reacts like Br⁺
    • Bromine, in the presence of a catalyst, is more electrophilic.
    • Intermediate is a non-aromatic carbocation, making it high in energy.

    Addition Intermediate in Bromination

    • Bromination occurs in two steps
    • In the first step, π electrons attack Br₂.
    • This forms a cationic addition intermediate from benzene and bromine
    • The intermediate is not aromatic and is high in energy

    Formation of Product from Intermediate

    • The cationic addition intermediate transfers a proton to FeBr₄⁻ (from Br and FeBr₃).
    • This restores aromaticity in the product
    • This contrasts with addition in alkenes

    Other Aromatic Substitutions

    • Bromine reaction mechanism similar to other benzene reactions with electrophiles
    • The cationic intermediate was proposed by G. W. Wheland and is called the Wheland intermediate
    • Other reactions may require specific catalysts for substitution.

    Aromatic Halogenation: Chlorination and Iodination

    • Chlorine and iodine undergo aromatic substitution with additives
    • Chlorination requires FeCl₃ as a catalyst
    • Iodine needs an oxidising agent such as a peroxide or a copper salt to form a powerful I⁺ species to react.

    Aromatic Nitration

    • Nitric acid and sulfuric acid combine to produce the nitronium ion (NO₂⁺)
    • The reaction with benzene yields nitrobenzene

    Aromatic Sulfonation

    • Substitution of H by SO₃ (sulfonation) with fuming sulfuric acid and SO₃
    • The active species is sulfur trioxide or its conjugate acid
    • The reaction occurs via Wheland intermediate and can be reversible

    Aromatic Hydroxylation (Alkali Fusion)

    • Direct hydroxylation of an aromatic ring to yield a hydroxybenzene (phenol) is difficult
    • Sulfonic acids are useful as intermediates in the reaction
    • Heating with NaOH at 300°C followed by neutralization with acid replaces the SO₃H group with an OH group
    • An example is the synthesis of p-cresol

    Alkylation of Aromatic Rings: Friedel-Crafts Reaction

    • Aromatic substitution of a R⁺ for H.
    • Aluminum chloride promotes carbocation formation
    • Wheland intermediate is formed

    Limitations of Friedel-Crafts Alkylation

    • Only alkyl halides can be used (F, Cl, Br, I)
    • Aryl and vinylic halides are not reactive due to high energy carbocations

    Friedel-Crafts Reactions

    • Friedel-Crafts reactions don't succeed on aromatic rings that are substituted with strongly electron-withdrawing groups, such as carbonyl groups or basic amino groups
    • Control Problems: multiple alkylations can occur, difficult to stop reaction after a single substitution. High monoalkylation yield obtained only with benzene excess
    • Carbocation rearrangements occur during alkylation, especially with primary alkyl halides, that include hydride and alkyl shifts.

    Acylation of Aromatic Rings

    • Reaction of an acid chloride (RCOCl) and an aromatic ring in the presence of AlCl₃ introduces an acyl group (-COR).
    • Benzene and acetyl chloride yield acetophenone
    • Acylations never occur more than once on a ring because the product acylbenzene is less reactive than non-acylated starting material

    Mechanism of Friedel-Crafts Acylation

    • Similar to alkylation, with same limitations
    • Acyl cations are resonance-stabilized and do not rearrange

    Trisubstituted Benzenes: Additivity of Effects

    • With identical directing effects, result is additive (e.g. ortho to methyl and meta to nitro) producing a single product
    • This principle is used in product prediction.

    Substituents with Opposite Effects

    • If directing effects oppose each other, more powerful effect dominates, usually forming mixtures of products.
    • OH usually more powerful than CH₃

    Meta-Disubstituted Compounds Are Unreactive

    • Further substitution rarely occurs between meta disubstituted groups due to steric hindrance.

    Aromatic Ring Oxidation

    • The benzene ring is generally inert to strong oxidizing agents.
    • Alkyl side chains can oxidize to CO₂H if there is a C-H bond next to the ring
    • This converts an alkylbenzene to a benzoic acid

    Bromination of Alkylbenzene Side Chains

    • Using N-bromo-succinimide (NBS) and benzoyl peroxide, introduces Br into the side chain at the benzylic position

    Benzylic Radical Reaction

    • Abstraction of a benzylic hydrogen atom generates an intermediate benzylic radical.
    • Reacts with Br₂ to yield product Br. radical cycles back into reaction.

    Nucleophilic Aromatic Substitution

    • Withdrawing substituents allow nucleophilic substitution to occur, via anion stabilization.
    • Aryl halides (Cl) with substituents such as nitro groups can displace the halogen (via addition/elimination).

    Benzyne Reactions

    • With halobenzenes, high temperature and pressure favour Cl elimination forming benzyne.
    • Subsequent nucleophilic addition yields products such as phenols.

    Reduction of Aromatic Compounds

    • Aromatic rings generally inert to catalytic hydrogenation.
    • To reduce alkene double bonds in the presence of aromatic rings and a carbonyl group.
    • Requires strong reducing conditions, high pressure with a catalyst like platinum or rhodium, to convert aromatic rings into cyclohexanes.
    • Reduction of aryl alkyl ketones proceeds the same way
    • Aromatic ring activation of neighboring C-H, and carbonyl, towards reduction.
    • Ketones to alkylbenzenes by catalytic hydrogenation.

    Synthesis Strategies

    • Synthesis planning and alternative routes required for complex synthesis.
    • Using this methodology to synthesize 4-bromo-2-nitrotoluene.

    Predicting Substitution Positions

    • Substituents affect orientation (e.g., ortho/para/meta) of further substitution, based on their respective inductive and resonance effects.
    • Relative strengths of activation/deactivation effects are key to predicting final product.

    Summary Table: Substituent Effects in Electrophilic Aromatic Substitution

    • Summarizes reactivity, orienting, inductive, and resonance effects of common substituents.

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    Description

    Test your knowledge on aromatic reactions and the Wheland intermediate in this quiz. Explore various elements and catalysts involved in electrophilic aromatic substitution. Assess your understanding of key concepts such as the role of Br2 in these reactions and the mechanisms at play.

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