Nomenclature of Aromatic Compounds
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Questions and Answers

What is the symbol used to represent the phenyl substituent?

  • CH3
  • C6H5 (correct)
  • CH2
  • Ph (correct)

    • What is the prefix used to name a disubstituted benzene with substituents on adjacent carbon atoms?

  • meta
  • ortho (correct)
  • para
  • iso

    • When naming a benzene with more than two substituents, how are the substituents presented in the name?

  • In order of electronegativity
  • In alphabetical order (correct)
  • In order of molecular weight
  • In order of reactivity

    • What is the empirical formula of benzene?

    <p>C6H6</p> Signup and view all the answers

    What is the structure of benzene according to the resonance theory?

    <p>A hybrid of two Kekulé structures</p> Signup and view all the answers

    What is the term used to describe each of the two equivalent structures of benzene?

    <p>Canonical form</p> Signup and view all the answers

    What is a characteristic of monosubstituted benzene?

    <p>It forms only one product</p> Signup and view all the answers

    What is a limitation of the Kekulé structure?

    <p>It fails to explain why benzene does not undergo addition reactions</p> Signup and view all the answers

    What is a result of the delocalization of the π system of electrons in benzene?

    <p>Identical electron density in all C-C bonds</p> Signup and view all the answers

    Why does benzene react slowly with Br2?

    <p>Because it requires a catalyst</p> Signup and view all the answers

    What is the hybridization of the carbon atoms in the benzene molecule?

    <p>sp2-hybridized</p> Signup and view all the answers

    What is the shape of the benzene molecule?

    <p>A regular hexagon</p> Signup and view all the answers

    What is the energy released when cis-Cyclooctene is hydrogenated to cyclooctane?

    <p>96 kJ mol-1</p> Signup and view all the answers

    What is the resonance energy of benzene?

    <p>150 kJ/mol</p> Signup and view all the answers

    What type of reagents are used in electrophilic fluorination instead of elemental fluorine?

    <p>Electrophilic fluorinating reagents</p> Signup and view all the answers

    What is the catalyst used in the reaction of Cl2 and Br2 with benzene?

    <p>FeCl3/FeBr3</p> Signup and view all the answers

    What is the limitation of Friedel-Crafts alkylation reactions with respect to aromatic rings?

    <p>Aromatic rings having electron withdrawing substituents or amino groups are not reactive.</p> Signup and view all the answers

    Which of the following substituents are ortho- and para- directors?

    <p>-OH and -NH2</p> Signup and view all the answers

    What is the reaction that involves the reduction of carbonyl compounds to alkanes?

    <p>Clemmensen's reduction</p> Signup and view all the answers

    What is required for iodine to react with aromatic rings?

    <p>An oxidizing agent</p> Signup and view all the answers

    What is the role of electrophilic aromatic halogenation in the biosynthesis of thyroxine?

    <p>It is involved in the biosynthesis of thyroxine in humans</p> Signup and view all the answers

    What is the product of the oxidation of alkyl benzenes with strong oxidizing agents such as acidified KMNO4?

    <p>Benzoic acid</p> Signup and view all the answers

    What is necessary for benzene to react with bromine?

    <p>Presence of Lewis acid catalysts</p> Signup and view all the answers

    What is the electrophile in aromatic nitration?

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

    What is the reason for using dilute HNO3 in the nitration of phenol?

    <p>To prevent oxidation of phenols</p> Signup and view all the answers

    What type of substituents make the aromatic ring more reactive in electrophilic aromatic substitution reactions?

    <p>Activating substituents</p> Signup and view all the answers

    Which of the following is NOT a meta-directing substituent?

    <p>-OH</p> Signup and view all the answers

    How is the electrophile for sulfonation of benzene generated?

    <p>When one molecule of H2SO4 protonates another and loses a molecule of water</p> Signup and view all the answers

    What is the catalyst used in Friedel-Crafts acylation reactions?

    <p>AlCl3</p> Signup and view all the answers

    What type of reaction occurs in the biosynthetic pathways of phylloquinone (Vit.K)?

    <p>Aromatic alkylation reaction</p> Signup and view all the answers

    What is the role of aromatic hydroxylation in drug metabolism?

    <p>It leads to excretion of drug metabolites</p> Signup and view all the answers

    How many bromine atoms are present in the product formed when phenol reacts with bromine?

    <p>3</p> Signup and view all the answers

    What is the purpose of aromatic sulfonation in industry?

    <p>It is used in the synthesis of dyes and pharmaceuticals</p> Signup and view all the answers

    What is the reaction that involves the formation of alkyl substituted benzenes from aryl halides and alkyl halides?

    <p>Wurtz-Fittig reaction</p> Signup and view all the answers

    What is the factor that controls the electron donation or electron withdrawal ability of substituents?

    <p>Interaction between inductive and resonance effects</p> Signup and view all the answers

    What is the limitation of Friedel-Crafts alkylation reactions with respect to alkyl halides?

    <p>Only alkyl halides can be used for this reaction.</p> Signup and view all the answers

    Study Notes

    Nomenclature of Aromatic Compounds

    • The phenyl substituent is represented as Ph or C6H5-
    • Prefixes used for naming disubstituted benzenes are:
      • Ortho (o)
      • Meta (m)
      • Para (p)
    • When there are more than two substituents on the benzene ring, the point of attachment is chosen as carbon 1 and numbering is done to give lower locants to the substituents.
    • Substituents are presented in alphabetical order when writing the names.

    Structure and Stability of Benzene

    • The structure of benzene is described by two theories: resonance and molecular orbital theories.
    • Empirical formula of benzene is C6H6, with no structure assigned.
    • Kekulé proposed the structure of benzene as a six-membered carbon-atom ring with alternating single and double bonds.
    • The molecule oscillates between two equivalent structures (A and B) where the single and double bonds continuously interchange positions.
    • The actual structure of benzene is a hybrid of the two Kekulé structures (A and B), which leads to the resonance hybrid (C).
    • Each of structures (A) and (B) is called a canonical form of benzene.
    • Evidence in support of the Kekulé structure of benzene:
      • Monosubstituted benzene always forms only one product (isomer).
      • Disubstituted benzene forms three products (isomers).
    • Limitations of the Kekulé structure:
      • Does not explain why benzene fails to undergo addition reactions typical of alkenes.
      • Does not account for the equal length of carbon-carbon bonds (139 pm) in benzene.
      • Does not explain the unusual stability of benzene (150 kJ/mol resonance energy).

    Molecular Orbital Theory

    • The carbon atoms of the benzene molecule are sp2-hybridized with each C-atom having an unhybridized p-orbital perpendicular to the plane of the benzene ring.
    • Each p-orbital overlaps equally well with both neighboring p-orbitals.
    • The 6 π electrons of benzene are completely delocalized around the ring, with electron density in all C-C bonds being identical.
    • The delocalization of the π system of electrons affords benzene its unusual stability.
    • The benzene molecule is conjugated and planar with the shape of a regular hexagon.

    Why is Benzene not an Alkene?

    • Treatment of cyclohexene with Br2 rapidly leads to the addition product 1,2-dibromocyclohexane.
    • However, benzene reacts only slowly and requires a catalyst to produce the substitution product bromobenzene.
    • Comparison of heats of hydrogenation of cyclooctatetraene, cyclohexenes, and benzene:
      • When cis-cyclooctene is hydrogenated to cyclooctane, 96 kJ/mol of energy is released.

    Electrophilic Aromatic Substitution Reactions

    • Aromatic halogenation:
      • Cl2 and Br2 react with benzene in the presence of FeCl3/FeBr3 catalysts to form chloro/bromo benzenes.
      • Iodine is rather unreactive towards aromatic rings, but in the presence of oxidizing agents such as CuCl2 or H2O2, a reaction takes place leading to the formation of iodobenzene.
    • Aromatic nitration:
      • A mixture of conc. HNO3 and H2SO4 produces a nitronium ion (NO2+) which reacts as the electrophile.
      • Aromatic nitration does not occur in nature, but the nitro group can be reduced to produce corresponding arylamine (ArNH2) such as aniline.
    • Aromatic sulfonation:
      • The electrophile for sulfonation of benzene can be generated when one molecule of H2SO4 protonates another and loses a molecule of water.
      • Aromatic sulfonation can also be achieved when the aromatic ring reacts with fuming sulfuric acid (a mixture of H2SO4/SO3).
      • Aromatic sulfonation does not occur in nature, but it is utilized in the synthesis of dyes and pharmaceuticals such as sulfanilamide.
    • Aromatic hydroxylation:
      • Hydrolysis of benzene diazonium salts to produce phenols.
      • Aromatic hydroxylation is an important reaction in drug metabolism leading to excretion of drug metabolites.

    Aromatic Alkylation

    • Synthesis of aromatic hydrocarbons can be achieved through one of the following named reactions:
      • Friedel-Crafts alkylation reaction
      • Clemensen's reduction
      • Wurtz-Fittig reaction
    • Mechanism of aromatic alkylation:
      • Limitations of Friedel-Crafts alkylation reactions:
        • Only alkyl halides can be used for this reaction.
        • The reaction is not feasible with aromatic rings already having electron-withdrawing substituents or amino groups.
        • It is difficult to control polyalkylation.
        • There can be skeletal rearrangement of the alkyl electrophile into a more stable carbocation.
    • Aromatic alkylation reactions occur in a number of biosynthetic pathways such as the synthesis of phylloquinone (Vit. K).

    Friedel-Crafts Acylation Reactions

    • An aromatic compound is treated with an alkyl chloride (RCl) in the presence of AlCl3 catalyst.
    • The electrophile is generated by AlCl3-assisted dissociation of an alkyl halide.
    • The aromatic ring is acylated with acyl chloride (ROCl) in the presence of AlCl3.

    Clemmensen's Reduction

    • Reduction of carbonyl compounds to alkanes using Zn/Hg and HCl.

    Wurtz-Fittig Reaction

    • The reaction involves aryl halides and alkyl halides in the presence of Na in dry ether to form alkyl-substituted benzenes.

    Oxidation of Alkyl Benzenes

    • Oxidation of alkyl benzenes with strong oxidizing agents such as acidified KMNO4 produces benzoic acid.

    Effect of Substituents in Electrophilic Aromatic Substitution Reactions

    • Reactivity of the aromatic ring:
      • Activating substituents make the aromatic ring more reactive (e.g. –NH2, -OH, -OCH3, etc.).
      • Deactivating substituents render the aromatic ring less reactive than benzene (e.g. –CHO, -NO2, -CO2H, -CO2R, -NR3+, etc.).
    • Substituents influence the orientation of the reaction by being either ortho/para- directors or meta- directors.
    • Generally, substituents can be classified into ortho- and para-directing activators, ortho- and para-directing deactivators, and meta-directing deactivators.

    Electrophilic Substitution on Phenols

    • Benzene reacts with bromine only in the presence of Lewis acid catalysts.
    • Phenols react rapidly (3x faster) without a catalyst with bromine to form a product containing three bromine atoms (2,4,6-tribromophenol) at specific positions.

    Nitration of Phenol

    • Nitration of phenol is problematic under normal nitration conditions since concentrated HNO3 oxidizes phenols.
    • Dilute HNO3 is used instead.

    Preparation of Paracetamol and Aspirin from Phenol

    • How to prepare paracetamol from phenol.
    • How to prepare salicylic acid and acetylsalicylic acid (aspirin) from phenol.

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    Description

    Learn about the rules for naming aromatic compounds, including the use of prefixes for disubstituted benzenes and the method for naming compounds with multiple substituents.

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