Organic Chemistry: Halogenation of Furan
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Questions and Answers

What is the reaction outcome of furan with iodine at room temperature?

  • Polyhalogenated products are formed
  • Monohalogenated products are formed
  • Furan decomposes
  • No reaction occurs (correct)

    • What is required for the formation of monobromo- and monochloro-furans?

  • Presence of a catalyst
  • High temperature
  • Milder conditions (correct)
  • Harsh conditions

    • What is the product of bromination of furan by bromine in dimethylformamide at room temperature?

  • Monochloro-furans
  • Polybromo-furans
  • 2-Bromo- or 2,5-dibromo-furans (correct)
  • 2,5-Dibromo-furans

    • What is the role of dioxane in the bromination of furan?

    <p>It forms a complex with bromine</p> Signup and view all the answers

    What is the mechanism involved in the bromination of furan substituted with an electron-withdrawing group at the position-2?

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

    What is the product of bromination of furan substituted with an electron-withdrawing group at the position-2?

    <p>5-Bromo derivative</p> Signup and view all the answers

    What is the order of reactivity in five-membered heterocycles?

    <p>pyrrole &gt; furan &gt; thiophene &gt; benzene</p> Signup and view all the answers

    Why is pyrrole more reactive towards electrophiles than furan?

    <p>because nitrogen has a greater electron releasing ability than oxygen</p> Signup and view all the answers

    What is the reason for the less reactive nature of furan compared to pyrrole?

    <p>the positive charge is less readily accommodated by oxygen atom than by nitrogen atom</p> Signup and view all the answers

    Why is thiophene less reactive than furan towards electrophiles?

    <p>because the +M effect of sulfur is smaller than that of oxygen</p> Signup and view all the answers

    What is the order of aromaticity in five-membered heterocycles?

    <p>benzene &gt; thiophene &gt; furan &gt; pyrrole</p> Signup and view all the answers

    Why is nitrogen more effective in stabilizing the transition state than oxygen?

    <p>because the positive charge is more readily accommodated by nitrogen atom than by oxygen atom</p> Signup and view all the answers

    What is the reason for the greater reactivity of pyrrole towards electrophiles?

    <p>because of the greater electron releasing ability of trivalent nitrogen</p> Signup and view all the answers

    Why is thiophene less reactive than pyrrole towards electrophiles?

    <p>because the +M effect of sulfur is smaller than that of nitrogen</p> Signup and view all the answers

    What is the effect of electron-releasing substituents at C-2 position on the incoming electrophile?

    <p>It directs the electrophile to C-5 position</p> Signup and view all the answers

    What is the effect of strongly activating substituents at C-2 position?

    <p>Substitution may occur at C-3 position</p> Signup and view all the answers

    What is the effect of electron-withdrawing substituents at C-2 position?

    <p>It directs the electrophile to C-4 position</p> Signup and view all the answers

    What is the reason for the formation of 2,5-disubstituted furan?

    <p>The powerful directing effect of oxygen in furan</p> Signup and view all the answers

    What is the effect of weakly electron-releasing substituents at C-2 position?

    <p>Substitution may occur at C-5 position</p> Signup and view all the answers

    What is the competing effect in the presence of electron-withdrawing substituents at C-2 position?

    <p>The -directing effect of heteroatom</p> Signup and view all the answers

    What is similar to the effect of electron-releasing substituents at C-2 position?

    <p>The ortho-para directing effect in benzene</p> Signup and view all the answers

    What is the effect of electron-withdrawing substituents at C-2 position on the incoming electrophile?

    <p>It directs the electrophile to C-4 position</p> Signup and view all the answers

    What is the product of the oxidation of thiophene by peracids?

    <p>Both thiophene-1-oxide and 2-hydroxythiophene oxide</p> Signup and view all the answers

    What happens when thiophene is reduced under certain conditions?

    <p>It undergoes desulphurisation with ring opening</p> Signup and view all the answers

    What is the role of Raney Ni/Hy in the oxidation of thiophene?

    <p>It acts as a catalyst</p> Signup and view all the answers

    What is the outcome of the electrophilic addition of bromine in presence of methanol?

    <p>2,5-Dihydro-2,5-dimethoxyfuran is formed</p> Signup and view all the answers

    What is the limitation of reducing thiophene?

    <p>It is poisoned by sulphur</p> Signup and view all the answers

    What is the process involved in the formation of 2-Methyltetrahydrofuran?

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

    What is the reactant in the formation of 2,5-Dihydro-2,5-dimethoxyfuran?

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

    What is the catalyst used in the formation of 2,5-Dihydro-2,5-dimethoxyfuran?

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

    Why is benzene less reactive towards electrophiles than five-membered heterocycles?

    <p>Due to the greater resonance stabilization energy of benzene</p> Signup and view all the answers

    What is the order of aromaticity among benzene, thiophene, furan, and pyrrole?

    <p>Benzene &gt; thiophene &gt; furan &gt; pyrrole</p> Signup and view all the answers

    Why does furan tend to give addition products instead of substitution products?

    <p>Because of its low aromaticity</p> Signup and view all the answers

    What is the correct order of reactivity in electrophilic substitution?

    <p>Pyrrole &gt; furan &gt; thiophene &gt; benzene</p> Signup and view all the answers

    Why are the 2 & 5 positions in pyrrole more reactive than the 3 & 4 positions?

    <p>Because the intermediate from attack at C2 can be stabilized by three resonance structures</p> Signup and view all the answers

    What is the main reason for the difference in reactivity between benzene and five-membered heterocycles?

    <p>The difference in resonance stabilization energy</p> Signup and view all the answers

    What is the role of the transition state in determining the reactivity of benzene and five-membered heterocycles?

    <p>It influences the reactivity of both benzene and five-membered heterocycles</p> Signup and view all the answers

    Why is nitration with a mild nitrating agent possible only in the furan nucleus at C-2?

    <p>Because of the specific conditions of the reaction</p> Signup and view all the answers

    The acylation of furan results in exclusively 2-acylfuran with acid anhydrides in the presence of mild catalysts.

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

    Trifluoroacetic anhydride requires a catalyst for the acylation of furan.

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

    Friedel-Crafts acylation can be performed with Ac2O-SnCl3.

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

    Phosphoric acid is a strong catalyst for the acylation of furan.

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

    Boron trifluoride etherate is a catalyst used in the Friedel-Crafts acylation of furan.

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

    The acylation of furan can only be performed with trifluoroacetic anhydride.

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

    Furan can be acylated with acid anhydrides in the absence of a catalyst.

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

    The Friedel-Crafts acylation of furan results in a mixture of 2-acylfuran and 3-acylfuran.

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

    Electron-releasing substituents at C-2 position tend to direct incoming electrophile to C-3 position.

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

    The presence of strongly activating substituents (-OCH3, -NHCOCH3) at C-2 position causes substitution to occur at C-5.

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

    Electron-withdrawing substituents at C-2 position cause substitution to occur at C-3.

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

    The -directing effect of the heteroatom always dominates the directing effect of the substituent in furan.

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

    The formation of 2,5-disubstituted furan is rationalized by the most powerful directing effect of sulfur in furan.

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

    Weakly electron-releasing substituents (-CI, -CH3) at C-2 position cause substitution to occur at C-3.

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

    The effect of electron-releasing substituents at C-2 position is similar to the meta-directing effect in benzene.

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

    The directing effect of electron-withdrawing substituents at C-2 position is more powerful than that of the heteroatom in furan.

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

    Electron-releasing substituents at C-3 reinforce the meta-directing effect of ring heteroatom.

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

    Electron-withdrawing substituents at C-3 have a similar effect to the ortho-directing effect in benzene.

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

    The reactivity of five-membered heterocycles towards electrophiles depends only on the electron availability on the ring carbon atoms.

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

    Furan is more reactive towards electrophiles than thiophene.

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

    The presence of electron-releasing substituents at C-3 restricts the substitution at C-2.

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

    Electron-withdrawing substituents at C-3 direct the incoming electrophile to C-2.

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

    The reactivity of five-membered heterocycles towards electrophiles is independent of the electron availability on the ring carbon atoms.

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

    The combined effect of meta-directing effect of the substituent and the directing effect of the heteroatom causes substitution to occur at C-2.

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

    Pyrrole is less reactive towards electrophiles than furan.

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

    Thiophene is more reactive than furan towards electrophiles.

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

    Benzene is more reactive towards electrophiles than five-membered heterocycles.

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

    The order of aromaticity is pyrrole > thiophene > furan > benzene.

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

    Furan is more reactive towards electrophiles than pyrrole at C-2.

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

    The +M effect of sulfur is greater than that of oxygen.

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

    Nitrogen is less effective in stabilizing the transition state than oxygen.

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

    Thiophene is more aromatic than furan.

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

    Thiophene is more reactive than furan towards electrophiles.

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

    The Diels-Alder reaction of thiophene with maleic anhydride requires mild conditions.

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

    Furan can be nitrated with concentrated nitric acid.

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

    The order of reactivity in the Diels-Alder reaction is furan > pyrrole > thiophene.

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

    The intermediate 2,5-addition product can be isolated in the nitration of furan.

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

    Acetyl nitrate is a mild nitrating agent.

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

    Thiophene is more aromatic than pyrrole.

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

    Raney Ni/H is used in the reduction of thiophene.

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

    Study Notes

    Halogenation of Furan

    • Furan reacts vigorously with chlorine and bromine at room temperature to give polyhalogenated products.
    • Furan reacts with bromine in dimethylformamide at room temperature to produce 2-bromo-furan or 2,5-dibromo-furan.
    • Bromination of furan by bromine in dioxane-dibromide at -5°C gives 2-bromo-furan.

    Mechanism of Bromination

    • Bromination of furan involves an electrophilic substitution mechanism.

    Effect of Electron-Releasing Substituents at C-2

    • Electron-releasing substituents at C-2 position direct incoming electrophile to C-5 position.
    • Weakly electron-releasing substituents (-Cl, -CH3) at C-2 position tend to direct incoming electrophile to C-5 position.
    • Strongly activating substituents (-OCH3, -NHCOCH3) may lead to substitution at C-3 due to the ortho-effect of electron-releasing substituents.

    Effect of Electron-Withdrawing Substituents at C-2

    • Electron-withdrawing substituents at C-2 position cause substitution at C-4 due to directing influence of the substituent.
    • The directing effect of substituent competes with the -directing effect of heteroatom, causing substitution to occur at C-5.

    Reactivity Comparison with Benzene

    • The order of reactivity in five-membered heterocycles is: pyrrole > furan > thiophene > benzene.
    • The order of aromaticity is: benzene > thiophene > furan > pyrrole.
    • Furan is less reactive than pyrrole towards electrophiles due to oxygen being more electronegative than nitrogen.
    • Thiophene is less reactive than furan towards electrophiles due to sulfur being less electronegative than oxygen.

    Synthesis of Furan and Thiophene

    • Furan and thiophene can be synthesized from dicarbonyl compounds in the presence of a weak base.

    Addition Reactions

    • Furan undergoes electrophilic addition reactions, such as oxidation with bromine in methanol to produce 2,5-dihydro-2,5-dimethoxyfuran.

    Thiophene Oxidation and Reduction

    • Thiophene can be oxidized by peracids to thiophene-1-oxide and 2-hydroxythiophene oxide.
    • Thiophene cannot be reduced under normal conditions due to sulfur poisoning the catalyst, but partial reduction can occur with metals in acidic medium.

    Friedel-Crafts Acylation

    • Furan can be acylated with acid anhydrides in the presence of mild catalysts such as phosphoric acid or boron trifluoride etherate to form exclusively 2-acylfuran.
    • Acylation can also be performed with Ac2O-SnCl4, although trifluoroacetic anhydride does not require a catalyst.

    Effect of Electron-Releasing Substituents at C-2

    • Electron-releasing substituents at C-2 position direct the incoming electrophile to C-5 position.
    • Weakly electron-releasing substituents (-Cl, -CH3) tend to direct the electrophile to C-5 position.
    • Strongly activating substituents (-OCH3, -NHCOCH3) may cause substitution to occur at C-3 due to the ortho-effect of electron-releasing substituents.

    Effect of Electron-Withdrawing Substituents at C-2

    • Electron-withdrawing substituents at C-2 cause substitution at C-4 due to the directing influence of the substituent.
    • The directing effect of the substituent competes with the -directing effect of the heteroatom, which may cause substitution to occur at C-5.

    Effect of Electron-Releasing Substituents at C-3

    • Electron-releasing substituents at C-3 reinforce the -directing effect of the ring heteroatom with substitution at C-2.
    • If the electron-releasing substituent is alkyl, the reactivity of -positions (C-2 and C-5) does not differ appreciably.
    • Sterically hindered alkyl groups or incoming electrophiles may restrict substitution at C-2 and cause it to occur at C-5.

    Effect of Electron-Withdrawing Substituents at C-3

    • Electron-withdrawing substituents at C-3 have a similar effect to meta-directing substituents in benzene.
    • The combined effect of both the meta-directing effect of the substituent and the -directing effect of the heteroatom causes substitution to occur at C-5 most favourably.

    Furan and Thiophene

    • The reactivity of five-membered heterocycles towards electrophiles depends on the electron availability on the ring carbon atoms and the mesomerically electron release from the heteroatom.
    • The order of reactivity in five-membered heterocycles is: pyrrole > furan > thiophene > benzene.
    • The order of aromaticity is: benzene > thiophene > furan > pyrrole.

    Diels-Alder Reaction

    • The reaction of thiophene with maleic anhydride requires more drastic conditions than in the case of furan and pyrrole (high pressure and temperature).
    • The order of reactivity in Diels-Alder reaction is: furan > N-alkyl pyrrole > pyrrole > thiophene.

    Electrophilic Aromatic Substitution Reactions of Furan

    • Nitration of furan can be performed with mild nitrating agents, such as acetyl nitrate, at low temperature.
    • The reaction proceeds by an addition-elimination mechanism involving an intermediate, 2,5-addition product.
    • In certain cases, the intermediate 2,5-addition product may be isolated if a base (pyridine) is not used to eliminate acetic acid.

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    Description

    This quiz covers the halogenation reaction of furan with chlorine, bromine, and iodine, including the conditions required for monohalogenation.

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