78 Questions
What is the reaction outcome of furan with iodine at room temperature?
No reaction occurs
What is required for the formation of monobromo- and monochloro-furans?
Milder conditions
What is the product of bromination of furan by bromine in dimethylformamide at room temperature?
2-Bromo- or 2,5-dibromo-furans
What is the role of dioxane in the bromination of furan?
It forms a complex with bromine
What is the mechanism involved in the bromination of furan substituted with an electron-withdrawing group at the position-2?
Electrophilic substitution
What is the product of bromination of furan substituted with an electron-withdrawing group at the position-2?
5-Bromo derivative
What is the order of reactivity in five-membered heterocycles?
pyrrole > furan > thiophene > benzene
Why is pyrrole more reactive towards electrophiles than furan?
because nitrogen has a greater electron releasing ability than oxygen
What is the reason for the less reactive nature of furan compared to pyrrole?
the positive charge is less readily accommodated by oxygen atom than by nitrogen atom
Why is thiophene less reactive than furan towards electrophiles?
because the +M effect of sulfur is smaller than that of oxygen
What is the order of aromaticity in five-membered heterocycles?
benzene > thiophene > furan > pyrrole
Why is nitrogen more effective in stabilizing the transition state than oxygen?
because the positive charge is more readily accommodated by nitrogen atom than by oxygen atom
What is the reason for the greater reactivity of pyrrole towards electrophiles?
because of the greater electron releasing ability of trivalent nitrogen
Why is thiophene less reactive than pyrrole towards electrophiles?
because the +M effect of sulfur is smaller than that of nitrogen
What is the effect of electron-releasing substituents at C-2 position on the incoming electrophile?
It directs the electrophile to C-5 position
What is the effect of strongly activating substituents at C-2 position?
Substitution may occur at C-3 position
What is the effect of electron-withdrawing substituents at C-2 position?
It directs the electrophile to C-4 position
What is the reason for the formation of 2,5-disubstituted furan?
The powerful directing effect of oxygen in furan
What is the effect of weakly electron-releasing substituents at C-2 position?
Substitution may occur at C-5 position
What is the competing effect in the presence of electron-withdrawing substituents at C-2 position?
The -directing effect of heteroatom
What is similar to the effect of electron-releasing substituents at C-2 position?
The ortho-para directing effect in benzene
What is the effect of electron-withdrawing substituents at C-2 position on the incoming electrophile?
It directs the electrophile to C-4 position
What is the product of the oxidation of thiophene by peracids?
Both thiophene-1-oxide and 2-hydroxythiophene oxide
What happens when thiophene is reduced under certain conditions?
It undergoes desulphurisation with ring opening
What is the role of Raney Ni/Hy in the oxidation of thiophene?
It acts as a catalyst
What is the outcome of the electrophilic addition of bromine in presence of methanol?
2,5-Dihydro-2,5-dimethoxyfuran is formed
What is the limitation of reducing thiophene?
It is poisoned by sulphur
What is the process involved in the formation of 2-Methyltetrahydrofuran?
Reduction
What is the reactant in the formation of 2,5-Dihydro-2,5-dimethoxyfuran?
Furan
What is the catalyst used in the formation of 2,5-Dihydro-2,5-dimethoxyfuran?
HCl
Why is benzene less reactive towards electrophiles than five-membered heterocycles?
Due to the greater resonance stabilization energy of benzene
What is the order of aromaticity among benzene, thiophene, furan, and pyrrole?
Benzene > thiophene > furan > pyrrole
Why does furan tend to give addition products instead of substitution products?
Because of its low aromaticity
What is the correct order of reactivity in electrophilic substitution?
Pyrrole > furan > thiophene > benzene
Why are the 2 & 5 positions in pyrrole more reactive than the 3 & 4 positions?
Because the intermediate from attack at C2 can be stabilized by three resonance structures
What is the main reason for the difference in reactivity between benzene and five-membered heterocycles?
The difference in resonance stabilization energy
What is the role of the transition state in determining the reactivity of benzene and five-membered heterocycles?
It influences the reactivity of both benzene and five-membered heterocycles
Why is nitration with a mild nitrating agent possible only in the furan nucleus at C-2?
Because of the specific conditions of the reaction
The acylation of furan results in exclusively 2-acylfuran with acid anhydrides in the presence of mild catalysts.
True
Trifluoroacetic anhydride requires a catalyst for the acylation of furan.
False
Friedel-Crafts acylation can be performed with Ac2O-SnCl3.
True
Phosphoric acid is a strong catalyst for the acylation of furan.
False
Boron trifluoride etherate is a catalyst used in the Friedel-Crafts acylation of furan.
True
The acylation of furan can only be performed with trifluoroacetic anhydride.
False
Furan can be acylated with acid anhydrides in the absence of a catalyst.
False
The Friedel-Crafts acylation of furan results in a mixture of 2-acylfuran and 3-acylfuran.
False
Electron-releasing substituents at C-2 position tend to direct incoming electrophile to C-3 position.
False
The presence of strongly activating substituents (-OCH3, -NHCOCH3) at C-2 position causes substitution to occur at C-5.
False
Electron-withdrawing substituents at C-2 position cause substitution to occur at C-3.
False
The -directing effect of the heteroatom always dominates the directing effect of the substituent in furan.
False
The formation of 2,5-disubstituted furan is rationalized by the most powerful directing effect of sulfur in furan.
False
Weakly electron-releasing substituents (-CI, -CH3) at C-2 position cause substitution to occur at C-3.
False
The effect of electron-releasing substituents at C-2 position is similar to the meta-directing effect in benzene.
False
The directing effect of electron-withdrawing substituents at C-2 position is more powerful than that of the heteroatom in furan.
False
Electron-releasing substituents at C-3 reinforce the meta-directing effect of ring heteroatom.
False
Electron-withdrawing substituents at C-3 have a similar effect to the ortho-directing effect in benzene.
False
The reactivity of five-membered heterocycles towards electrophiles depends only on the electron availability on the ring carbon atoms.
False
Furan is more reactive towards electrophiles than thiophene.
False
The presence of electron-releasing substituents at C-3 restricts the substitution at C-2.
False
Electron-withdrawing substituents at C-3 direct the incoming electrophile to C-2.
False
The reactivity of five-membered heterocycles towards electrophiles is independent of the electron availability on the ring carbon atoms.
False
The combined effect of meta-directing effect of the substituent and the directing effect of the heteroatom causes substitution to occur at C-2.
False
Pyrrole is less reactive towards electrophiles than furan.
False
Thiophene is more reactive than furan towards electrophiles.
False
Benzene is more reactive towards electrophiles than five-membered heterocycles.
False
The order of aromaticity is pyrrole > thiophene > furan > benzene.
False
Furan is more reactive towards electrophiles than pyrrole at C-2.
False
The +M effect of sulfur is greater than that of oxygen.
False
Nitrogen is less effective in stabilizing the transition state than oxygen.
False
Thiophene is more aromatic than furan.
False
Thiophene is more reactive than furan towards electrophiles.
False
The Diels-Alder reaction of thiophene with maleic anhydride requires mild conditions.
False
Furan can be nitrated with concentrated nitric acid.
False
The order of reactivity in the Diels-Alder reaction is furan > pyrrole > thiophene.
False
The intermediate 2,5-addition product can be isolated in the nitration of furan.
True
Acetyl nitrate is a mild nitrating agent.
True
Thiophene is more aromatic than pyrrole.
True
Raney Ni/H is used in the reduction of thiophene.
True
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.
This quiz covers the halogenation reaction of furan with chlorine, bromine, and iodine, including the conditions required for monohalogenation.
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