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Reactions of Pyrrole, Furan, and Thiophene quiz

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  • What type of reaction do pyrrole, furan, and thiophene undergo similarly to benzene?
    They undergo electrophilic aromatic substitution (EAS) reactions.
  • Which atoms serve as the heteroatoms in pyrrole, furan, and thiophene?
    Pyrrole contains nitrogen, furan contains oxygen, and thiophene contains sulfur.
  • Why do pyrrole, furan, and thiophene undergo EAS more readily than benzene?
    Their heteroatoms stabilize the intermediate, making them more reactive than benzene.
  • At which carbon does EAS preferentially occur in pyrrole, furan, and thiophene?
    EAS preferentially occurs at carbon 2 (the alpha position) in these compounds.
  • What is the role of the pi bond in the EAS reaction of these heterocycles?
    The pi bond attacks the electrophile, forming a new bond and generating a carbocation intermediate.
  • How is aromaticity restored after the electrophile adds to the ring?
    A hydrogen is lost from the carbon where the electrophile added, restoring aromaticity.
  • What is the order of reactivity for EAS among pyrrole, furan, and thiophene?
    Pyrrole is more reactive than furan, which is more reactive than thiophene.
  • Why are milder conditions required for EAS in pyrrole, furan, and thiophene compared to benzene?
    Their increased reactivity due to heteroatom stabilization allows EAS to occur under milder conditions.
  • What is the major product of EAS in these heterocycles: substitution at carbon 2 or carbon 3?
    The major product results from substitution at carbon 2.
  • What is the minor product of EAS in pyrrole, furan, and thiophene?
    The minor product results from substitution at carbon 3.
  • How does resonance stabilization differ between substitution at carbon 2 and carbon 3?
    Substitution at carbon 2 allows more resonance structures that better stabilize the positive charge.
  • What happens to the heteroatom during resonance after electrophile addition?
    The heteroatom can become positively charged in one of the resonance forms.
  • Why is substitution at carbon 2 favored over carbon 3 in EAS?
    Because resonance structures for carbon 2 substitution place the positive charge on less electronegative atoms, stabilizing the intermediate.
  • What must happen to the hydrogen on the carbon where the electrophile added to restore aromaticity?
    The hydrogen must be lost, with its electrons helping to re-form the aromatic pi system.
  • What is the general mechanism type for EAS in these heterocycles?
    It is an addition-substitution mechanism: electrophile addition followed by hydrogen loss.