Problems and exercisesFive-membered aromatic heterocycles: pyrrole, furan and thiophene

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Exercise 1

Why is pyrrole (pK_aH = −3.8) a much weaker base than pyrrolidine (pK_aH = 11.3) and pyridine (5.2)? Where is it protonated and what happens to it in concentrated acid?

Exercise 2

Give the main product: a) furan with acetic anhydride and BF₃·OEt₂; b) thiophene with n-BuLi and then CO₂ and H₃O⁺; c) 1-methylpyrrole with DMF and POCl₃, then water. Why is AlCl₃ not used in a)?

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