Acylation of furan, thiophene and pyrrole: practice and mechanism
Furan, thiophene and pyrrole are very electron-rich and are acylated even with mild Lewis acids, at C2 next to the heteroatom (at C5 if C2 is taken).
Example
Five-membered heteroaromatics are far more electron-rich than benzene and react even with mild Lewis acids. Attack is at C2 (next to the heteroatom) because the σ-complex there is stabilised by three resonance forms (only two at C3). If C2 is taken, C5 reacts.
Mechanism step by step
- 1
BF₃ activates acetic anhydride to an acylium-like electrophile.
- 2
The ring attacks with C2. In the σ-complex the positive charge is spread over three atoms, including the heteroatom.
- 3
Loss of H⁺ from C2 restores aromaticity: 2-acylfuran, -thiophene or -pyrrole.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Attack at C3 is less favourable: the σ-complex there has only two resonance forms, at C2 three. Five-membered heteroaromatics therefore react at C2 (or C5).
- The pyrrole N–H is weakly acidic, but under these conditions the electron-rich ring reacts at carbon (C2), not at nitrogen.
- The acyl group deactivates the ring, so no second one is added. The monoacyl product forms.
Often confused with: Vilsmeier–Haack formylation · Friedel–Crafts acylation
Frequently asked questions
How reactive are the five-membered heteroaromatics?
Pyrrole > furan > thiophene > benzene. The heteroatom pushes its lone pair into the ring; N donates most easily, S least (poorer orbital overlap). Pyrrole polymerises with strong acids, hence mild conditions.
Why not AlCl₃ as with benzene?
AlCl₃ is too strong: furan and pyrrole decompose or polymerise with it. The rings are so reactive that milder Lewis acids suffice.