Paal–Knorr synthesis (pyrrole): practice and mechanism
With NH₃ a 1,4-dicarbonyl compound becomes a pyrrole: the N links the two former carbonyl carbons, water is lost twice and the five-membered ring is aromatic.
Example
Ammonia adds to both carbonyl groups of a 1,4-dicarbonyl compound. After losing water twice a five-membered ring with N closes and becomes aromatic: a pyrrole. The two carbonyl carbons become C2 and C5 of the ring.
Mechanism step by step
- 1
NH₃ adds to one carbonyl group, giving a hemiaminal.
- 2
The nitrogen attacks the second carbonyl group: a five-membered ring with two OH groups closes.
- 3
Water is eliminated twice; the double bonds and the N lone pair form the aromatic pyrrole.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- That would be the saturated pyrrolidine. Without a reducing agent the double bonds remain: after loss of water the aromatic pyrrole ring forms.
Often confused with: Paal–Knorr synthesis (furan) · Paal–Knorr synthesis (thiophene) · Intramolecular aldol condensation
Frequently asked questions
How do I spot a Paal–Knorr synthesis in reverse?
A 2,5-disubstituted five-membered heteroaromatic (pyrrole, furan, thiophene): "cut" between the heteroatom and both α-carbons and put a C=O at C2 and C5. That gives the 1,4-dicarbonyl compound.
Can N-substituted pyrroles be made too?
Yes, with a primary amine instead of NH₃: R–NH₂ gives the N-R pyrrole.