Pyrrole

Pyrrole

Names
Preferred IUPAC name 1H-Pyrrole
Other names Azole Imidole
Properties
Chemical formula	C4H5N
Molar mass	67.09 g·mol−1
Acidity (pKa)	17.5
Basicity (pKb)	13.6
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Pyrrole is an organic compound. It is a cyclic compound with four carbon atoms and a nitrogen atom, each bonded to one hydrogen atom. The chemical formula of pyrrole is C₄H₅N. Chemicals related to pyrrole, replacing one or more hydrogen atoms with other atoms or functional groups, are called pyrroles.

It is similar in structure to furan, but with the ether functional group replaced by amine. Like furan, it is an aromatic compound.

Preparation

Early ideas for making pyrrole included pyrolysis of biological materials like leather scraps and bone tar, and a reaction between acetylene and ammonia. Pyrolysis of ammonium mucate with glycerol gives pyrrole in about 40% yield. It was then discovered that alumina and silica could be catalysts for a reaction between furan and ammonia:

This reaction is the main way that pyrrole is made in the chemical industry today.

Preparation of pyrrole derivatives

Template:More sources section Derivative (chemistry)s of pyrrole where hydrogen has been replaced by other functional groups are made by more complicated organic synthesis reactions. Most of these reactions start with nucleophilic attack by a nitrogen atom on a carbonyl compound.

Paal–Knorr synthesis

The Paal–Knorr synthesis makes pyrrole derivatives from 1,4-dicarbonyl compounds and ammonia (or primary amines). The reaction makes pyrroles if done with a weak acid, but a strong acid will make ammonium salts and furans instead.

This reaction is an efficient and easy way to make pyrroles, but it relies on an acid catalyst. This can make it a bad choice if the reagents have other functional groups that react with acids.

Knorr synthesis

The Knorr pyrrole synthesis uses an oxime and a ketone with some other electron-withdrawing group like an ester. The oxime is converted to an enamine by Neber rearrangement, and the nitrogen atom then attacks the ketone to make the ring. Water is the main byproduct.

Hantzsch synthesis

The Hantzsch pyrrole synthesis uses ammonia, a 1,3-ketoester, and a 1,2-haloketone. This reaction makes the pyrrole-2-carboxylic acid ester with the same alcohol parent as the ketoester. The ammonium salt that matches the haloketone (usually ammonium chloride) and water are the byproducts.

Barton–Zard synthesis

The Barton–Zard synthesis makes carbonylated pyrroles from nitroalkenes and carbonyl isocyanides.

The Barton–Zard synthesis notably does not involve a nucleophilic nitrogen atom. Instead, it uses a basic catalyst, changing the isocyanide's carbonyl into an enolate that acts as a Michael donor.