Pyrrole

Heterocyclic Compounds
Heterocyclic Compounds
Those cyclic compounds in which one or more of the ring carbons are
replaced by another atoms (referred as heteroatoms).
The most common heteroatoms are Nitrogen, Oxygen and Sulphur.
But,
Other atoms such as Boron, Phosphorous, or Silicon can also be
members of heterocyclic rings.
A variety of heterocyclic compounds of different ring sizes are known,
the most important ones are made of five and six-membered rings.
Five-membered Rings
Six-membered Rings
Notice that the rings containing nitrogen usually end with -ole if five-
membered and with -ine if six-membered. The hetero atom is always
numbered as 1 (isoquinoline is an exception
The carbon atoms next to the hetero atom are sometimes referred to
as the α-carbon atoms and those further away as β- and ϒ-carbon
atoms
Pyrrole
It is an important five-membered heterocyclic compound because many
naturally occurring substances contain the pyrrole ring e.g., chlorophyll,
hemoglobin and some of the alkaloids.
Occurrence
Pyrrole occurs in coal-tar and in bone oil (Dippel's oil). The latter is
obtained by the dry distillation, or pyrolysis, of animal by-products such as
horns, hooves, and bones.
It may be isolated from bone oil by first washing it with dilute sulphuric
acid to remove the basic substances, and then with dilute alkali to
remove the acidic substances. It is next subjected to fractional
distillation. The fraction passing over between 100°C to 150°C contains
pyrrole, which may be removed by boiling with potassium hydroxide. The
potassium salt is formed which on steam distillation gives pyrrole. This is
finally purified by distillation.
Preparation Methods
1. By passing a mixture of acetylene and ammonia through a red-hot tube
2. By heating ammonium mucate with glycerol at 200 degrees. At
this temperature, ammonium mucate is dissociated into mucic acid
and ammonia. The acid then undergoes dehydration,
decarboxylation and ring-closure by reaction with ammonia.
3. By heating Succinimide with zinc dust.
4. By warming succinic dialdehyde with ammonia
5. Commercial Method: By passing a mixture of furan, ammonia,
and steam over aluminium oxide catalyst at 480-490°C.
Structure of Pyrrole
Physical Properties of Pyrrole
• Pyrrole is colorless liquid,
• Boiling point 131°C, which rapidly turns brown on exposure to
air.
• Its odour is like that of chIoroform.
• Pyrrole is sparingly soluble in water but dissolves in ethanol
and ether.
Chemical Properties of Pyrrole
1. Basic Character:
Pyrrole reacts with dilute hydrochloric acid to give a crystalline
hydrochloride.
2. Acidic Character
Pyrrole is not only a weak base but also a very weak acid. This is
shown by its reactions with potassium hydroxide and Grignard
reagents.
3. Electrophilic Substitution Reactions
Pyrrole undergoes electrophilic substitution reactions at C-2
because three resonance forms can be written for the
intermediate obtained from attack at C-2, whereas only two
such forms are possible for substitution at C-3.
Consequently the C-2 intermediate is more stable and the

product with a substituent at C-2 predominates. Substitution at
C-3 occurs only when both the 2-positions (that is, α and α') are
blocked.
a. Nitration
Pyrrole can be nitrated by a cold solution of nitric acid in acetic anhydride
to give 2-nitropyrrole.
b. Halogenation
c. Sulphonation
Pyrrole may be sulphonated with sulphur trioxide in pyridine at about
100°C to yield 2-pyrrolesulfonic acid.
d. Friedel-Craft Acylation
Pyrrole may be acetylated with acetic anhydride at 250°C to give
2-acetylpyrrole. Notice that no catalyst is required in this reaction.
4. Oxidation
Pyrrole is oxidized by chromium trioxide in acetic acid to give the imide of
maleic acid.
5. Reduction
Mild reduction of pyrrole with zinc and acetic acid yields 3-pyrroline (2,5-
dihydropyrrole). Catalytic reduction completely hydrogenates the ring
system and produces pyrrolidine.
6. Ring Expansion Reaction
When treated with sodium methoxide and Methylene iodide, pyrrole
undergoes ring expansion forming pyridine.
7. Ring Opening Reaction
When treated with hot ethanolic hydroxylamine, pyrrole undergoes ring
opening forming the dioxime of succindialdehyde
7. Kolbe-Schmitt Carboxylation
Pyrrole reacts with aqueous potassium carbonate at 100°C to give pyrrole-
2-carboxylic acid.
8. Reimer-Tiemann Formylation
Pyrrole reacts with chloroform in the presence of alkali to yield pyrrole-2-
aldehyde (2-formylpyrrole) and 3-chloropyridine.
9. Diazo Coupling
Pyrrole couples with benzenediazonium chloride in a weakly acidic solution
to give 2-phenylazopyrrole.
Medicinal Importance

Pyrrole

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Heterocyclic Compounds

Consequently the C-2 intermediate is more stable and the

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