Chemistry, Reactivity, Properties and Synthesis of Pyrazole

Chemistry of Pyrazole

Session Objectives

By the end of this
session, students will be able to:

• Discuss the chemistry, reactivity, properties and method
of synthesis of pyrazole

Pyrazole

Pyrazole
is planar and structural isomer of imidazole

Pyrazole
is much weaker base than imidazole

Pyrazole
was first discovered by Buchner in 1889 during the decomposition of pyrazole
3,4,5-tricarboxylic acid

Benzo
derivative of pyrazole is benzopyrazole.

Pyrazole
is used as oxidants in fuels.

Orisul is
an example of pyrazole sulphonamide.

Dihydropyrazoles
are called pyrazolines

Pyrazolines
are less stable than corresponding pyrazoles but can be converted into the
latter by using mild

Oxidising
agents such as lead tetra acetate.

Properties of pyrazole

Colourless needles

MP 700C

BP 1880c

Soluble in water

Exists in the solid state and in concentrated
solution as a dimer with two intermolecular hydrogen bonds.

It
possesses a penetrating pleasant smell unlike most amines.

Introduction
of alkyl groups in the ring causes an increase in the boiling point.

Pyrazoles
which are unsubstituted at position 1 exhibit tautomerism.

The two
tautomeric forms are identical
entities and are in rapid equilibrium with each
other.

Two
nitrogen atoms are indistinguishable.

Synthesis of Pyrazole

1. Hydrazine
and alkyl- or arylhydrazines undergo cyclocondensation with 1,3-dicarbonyl
compounds to give pyrazoles 4.

2. The 1,3-dipolar cycloaddition of diazoalkanes to alkynes
leads to pyrazoles, e.g.:

Electrophilic substitution of
Pyrazoles:

Pyrazoles
are subjected to electrophilic substituion and the attack takes place at
position 4.

Chlorination
of SO2Cl2 of pyrazoles yields 4-chloropyrazoles.

Bromination
(Br2 / dioxane) occurs at position 4.

Nitration
occurs at 4 position but under more severe conditions.

Nucleophilic substitution of
Pyrazoles:

Direct
amination of pyrazoles with sodamide is not observed. It causes ring opening.

Metallation
of n-butyl lithium takes place in position5.

Methylation of Pyrazole:

The best procedure for methylation of pyrazole is
via the sodium salt which reacts with idomethane or dimethyl sulphate.

Formation
of metal complexes

• Many metal complexes with pyrazole as ligand have been
prepared with pyridine like N-atom acting as donor, for instance in
dichlorotetrapyrazole nickel (II).

• In another series of metal complexes, the pyrazole anion
is the ligand, e.g.: gold (I) Complex

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