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
