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 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

Post a Comment