Pyrazole

Pyrazole

Names
IUPAC name Pyrazole
Other names 1,2-Diazole
Identifiers
CAS Number	288-13-1 Y
ChEBI	CHEBI:17241 Y
ChEMBL	ChEMBL15967 Y
ChemSpider	1019 Y
DrugBank	DB02757 Y
Jmol 3D model	Interactive image
KEGG	C00481 Y
PubChem	1048
InChI InChI=1S/C3H4N2/c1-2-4-5-3-1/h1-3H,(H,4,5) Y Key: WTKZEGDFNFYCGP-UHFFFAOYSA-N Y InChI=1/C3H4N2/c1-2-4-5-3-1/h1-3H,(H,4,5) Key: WTKZEGDFNFYCGP-UHFFFAOYAW
SMILES n1cccn1
Properties
Chemical formula	C3H4N2
Molar mass	68.08 g·mol−1
Melting point	66 to 70 °C (151 to 158 °F; 339 to 343 K)
Boiling point	186 to 188 °C (367 to 370 °F; 459 to 461 K)
Basicity (pKb)	11.5
Vapor pressure	{{{value}}}
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y verify (what is YN ?)
Infobox references

Pyrazole is an organic compound with the formula C₃H₃N₂H. It is a heterocycle characterized by a 5-membered ring of three carbon atoms and two adjacent nitrogen atoms. Pyrazole is a weak base, with pK_b 11.5 (pK_a of the conjugated acid 2.49 at 25 °C). Pyrazoles are also a class of compounds that have the ring C₃N₂ with adjacent nitrogen atoms. A notable drug containing a pyrazole ring is celecoxib (Celebrex).

Preparation and reactions

Pyrazoles are synthesized by the reaction of α,β-unsaturated aldehydes with hydrazine and subsequent dehydrogenation:

Substituted pyrazoles are prepared by condensation of 1,3-diketones with hydrazine. For example, acetylacetone and hydrazine gives 3,5-dimethylpyrazole:

CH₃C(O)CH₂C(O)CH₃ + N₂H₄ → (CH₃)₂C₃HN₂H + 2 H₂O

History

The term pyrazole was given to this class of compounds by German Chemist Ludwig Knorr in 1883. In a classical method developed by German chemist Hans von Pechmann in 1898, pyrazole was synthesized from acetylene and diazomethane.

Conversion to scorpionates

Pyrazoles react with potassium borohydride to form a class of ligands known as scorpionate. Pyrazole itself reacts with potassium borohydride at high temperatures (~200 °C) to form a tridentate ligand known as Tp ligand:

KBH₄ + 3 C₃H₃N₂H → KBH(C₃H₃N₂)₃ + 3 H₂

Occurrence and uses

Celecoxib, a pyrazole derivative used as an analgesic

In medicine, derivatives of pyrazoles are used for their analgesic, anti-inflammatory, antipyretic, antiarrhythmic, tranquilizing, muscle relaxing, psychoanaleptic, anticonvulsant, monoamineoxidase inhibiting, antidiabetic and antibacterial activities.

In 1959, the first natural pyrazole, 1-pyrazolyl-alanine, was isolated from seeds of watermelons

Related heterocycles

Imidazole is an analog of pyrazole with two non-adjacent nitrogen atoms. In isoxazole, another analog, the nitrogen atom in position 1 replaced by oxygen.