Pyrazole

Pyrazole
Names
	Preferred IUPAC name 1H-Pyrazole
	Systematic IUPAC name 1,2-Diazacyclopenta-2,4-diene
	Other names 1,2-Diazole
Identifiers
CAS Number	288-13-1 ;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:17241 ;
ChEMBL	ChEMBL15967 ;
ChemSpider	1019 ;
DrugBank	DB02757 ;
ECHA InfoCard	100.005.471
KEGG	C00481 ;
PubChem CID	1048;
UNII	3QD5KJZ7ZJ ;
CompTox Dashboard (EPA)	DTXSID2059774 ;
	InChI InChI=1S/C3H4N2/c1-2-4-5-3-1/h1-3H,(H,4,5) Key: WTKZEGDFNFYCGP-UHFFFAOYSA-N ; InChI=1/C3H4N2/c1-2-4-5-3-1/h1-3H,(H,4,5) Key: WTKZEGDFNFYCGP-UHFFFAOYAW ;
	SMILES c1cn[nH]c1;
Properties
Chemical formula	C3H4N2
Molar mass	68.079 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
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

Pyrazole is an organic compound of azole group with the formula C₃H₃N₂H. It is a heterocycle characterized by a 5-membered ring of three carbon atoms and two adjacent nitrogen atoms, which are in ortho-substitution. Pyrazole is a weak base, with pK_b 11.5 (pK_a of the conjugate acid 2.49 at 25 °C). Pyrazoles are also a class of compounds that have the ring C₃N₂ with adjacent nitrogen atoms. Notable drugs containing a pyrazole ring are celecoxib (celebrex) and the anabolic steroid stanozolol.

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 (Knorr-type reactions). 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:

3,5-Diphenyl-1H-pyrazole

3,5-Diphenyl-1H-pyrazole is produced when (E)-1,3-diphenylprop-2-en-1-one is reacted with hydrazine hydrate in the presence of elemental sulfur or sodium persulfate, or by using a hydrazone in which case an azine is produced as a by-product.

Occurrence and uses

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

In medicine, derivatives of pyrazole are widely used, including celecoxib and similar COX-2 inhibitors, zaleplon, betazole, and CDPPB.

The pyrazole ring is found within a variety of pesticides as fungicides, insecticides and herbicides, including fenpyroximate, fipronil, tebufenpyrad and tolfenpyrad. Pyrazole moieties are listed among the highly used ring systems for small molecule drugs by the US FDA

3-(Difluoromethyl)-1-methyl-1H-pyrazole-4-carboxylic acid is used in the manufacture of six commercial fungicides which are inhibitors of succinate dehydrogenase.