Contents

Pyrazole

Pyrazole

Names
Preferred IUPAC name 1H-Pyrazole[1]
Systematic IUPAC name 1,2-Diazacyclopenta-2,4-diene
Other names 1,2-Diazole
Identifiers
CAS Number	288-13-1 Y
3D model (JSmol)	Interactive image
Beilstein Reference	103775
ChEBI	CHEBI:17241 Y
ChEMBL	ChEMBL15967 Y
ChemSpider	1019 Y
DrugBank	DB02757 Y
ECHA InfoCard	100.005.471
EC Number	206-017-1
Gmelin Reference	1360
KEGG	C00481 Y
PubChem CID	1048
UNII	3QD5KJZ7ZJ Y
CompTox Dashboard (EPA)	DTXSID2059774
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 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
Hazards
GHS labelling:[2]
Pictograms
Signal word	Danger
Hazard statements	H302, H311, H315, H318, H319, H335, H372, H412
Precautionary statements	P260, P261, P262, P264, P264+P265, P270, P271, P273, P280, P301+P317, P302+P352, P304+P340, P305+P351+P338, P305+P354+P338, P316, P317, P319, P321, P330, P332+P317, P337+P317, P361+P364, P362+P364, P403+P233, P405, P501
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 of azole group with the formulaC₃H₃N₂H. 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).^[3] Pyrazoles are also a class of compounds that have the ring C₃N₂ with adjacent nitrogen atoms.^[4] Notable drugs containing a pyrazole ring are celecoxib (celebrex) and the anabolic steroid stanozolol.

Preparation and reactions[edit]

Pyrazoles are synthesized by the reaction of α,β-unsaturated aldehydes with hydrazine and subsequent dehydrogenation:^[5]

Substituted pyrazoles are prepared by condensation of 1,3-diketones with hydrazine (Knorr-type reactions).^[6] For example, acetylacetone and hydrazine gives 3,5-dimethylpyrazole:^[7]

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

History[edit]

The term pyrazole was given to this class of compounds by German Chemist Ludwig Knorr in 1883.^[8] In a classical method developed by German chemist Hans von Pechmann in 1898, pyrazole was synthesized from acetylene and diazomethane.^[9]

Conversion to scorpionates[edit]

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[edit]

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^[10]orsodium persulfate,^[11] or by using a hydrazone in which case an azine is produced as a by-product.^[12]

Occurrence and uses[edit]

In 1959, the first natural pyrazole, 1-pyrazolyl-alanine, was isolated from seeds of watermelons.^[13][14]

In medicine, derivatives of pyrazole are widely used,^[15] including celecoxib and similar COX-2 inhibitors, zaleplon, betazole, and CDPPB.^[16]

The pyrazole ring is found within a variety of pesticides as fungicides, insecticides and herbicides,^[15] including fenpyroximate, fipronil, tebufenpyrad and tolfenpyrad.^[17] Pyrazole moieties are listed among the highly used ring systems for small molecule drugs by the US FDA^[18]

3-(Difluoromethyl)-1-methyl-1H-pyrazole-4-carboxylic acid is used in the manufacture of six commercial fungicides which are inhibitors of succinate dehydrogenase.^[19][20]

See also[edit]

• 3,5-dimethylpyrazole
• Pyrazolidine, fully saturated analogue
• imidazole, structural analogue of pyrazole with two non-adjacent nitrogen atoms.
• isoxazole, another analogue, the nitrogen atom in position 1 replaced by oxygen.

References[edit]

Further reading[edit]

A. Schmidt; A. Dreger (2011). "Recent Advances in the Chemistry of Pyrazoles. Part 2. Reactions and N-Heterocyclic Carbenes of Pyrazole". Curr. Org. Chem. 15 (16): 2897–2970. doi:10.2174/138527211796378497.