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 pKb 11.5 (pKa 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.
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Names | |||
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Preferred IUPAC name
1H-Pyrazole[1] | |||
Systematic IUPAC name
1,2-Diazacyclopenta-2,4-diene | |||
Other names
1,2-Diazole
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Identifiers | |||
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3D model (JSmol)
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103775 | |||
ChEBI |
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ChEMBL |
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ChemSpider |
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DrugBank |
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ECHA InfoCard | 100.005.471 | ||
EC Number |
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1360 | |||
KEGG |
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PubChem CID
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UNII |
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CompTox Dashboard (EPA)
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Properties | |||
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] | |||
Danger | |||
H302, H311, H315, H318, H319, H335, H372, H412 | |||
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).
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Infobox references
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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]
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]
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 is produced when (E)-1,3-diphenylprop-2-en-1-one is reacted with hydrazine hydrate in the presence of elemental sulfur[10] or sodium persulfate,[11] or by using a hydrazone in which case an azine is produced as a by-product.[12]
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]
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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.