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Pyrazole
Full structural formula Skeletal formula with numbers
Ball-and-stick model Space-filling model
Identifiers
CAS number 288-13-1 YesY
PubChem 1048
ChemSpider 1019 YesY
DrugBank DB02757
KEGG C00481 YesY
ChEBI CHEBI:17241 YesY
ChEMBL CHEMBL15967 YesY
Jmol-3D images Image 1
Properties
Molecular formula C3H4N2
Molar mass 68.08 g mol−1
Melting point 66–70 °C
Boiling point 186–188 °C
Acidity (pKa) 14.0
Basicity (pKb) 2.5
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY (verify) (what is: YesY/N?)
Infobox references

Pyrazole is the organic compound with the formula C3H3N2H. It is a heterocycle characterized by a 5-membered ring of three carbon atoms and two adjacent nitrogen centres. Pyrazoles are also the class of compounds that have the ring C3N2 with adjacent nitrogen centres.[1] Notable drug that is a pyrazole is Celebrex.

Preparation and reactions[edit]

Pyrazoles are synthesized by the reaction of α,β-unsaturated aldehydes with hydrazine and subsequent dehydrogenation:[2]

Pyrazoles synthesis.png

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

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.[citation needed] In a classical method developed by German chemist Hans von Pechmann in 1898, pyrazole was synthesized from acetylene and diazomethane.[4]

Conversion to scorpionates[edit]

Pyrazoles react with potassium borohydride to form a class of ligands known as scorpionate. Pyrazole itself reacts with potassium borohydride to form a tridentate ligand known as Tp ligand:

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

Occurrence and uses[edit]

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[5][6]

Related heterocycles[edit]

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.

References[edit]

  1. ^ Eicher, T.; Hauptmann, S. (2003). The Chemistry of Heterocycles: Structure, Reactions, Syntheses, and Applications (2nd ed.). Wiley-VCH. ISBN 3-527-30720-6. 
  2. ^ Review article (properties, biological activities, syntheses of pyrazoles): A. Schmidt, A. Dreger (2011). "Recent Advances in the Chemistry of Pyrazoles. Properties, Biological Activities, and Syntheses". Curr. Org. Chem 15 (9): 1423–1463. doi:10.2174/138527211795378263. 
  3. ^ William S. Johnson and Robert J. Highet (1963), "3,5-Dimethylpyrazole", Org. Synth. ; Coll. Vol. 4: 351 
  4. ^ Pyrazol aus Acetylen und Diazomethan H. v. Pechmann Berichte der deutschen chemischen Gesellschaft Volume 31 Issue 3, Pages 2950–51, 1898 doi:10.1002/cber.18980310363
  5. ^ Fowden; Noe, Ridd and White, (1959). Proc. Chem. SOC.,: 131. 
  6. ^ Noe, F F; L Fowden (1959). "alpha-Amino-beta-(pyrazolyl-N) propionic acid: a new amino-acid from Citrullus vulgaris (water melon)". Nature 184: 69–70. ISSN 0028-0836. 

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