Reactions of Pyrazole

Reactions of Pyrazole mainly include electrophilic substitution at the 4-position and oxidation or reduction pathways.

Reactions of Pyrazole

1. Tautomerism:

   • Pyrazole exhibits prototropic tautomerism between N-1 and N-2.
     • 1H-pyrazole    ⇌    2H-pyrazole
   • This affects reactivity at the nitrogen atoms and the electron density distribution on the ring.

2. Electrophilic Substitution Reactions (EAS):

   • Due to the electron-rich ring, EAS is favored at position-4 (C-4).
     1. Nitration:
        • Reagents: HNO₃ + H₂SO₄
        • Major product: 4-nitropyrazole
     2. Halogenation:
        • Br₂/FeBr₃ → 4-bromopyrazole
     3. Sulfonation:
        • Rare, only under harsh conditions.

3. Nucleophilic Substitution Reactions (NAS):

   • Less common due to electron-rich ring.
   • Possible if electron-withdrawing groups (e.g., NO₂) are at C-3 or C-5.

4. Metal Complex Formation:

   • Pyrazole can coordinate metals through N-2.
   • Often used in ligands like tris(pyrazolyl)borates for transition metal complexes.

5. Ring Functionalization:

   • Lithiation at C-4 or C-5 followed by electrophile quench (e.g., CO₂, RCHO).
   • C-alkylation possible via deprotonation with strong base (LDA).

6. Oxidation & Reduction:

   • Not easily reduced due to aromaticity.
   • Oxidation at N possible under strong conditions → N-oxides.

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