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ChemistryMediumClass 12
Hell-Volhard-Zelinsky (HVZ) Reaction
Aldehydes, Ketones & Carboxylic Acids
17
JEE Qs
8%
Hard
40
min
Always check for the presence of alpha-hydrogens in the carboxylic acid before applying HVZ reaction, as it dictates the feasibility and the specific site of halogenation.
🧮 Key Formulas
R-CH2-COOH + X2 (P/PX3) --(H2O)--> R-CH(X)-COOH + HX
Where X = Cl or Br
P + 3/2 X2 -> PX3 (in situ)
✅ Key Points for JEE
- 1The HVZ reaction is specific for carboxylic acids having at least one alpha-hydrogen.
- 2Halogenation occurs exclusively at the alpha-carbon, replacing an alpha-hydrogen with a halogen (Cl or Br).
- 3Red phosphorus (P) reacts with the halogen (X2) in situ to form PX3, which converts the carboxylic acid into a more reactive acyl halide (RCOX).
- 4The reaction proceeds via the enol form of the acyl halide, which is highly reactive towards electrophilic halogenation.
- 5The final product is an alpha-halo carboxylic acid, which serves as a versatile intermediate for synthesizing alpha-hydroxy acids (via hydrolysis) or alpha-amino acids (via ammonolysis).
⚠️ Common Mistakes
- ✕Attempting the reaction on carboxylic acids that do not possess any alpha-hydrogens (e.g., benzoic acid, pivalic acid).
- ✕Incorrectly identifying the site of halogenation; the halogen replaces a hydrogen *only* on the alpha-carbon, not on any other carbon or an aromatic ring.
- ✕Forgetting the critical role of red phosphorus (P) or PX3 in converting the carboxylic acid to its acyl halide, which is essential for enolization and subsequent halogenation.
NCERT Chapters
- Class 12 Chemistry Ch 12: Aldehydes, Ketones and Carboxylic Acids