The Intuition: Why Carboxylic Acids Are Hard to Halogenate
Imagine you want to attach a halogen atom (like bromine) to a carbon chain. With an alkane, you can just use Br₂ and light — the reaction is straightforward. But with a carboxylic acid, things get tricky. The carboxyl group (−COOH) is strongly electron-withdrawing. It pulls electron density away from the carbon next to it (the α-carbon), making that C–H bond less reactive toward radical halogenation. Direct bromination of a carboxylic acid with Br₂ alone is painfully slow and gives messy mixtures.
So how do chemists get around this? They use a clever trick: convert the acid into a more reactive intermediate first.
The HVZ Reaction: What It Does
The Hell-Volhard-Zelinsky (HVZ) reaction is a method to introduce a halogen atom specifically at the α-position of a carboxylic acid. The reagents are:
A carboxylic acid (with at least one α-hydrogen)
Cl₂ or Br₂ (chlorine or bromine)
A catalytic amount of red phosphorus (P4)
The product is an α-halo carboxylic acid.
R−CH2−COOHBr2red PR−CHBr−COOH+HBr
The reaction works for any carboxylic acid that has at least one hydrogen on the carbon adjacent to the carboxyl group. If the α-carbon is fully substituted (no α-H), the reaction does not occur.
The Mechanism: Step by Step
The magic happens in three stages. Let's trace it with propanoic acid (CH3CH2COOH) and bromine.
Step 1: Formation of the acyl bromide. Red phosphorus reacts with Br₂ to form PBr₃ (phosphorus tribromide). This PBr₃ then converts some of the carboxylic acid into an acyl bromide:
3R−COOH+PBr3→3R−COBr+H3PO3
The acyl bromide is the key intermediate. It is much more reactive than the original acid.
Step 2: Enolization and bromination. The acyl bromide has a slightly acidic α-hydrogen (more acidic than in the acid itself). A trace of HBr (from step 1) catalyzes the formation of an enol:
R−CH2−COBrHBrR−CH=C(OH)Br
This enol attacks Br₂, giving the α-bromo acyl bromide:
R−CH=C(OH)Br+Br2→R−CHBr−COBr+HBr
The HBr released continues the catalytic cycle.
Step 3: Hydrolysis. Finally, water (added at the end of the reaction) hydrolyzes the α-bromo acyl bromide back to the α-bromo carboxylic acid:
R−CHBr−COBr+H2O→R−CHBr−COOH+HBr
Note
The red phosphorus is catalytic — it is consumed to make PBr₃, but PBr₃ is regenerated in the cycle. Only a small amount is needed.
Both parts are named reactions defined by their specific reagents and selectivity. The Hell-Volhard-Zelinsky reaction uses red phosphorus with a halogen to substitute the alpha carbon of a carboxylic acid, while Rosenmund reduction uses a poisoned palladium catalyst to stop cleanly at the aldehyde. …
(a) HVZ selectively halogenates the α-carbon of a carboxylic acid; (b) Rosenmund reduction selectively reduces an acid chloride to an aldehyde using a poisoned Pd catalyst.
(a) Hell–Volhard–Zelinsky (HVZ) reaction: Carboxylic acids possessing an α-hydrogen atom react with chlorine or bromine in the presence of a small amount of red phosphorus to give the corresponding α-halocarboxylic acid. Red phosphorus first converts a little acid to the acid halide, which enolises and is halogenated at the α-carbon; the α-halo acid halide then reacts with more acid to regenerate the α-halo acid and continue the cycle.
CH3COOH+Cl2red PClCH2COOH+HCl
(b) Rosenmund reduction: Acid chlorides are selectively reduced to aldehydes by hydrogenation (H2 gas) using a palladium catalyst supported on barium sulphate (Pd/BaSO4), which is partially 'poisoned' (deactivated) so that the reaction stops cleanly at the aldehyde stage instead of proceeding further to the alcohol.
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2025Set ANNUAL1 mark
Q.Acetic acid reacts with chlorine in presence of red phosphorus to give chloroacetic acid. Name the reaction.
›Reveal solutionSolution
Halogenation of a carboxylic acid's α-carbon using Cl₂/Br₂ with red phosphorus is the classic HVZ reaction.
Carboxylic acids possessing an α-hydrogen undergo selective halogenation at the α-carbon when treated with chlorine or bromine in the presence of a small amount of red phosphorus (which generates the reactive phosphorus trihalide catalytically in situ). This substitution of an α-hydrogen by a halogen is called the Hell–Volhard–Zelinsky (HVZ) reaction:
Red phosphorus converts a bit of the halogen into a reactive phosphorus trihalide, which activates the carboxylic acid's alpha-hydrogen for substitution by a halogen.
The Hell-Volhard-Zelinsky (HVZ) reaction is used to introduce a halogen atom at the alpha-carbon (the carbon adjacent to -COOH) of a carboxylic acid that has at least one α-hydrogen. The carboxylic acid is treated with chlorine or bromine (X2) in the presence of a small amount of red phosphorus (which reacts with the halogen to generate phosphorus trihalide, PX3, in situ — this converts the acid to its more reactive acid halide intermediate, allowing enolisation and halogenation at the alpha carbon):
Red phosphorus converts a small amount of the acid to its acyl halide, which enolises and gets halogenated at the α-carbon; the halogen is then relayed to fresh acid molecules, α-halogenating the whole batch.
The Hell–Volhard–Zelinsky (HVZ) reaction is used to introduce a halogen atom at the α-carbon (the carbon next to the −COOH group) of a carboxylic acid that has at least one α-hydrogen.
Procedure: The carboxylic acid is treated with chlorine or bromine (Cl2 or Br2) in the presence of a small (catalytic) amount of red phosphorus.