Q.(a) Write Hell Volhard-Zelinsky (H.V.Z.) reaction.

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 $\alpha$-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 $\alpha$-position of a carboxylic acid. The reagents are:

• A carboxylic acid (with at least one $\alpha$-hydrogen)
• Cl₂ or Br₂ (chlorine or bromine)
• A catalytic amount of red phosphorus ($P_4$)

The product is an $\alpha$-halo carboxylic acid.

The reaction works for any carboxylic acid that has at least one hydrogen on the carbon adjacent to the carboxyl group. If the $\alpha$-carbon is fully substituted (no $\alpha$-H), the reaction does not occur.

The Mechanism: Step by Step

The magic happens in three stages. Let's trace it with propanoic acid ($CH_3CH_2COOH$) 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:

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 $\alpha$-hydrogen (more acidic than in the acid itself). A trace of HBr (from step 1) catalyzes the formation of an enol:

This enol attacks Br₂, giving the $\alpha$-bromo acyl bromide:

The HBr released continues the catalytic cycle.

Step 3: Hydrolysis. Finally, water (added at the end of the reaction) hydrolyzes the $\alpha$-bromo acyl bromide back to the $\alpha$-bromo carboxylic acid:

Why This Matters …