Hell Volhard Zelinsky Reaction - Mechanism, Uses, Enolization of acyl bromide and Bromination of Enol
 

Introduction

Carl Magnus von Hell, Jacob Volhard, and Nikolay Zelinsky are the three chemists named the HVZ reaction. It is a halogenation reaction. At the alpha carbon, the process is utilized to halogenate carboxylic acids.

Hell- Volhard- Zelinsky Reaction

The Hell- Volhard- Zelinsky (HVZ) reaction is an organic reaction that uses a phosphorous catalyst, halogen gas, and water to convert alpha hydrogen carboxylic acid to an alpha halo carboxylic acid. It is a form of a substitution reaction. For example, red phosphorus and bromine are often treated with carboxylic acid and then hydrolyzed to produce alpha -Bromo carboxylic acid.

The Hell- Volhard- Zelinsky reaction has harsh reaction conditions, including reaction temperatures above 373 K and a longer reaction time. In most cases, less than one phosphorous equivalent is required for the reaction.

The Hell- Volhard- Zelinsky Reaction

You can make an alpha-Bromo acid by treating a carboxylic acid like the one below with PBr3 and excess Br2, then adding water.

This is the general reaction of the Hell-Volhard-Zelinsky Reaction. The C-H bond on the alpha carbon of a carboxylic acid, as shown above, is replaced with a C-Br bond.

• PBr3 takes up the function to convert the initial carboxylic acid to an acyl bromide.
• Br2 has the function of converting acyl bromide to an alpha-halo acyl bromide.
• Hydrolysis in step 2 converts the acyl bromide back to a carboxylic acid.

Mechanism of Hell- Volhard- Zelinsky Reaction

The following are the steps involved in the Hell- Volhard- Zelinsky Reaction
1. The carboxylic acid is converted to acyl bromide.
2. The acyl bromide is tautomerize to its enol form (breaking C-H)
3. Bromination of the enol's alpha-carbon (forming C-Br)
4. The carboxylic acid is formed once the acid bromide is hydrolyzed.

This is essentially a substitution reaction because a C-OH bond is substituted for a C-Br bond. This is a nucleophilic acyl replacement process because the substitution happens on an acyl (RC(O)-) carbon molecule.

The goal of PBr3, in this case, is to convert the carboxylic acid's -OH into a suitable departing group, with the development of the solid P-O bond acting as the key driver.

Step 2: Enolization of acyl bromide
Remember that the "alpha" carbon is the carbon molecule next to a carbonyl carbon. All compounds containing alpha carbons that include a C-H bond can form an enol, a structural isomer known as a tautomer. Tautomers are interconvertible fundamental isomers.

The acid bromide and its enol form are in equilibrium. Although the balance is on the acid bromide's end, the enol form is still attainable. A tiny drop of a strong acid will assist in speeding up the pace of keto-enol interconversion in practice. This is because the C-H bond on the alpha carbon breaks at this point.

The enol state is far more susceptible towards the acid bromide than the carboxylic acid, which explains why carboxylic acids do not perform straightforward alpha-bromination.

Step 3: Bromination of Enol
It should go without saying that enols are nucleophiles, given that their basic form contains a negative charge on carbon molecules.

Bromine (Br2) rapidly combines with enols, culminating in creating a C-Br bond upon the alpha carbon molecule.

Step 4: Hydrolysis of acid bromide to form carboxylic acid
The acid bromide is hydrolyzed to carboxylic acid in the HVZ's last phase.
This is usually accomplished by adding H2O after the initial carboxylic acid has been absorbed to reach our objective. The C-Br bond is broken, and the C-O bond is formed during the hydrolysis process.

Summary of HVZ Reaction

We have seen that there are four steps in the HVZ reaction.
1. The entire reaction is a nucleophilic acyl substitution. It involves the conversion of carboxylic acid to haloacid bromide.
2. The acid bromide produced in Step 1 helps in the formation of an enol. The process of formation of such an enol is                                  called enolization.
3. The formed enol undergoes an electrophilic addition reaction with Br2. This process is called bromination and results in        the              formation of an alpha- Bromo acid bromide product.
4. The last and final step involves the hydrolysis of the alpha- Bromo acid bromide in the third step, creating the final                product,      alpha- Bromo carboxylic acid.

Uses of HVZ Reaction

• When phenylacetic acid is treated with PBr3, the Hell-Volhard-Zelinsky reaction is utilized to convert it to 2-Bromo-2-phenylacetic acid.
• The HVZ reaction is employed to produce alanine, an amino acid from 2-bromopropanoic acid, following ammonolysis.