Hell Volhard Zelinsky Reaction Mechanism - Significance, Application with FAQs

Q: Which catalyst is used in the hvz reaction?

A catalytic amount of phosphorus tribromide PBr 3 followed by one molar equivalent of bromide Br 2 . Tribromide is formed by the reaction of red phosphorus and bromine gas Br 2 . PBr 3 is very reactive and readily reacts with water vapour present in the atmosphere to form P-O bond, when left out in air. In hvz reaction, PBr 3 has the responsibility of converting carboxylic acid into acyl bromides.Br 2 is added in excess amount in hvz reaction to form PBr 3 . This is because PBr 3 by itself is not sufficient enough to perform two duties; role as a catalyst and participate in halogenation. Addition of Br 2 in excess amount, in hvz, ensures that there is at least one extra equivalent of Br 2 available to carry out the addition of halogen at the α-carbon. Hvz reaction cannot be used for fluorination and bromination of carboxylic groups.

Hell Volhard Zelinsky Reaction Mechanism - Significance, Application with FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:57 PM IST

In this article we will discuss about hell volhard zelinsky reaction (or hvz reaction in short), hell volhard zelinsky reaction mechanism( or hvz reaction mechanism), hell volhard zelinsky reaction example hvz reaction example, what hvz reaction is used to prepare, hvz reaction definition and everything related to hell volhard zelinsky reaction (or hvz reaction class 12).

Hell Volhard Zelinsky reaction

Nikolay Zelinsky in 1923

The name hell volhard zelinsky (or hvz) is named after three chemists. They were Carl Magnus von Hell; a German chemist, Nikolay Zelinsky; a Russian chemist, and Jacob Volhard; a German chemist. Hvz reaction is a chemical reaction used to halogenate α-carbon of carboxylic acids. Along with water, Phosphorus as a catalyst is used in the hvz reaction.

Hell Volhard Zelinsky reaction

The catalyst used in hvz reaction

This is because PBr₃by itself is not sufficient enough to perform two duties; role as a catalyst and participate in halogenation. Addition of Br₂ in excess amount, in hvz, ensures that there is at least one extra equivalent of Br₂ available to carry out the addition of halogen at the α-carbon. Hvz reaction cannot be used for fluorination and bromination of carboxylic groups.

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Significance of hvz reaction

As we know, aldehyde and ketones with α-H can undergo electrophilic addition reaction to form an enol tautomer which is in equilibrium with the substrate aldehyde or ketone. However, this is not quite the case with carboxylic acids. Carboxylic acids generally do not form stable enols which mean addition at α C becomes difficult.

Thus, α addition is not as effortless for carboxylic acid as it is for aldehydes and ketones. This is where the requirement of hvz reaction comes into play. The significance of the hvz reaction is to form a derivative, by conversion of the carboxylic acid, which can undergo tautomerization and is capable of executing α addition reaction.

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Hvz reaction mechanism

The hvz reaction initiates with addition of phosphorus tribromide. When PBr₃is added, oxygen of the carboxylic group attacks the phosphorus of PBr₃ which results in breaking of C-O pi bond (1). This breaking of the C=O bond influences the hydroxyl OH group to become a good leaving group. With withdrawal of hydroxide group, a tetrahedral intermediate is formed (2). The orbitals of oxygen which were first employed to form pi bonds are now participating in bond formation with phosphorus.

The hvz reaction proceeds with deport of HOPBr₂ from the previously formed intermediate to give an acyl bromide (3). The previous C-O bond in carboxylic acid is broken to form a new C-Br bond.

As we discussed previously, addition of halogen takes place at α-carbon. The carbon next to the carbonyl group is known as alpha carbon. Carbonyl compounds with α-carbon possessing α-hydrogen are capable of undergoing enolization and hence form enols. Enols are basically olefins with a hydroxyl group at one end (4).

The reaction moves forward with breaking of the carbon-oxygen pi bond present in acyl bromide. The formation of a new bond between carbonyl carbon and alpha carbon takes place.

This step of breaking of C-O pi bond and formation of C-C pi bond between acyl bromide (3) and enol form (4) is in equilibrium.

Since enols are nucleophilic in nature, they readily react with Br₂when Br₂is added in excess. In this step, halogenation takes place. The C=C bond breaks with addition of Br to the alpha carbon (5). This induces a positive charge on hydroxide oxygen which in turn results in expel of hydrogen. Overall, a molecule of HBr is removed in this step of the hvz reaction.

Hvz reaction comes to an end with hydrolysis of newly generated acid bromide (5) to form a carboxylic acid derivative. Water being a nucleophile, when added, attacks the carbonyl oxygen and attaches itself to the carbonyl carbon (6). This nucleophilic acyl substitution results in elimination of HBr and regeneration of neutral carboxylic group (7).

Hvz reaction mechanism

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Application of hvz reaction

Hvz reaction is often used in preparation of alanine. Synthesis of various amino acids is also an important application of hvz reaction. The hvz reaction for synthesis of amino acids proceeds with displacement of halogens by using ammonia. An example of such hvz reaction is given below:

hvz reaction

Alpha bromination of carboxylic acid remains the most important application of hvz reaction. (This article covers details of alpha bromination of carboxylic acid.)

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Frequently Asked Questions (FAQs)

1. Explain hvz reaction.

Hvz reaction is a chemical reaction used to halogenate α-carbon of carboxylic acids. Along with water, Phosphorus as a catalyst is used in the hvz reaction.

2. The product of hell volhard zelinsky reaction is _.

The product of hell volhard zelinsky reaction is a carboxylic acid bromide.

3. Which reaction is used in alpha halogenation of carboxylic acids?

Hell volhard zelinsky reaction is used in alpha halogenation of carboxylic acids.

4. Which catalyst is used in the hvz reaction?

A catalytic amount of phosphorus tribromide PBr₃ followed by one molar equivalent of bromide Br₂. Tribromide is formed by the reaction of red phosphorus and bromine gas Br₂. PBr₃ is very reactive and readily reacts with water vapour present in the atmosphere to form P-O bond, when left out in air. In hvz reaction, PBr₃ has the responsibility of converting carboxylic acid into acyl bromides.Br₂ is added in excess amount in hvz reaction to form PBr₃. This is because PBr₃ by itself is not sufficient enough to perform two duties; role as a catalyst and participate in halogenation. Addition of Br₂ in excess amount, in hvz, ensures that there is at least one extra equivalent of Br₂ available to carry out the addition of halogen at the α-carbon. Hvz reaction cannot be used for fluorination and bromination of carboxylic groups.

5. Can the HVZ reaction be used with all carboxylic acids?

The HVZ reaction works best with carboxylic acids that have at least one α-hydrogen. It is less effective or may not work at all with carboxylic acids lacking α-hydrogens, such as tertiary carboxylic acids.

6. How does the HVZ reaction compare to other methods of α-halogenation?

The HVZ reaction is generally more selective and efficient for α-halogenation of carboxylic acids compared to other methods. It avoids the use of strong bases or harsh conditions that might be required in alternative approaches, making it a preferred method in many synthetic scenarios.

7. Can the HVZ reaction be used to introduce multiple halogen atoms?

Yes, the HVZ reaction can be used to introduce multiple halogen atoms at the α-position of a carboxylic acid. By using excess halogen and extending the reaction time, it's possible to achieve di- or even tri-halogenation, depending on the number of available α-hydrogens.

8. How does the HVZ reaction differ from direct halogenation of alkanes?

The HVZ reaction is more selective than direct halogenation of alkanes. It specifically targets the α-carbon of carboxylic acids, whereas direct halogenation of alkanes can lead to multiple products due to free radical mechanisms. The HVZ reaction provides better control over the position of halogenation.

9. What is the significance of α-halo carboxylic acids in organic synthesis?

α-Halo carboxylic acids are versatile intermediates in organic synthesis. They can undergo various transformations such as nucleophilic substitution, elimination reactions, and coupling reactions. This versatility makes them valuable building blocks for synthesizing more complex organic compounds.

10. What is the Hell-Volhard-Zelinsky (HVZ) reaction?

The Hell-Volhard-Zelinsky (HVZ) reaction is a chemical process used to α-halogenate carboxylic acids. It involves the reaction of a carboxylic acid with halogen (usually chlorine or bromine) in the presence of a catalyst like phosphorus or thionyl chloride to produce an α-halo carboxylic acid.

11. What precautions should be taken when performing the HVZ reaction in a laboratory setting?

When performing the HVZ reaction, important precautions include:

12. What is the role of phosphorus in the HVZ reaction?

Phosphorus acts as a catalyst in the HVZ reaction. It helps convert the carboxylic acid into an acid chloride intermediate, which is more reactive than the original acid. This increased reactivity facilitates the subsequent halogenation step.

13. What are the main steps in the HVZ reaction mechanism?

The main steps in the HVZ reaction mechanism are:

14. Why is the HVZ reaction often carried out in the presence of thionyl chloride?

Thionyl chloride (SOCl2) is used in the HVZ reaction to convert the carboxylic acid into an acid chloride. This step increases the reactivity of the compound, making it more susceptible to α-halogenation. Thionyl chloride also helps remove water from the reaction, preventing unwanted side reactions.

15. What determines the regioselectivity in the HVZ reaction?

The regioselectivity in the HVZ reaction is determined by the formation of the enol intermediate. The halogen preferentially attacks the α-carbon because it is the most electron-rich site in the enol form of the acid chloride.

16. What factors affect the yield of the HVZ reaction?

Several factors can affect the yield of the HVZ reaction:

17. Why is the HVZ reaction considered significant in organic chemistry?

The HVZ reaction is significant because it provides a selective method to introduce a halogen atom at the alpha position of a carboxylic acid. This α-halogenation creates a versatile intermediate that can be used in various synthetic transformations, making it a valuable tool in organic synthesis.

18. How does the HVZ reaction contribute to green chemistry principles?

The HVZ reaction aligns with some green chemistry principles by offering good atom economy and selectivity, potentially reducing waste. However, it uses hazardous halogens and generates acidic by-products. Green chemistry improvements might focus on using less hazardous reagents, improving catalysts, or developing solvent-free conditions.

19. What is the significance of the enol intermediate in the HVZ reaction?

The enol intermediate is crucial in the HVZ reaction because it determines the regioselectivity of halogenation. The enol form creates a nucleophilic center at the α-carbon, which then reacts with the electrophilic halogen. Understanding this intermediate helps explain why halogenation occurs specifically at the α-position rather than elsewhere on the molecule.

20. How does the choice of halogen (chlorine vs. bromine) affect the HVZ reaction?

The choice between chlorine and bromine can affect the reaction rate and product distribution in the HVZ reaction. Bromine is generally more reactive than chlorine, leading to faster reactions. However, chlorination is often preferred due to the lower cost and easier handling of chlorine reagents. The choice may also depend on the desired properties of the final product.

21. How does the presence of electron-withdrawing or electron-donating groups on the carboxylic acid affect the HVZ reaction?

Electron-withdrawing groups (EWGs) on the carboxylic acid can make the α-hydrogens more acidic, potentially increasing the rate of enolization and subsequent halogenation. Conversely, electron-donating groups (EDGs) can stabilize the carboxylic acid, potentially slowing down the reaction. The electronic effects can also influence the stability and reactivity of the enol intermediate.

22. What are some common side reactions in the HVZ reaction?

Common side reactions in the HVZ reaction include:

23. Can the HVZ reaction be used in industrial processes?

Yes, the HVZ reaction is used in industrial processes, particularly in the pharmaceutical and fine chemicals industries. It's valued for its selectivity and relatively mild conditions. However, the use of corrosive halogens and the generation of acid by-products can present challenges in large-scale applications, requiring careful process design and safety measures.

24. What are some alternatives to the HVZ reaction for α-halogenation of carboxylic acids?

Some alternatives to the HVZ reaction include:

25. How can the progress of the HVZ reaction be monitored?

The progress of the HVZ reaction can be monitored through various analytical techniques:

26. How does the HVZ reaction demonstrate the concept of kinetic vs. thermodynamic control?

The HVZ reaction demonstrates kinetic control rather than thermodynamic control. The halogenation occurs at the α-position because the enol intermediate forms rapidly (kinetically favored) at this position. Even though other positions might be thermodynamically more stable for halogenation, the reaction is controlled by the rate of enol formation at the α-carbon.

27. Can the HVZ reaction be used to synthesize optically active α-halo carboxylic acids?

The standard HVZ reaction produces racemic mixtures of α-halo carboxylic acids when starting from achiral carboxylic acids. However, modifications using chiral catalysts or starting from optically active carboxylic acids can lead to the synthesis of optically active α-halo carboxylic acids. This is an area of ongoing research in asymmetric synthesis.

28. How does the acidity of the carboxylic acid affect the HVZ reaction?

The acidity of the carboxylic acid can influence the HVZ reaction in several ways:

29. What role does solvent choice play in the HVZ reaction?

Solvent choice is important in the HVZ reaction for several reasons:

30. How does the HVZ reaction exemplify the concept of electrophilic substitution?

The HVZ reaction is an example of electrophilic substitution at the α-carbon of a carboxylic acid. The halogen acts as an electrophile, replacing the α-hydrogen. This substitution is facilitated by the formation of the enol intermediate, which provides the nucleophilic site for the electrophilic halogen to attack.

31. Can the HVZ reaction be applied to cyclic carboxylic acids?

Yes, the HVZ reaction can be applied to cyclic carboxylic acids. In these cases, the reaction often leads to α-halogenation of the ring carbon adjacent to the carboxylic acid group. This can be particularly useful in synthesizing functionalized cyclic compounds, which are important in many areas of organic chemistry and pharmaceutical development.

32. What are some common workup procedures for isolating the product of an HVZ reaction?

Common workup procedures for the HVZ reaction include:

33. How does the HVZ reaction compare to other named reactions in organic chemistry?

The HVZ reaction is similar to other named reactions in organic chemistry in that it provides a specific transformation under defined conditions. It's particularly comparable to other halogenation reactions like the Hunsdiecker reaction or the Sandmeyer reaction. However, the HVZ reaction is unique in its ability to selectively α-halogenate carboxylic acids, making it a valuable tool in synthetic organic chemistry.

34. What are some applications of α-halo carboxylic acids produced by the HVZ reaction?

α-Halo carboxylic acids produced by the HVZ reaction have numerous applications:

35. How does the HVZ reaction demonstrate the principle of chemoselectivity?

The HVZ reaction demonstrates chemoselectivity by specifically targeting the α-position of carboxylic acids for halogenation, even in the presence of other potentially reactive sites. This selectivity is achieved through the formation of the enol intermediate, which directs the halogenation to the α-carbon. This principle is crucial in complex molecule synthesis where selective functionalization is often required.

36. Can the HVZ reaction be performed on carboxylic acid derivatives like esters or amides?

The classical HVZ reaction is specific to carboxylic acids. However, modified versions of the reaction can be applied to certain carboxylic acid derivatives. For example, esters can undergo α-halogenation under similar conditions, often referred to as the Reformatsky reaction. Amides generally do not undergo the HVZ reaction directly due to their different electronic properties.

37. How does the presence of α-branching in the carboxylic acid affect the HVZ reaction?

α-Branching in the carboxylic acid can significantly affect the HVZ reaction:

38. What is the significance of using red phosphorus in some variations of the HVZ reaction?

Red phosphorus is sometimes used in the HVZ reaction as an alternative to white phosphorus or thionyl chloride. Its significance includes:

39. How does temperature control affect the outcome of the HVZ reaction?

Temperature control is crucial in the HVZ reaction:

40. What are some common errors or misconceptions students have about the HVZ reaction?

Common errors or misconceptions about the HVZ reaction include:

41. How does the HVZ reaction relate to the concept of umpolung in organic synthesis?

The HVZ reaction relates to umpolung (polarity inversion) in that it effectively changes the reactivity at the α-carbon of the carboxylic acid. Normally, this position is electrophilic, but the formation of the enol intermediate temporarily makes it nucleophilic. This inversion of polarity allows for the introduction of an electrophilic halogen at a typically electrophilic site, demonstrating the principle of umpolung.

42. Can the HVZ reaction be used in total synthesis of natural products?

Yes, the HVZ reaction is often used in the total synthesis of natural products. It provides a method to introduce a halogen at a specific position, which can then be used for further functionalization. This is particularly useful in synthesizing complex