Hell-Volhard-Zelinsky reaction
The Hell-Volhard-Zelinsky (HVZ) reaction is a chemical transformation used to halogenate carboxylic acids at the alpha (α) position, resulting in α-halo carboxylic acids. This reaction is particularly useful for the selective α-bromination of carboxylic acids. The HVZ reaction involves several key steps and reagents, including phosphorus tribromide (PBr3), bromine (Br2), and sometimes water for the final hydrolysis step.
Mechanism of the HVZ Reaction
- Formation of Acyl Bromide: The reaction begins with the conversion of the carboxylic acid into an acyl bromide. This is achieved by reacting the carboxylic acid with PBr3, which substitutes the -OH group with a bromide (Br), resulting in an acyl bromide. This step is a nucleophilic acyl substitution reaction[1][4][5].
- Enol Formation: The acyl bromide then tautomerizes to form an enol, a process facilitated by the acidic conditions of the reaction[4][5].
- Halogenation at the α-Position: The enol reacts with Br2, allowing for the halogenation at the α-position. This step results in the formation of an α-bromo acyl bromide[4][5].
- Hydrolysis: Finally, the α-bromo acyl bromide is hydrolyzed back to an α-bromo carboxylic acid. This step often involves the addition of water[1][5].
Applications and Examples
The HVZ reaction is widely used in organic synthesis for the preparation of α-halo carboxylic acids, which are valuable intermediates in the synthesis of various organic compounds. For example, the reaction can be used to convert phenylacetic acid to 2-bromo-2-phenylacetic acid[8]. Additionally, it has applications in the synthesis of amino acids, such as the preparation of alanine from α-bromopropionic acid through ammonolysis[10].
Key Points
- The HVZ reaction is named after the chemists Carl Magnus von Hell, Jacob Volhard, and Nikolay Zelinsky[10].
- It is a method for the α-halogenation of carboxylic acids, with a focus on bromination[7].
- The reaction involves the conversion of carboxylic acids to acyl bromides, enol formation, halogenation at the α-position, and hydrolysis to yield α-halo carboxylic acids[1][4][5].
- This reaction is significant for the synthesis of α-halo carboxylic acids, which serve as intermediates in various organic syntheses, including the preparation of amino acids[8][10].
The HVZ reaction exemplifies a strategic method in organic chemistry for modifying carboxylic acids, enabling the introduction of halogen atoms at positions adjacent to the carbonyl group, thereby expanding the versatility of carboxylic acid derivatives in synthetic applications.
Citations:
[1] https://www.masterorganicchemistry.com/2020/09/30/the-hell-volhard-zelinsky-reaction/
[2] https://youtube.com/watch?v=gSYClu63DDA
[3] https://youtube.com/watch?v=MeKixdRx5qY
[4] https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Reactions/Named_Reactions/Hell-Volhard-Zelinsky_reaction
[5] https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Carboxylic_Acids/Reactivity_of_Carboxylic_Acids/Hell-Volhard-Zelinskii_Reaction
[6] https://youtube.com/watch?v=0Hgq_a0Ivps
[7] https://www.organic-chemistry.org/namedreactions/hell-volhard-zelinsky-reaction.shtm
[8] https://www.chemistrylearner.com/hell-volhard-zelinsky-reaction.html
[9] https://byjus.com/chemistry/hell-volhard-zelinsky-reaction-mechanism/
[10] https://en.wikipedia.org/wiki/Hell%E2%80%93Volhard%E2%80%93Zelinsky_halogenation
[11] https://youtube.com/watch?v=V-fadpHppqI
[12] https://www.pw.live/concepts-hell-volhard-zelinsky-reaction
[13] https://collegedunia.com/exams/hell-volhard-zelinsky-reaction-chemistry-articleid-1941