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The Hell-Volhard Zelinsky Reaction is an organic reaction that halogenates carboxylic acids at the \(\alpha\)-carbon to produce \(\alpha\)-halo carboxylic acid. This reaction bears the names of three German chemists: Carl Magnus Von Hell, Jacob Volhard, and Nikolay Zelinsky. The mechanism of Hell-Volhard Zelinsky Reaction is different from the other halogenation mechanism as it takes place in the absence of a halogen carrier.
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Key Terms: Hell-Volhard Zelinsky Reaction, carboxylic acid, halogenation, carbon, compounds, Diatomic Bromine, Phosphorus tribromide
Hell-Volhard Zelinsky Reaction
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In the Hell-Volhard Zelinsky Reaction, carboxylic acid is converted into alpha-halo carboxylic acid. The reaction takes place in the presence of a catalytic quantity of Phosphorus tribromide (PBr3) and one molar equivalent of Diatomic Bromine (Br2). This reaction does not allow iodination or fluorination.
Hell Volhard Zelinsky Reaction
The Hell Volhard Zelinsky Reaction Mechanism differs from other halogenation reactions as it occurs in the lack of a halogen carrier. The process is utilised for the alpha carbon halogenation of carboxylic acids.
Hell Volhard Zelinsky Reaction is started by adding one molar equivalent of diatomic bromine and one molar equivalent of phosphorus tribromide (catalytic quantity).
The Hell Volhard Zelinsky reaction provides a technique for alpha addition using a carboxylic acid. The technique works by converting the carboxylic acid into a derivative that undergoes tautomerization and then performing alpha addition on that form.
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Mechanism of Hell-Volhard Zelinsky Reaction
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The steps below consist of mechanism of Hell Volhard Zelinsky Reaction:
Step-1: The hydroxide forms a suitable leaving group when oxygen attacks phosphorus. The bromide ion now attacks the carbonyl cation, causing the carbon-oxygen pi bond to break and a tetrahedral intermediate to form. Because the hydroxide ion is now a good leaving group, it is ejected from the tetrahedral intermediate, as demonstrated by the reactions below.
Step-2: The carbonyl oxygen receives a proton from the hydrogen bromide. Because the resultant bromide ion is a weak base, it receives the hydrogen at the alpha position. The additional electron is used to form a double bond with the neighbouring carbon atom and push the carbon-oxygen double bond's pi electrons towards the oxygen. The oxygen's positive charge is eliminated. The catalyst is renewed because the bromide ion recaptures a hydrogen atom. Thus, carboxylic acid’s keto-enol tautomerization was accomplished.
Step-3: The oxygen in the enol form contributes an electron pair for the formation of a double bond with the carbon. The existing carbon-carbon double bond attacks a bromine atom, resulting in enol bromination at the alpha carbon. Because there is no water in the system, the bromide ion also removes the hydrogen bound to the oxygen atom and produces another hydrogen bromide molecule, which evaporates from the system.
Step-4: The acyl bromide is hydrolyzed to carboxylic acid with the addition of water molecules. The oxygen present in water attacks the carbonyl group and forms an intermediate. The bromide ion is ejected during the reformation of the carbonyl. The hydrogen extracted from the water then returns to the carboxylic acid.
Applications of Hell-Volhard Zelinsky Reaction
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The uses and applications are listed below:
- Used in the bromination of carboxylic acids.
- Through the displacement of the halogen with ammonia, the HVZ is used as a technique for the synthesis of different amino acids (NH3).
Things to Remember
- Hell-Volhard-Zelinsky reaction occurs in the absence of a halogen carrier.
- Electrophiles are formed when halogen carriers grab molecules from other compounds.
- Phosphorus tribromide (PBr3) substitutes a bromide for the carboxylic OH. As a result, a Carboxylic acid Bromide is formed.
- In the next step tautomerization occurs, in which acyl bromide/ carboxylic acid bromide tautomerizes to Enol.
- This Enol easily reacts with Br2 to brominate at the -position, resulting in the formation of -bromoAcyl bromide.
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Important Questions
Ques: Is Hcooh affected by the HVZ reaction? (1 Mark)
Ans: Since it lacks an alkyl group, formic acid does not conduct this reaction. Reaction of Hell-Volhard Zelinsky Only occurs when -hydrogen is present.
Ques: What is the HVZ reaction? (2 Marks)
Ans: Hell-Volhard-Zelinsky reaction is abbreviated as HVZ reaction. It is a halogenation reaction, which is an addition process in which one or more halogens are added to the compound. The acyl bromide produced can then react with the remaining unreacted carboxylic acid, and the cycle continues until the conversion is complete.
Ques: What role does red phosphorus play in the HVZ reaction? (2 Marks)
Ans: Red phosphorus can be used. It has a good air stability (whereas white phosphorus combust in air almost spontaneously). Phosphorus combines with bromine to form phosphorus tribromide, which now serves as the reaction's catalyst.
Ques: Why does acetic acid produce HVZ while formic acid does not? (2 Marks)
Ans: When alpha hydrogen carboxylic acids are heated with halogens, they produce alpha halo acid, which is known as the H.V.Z. reaction. Because formic acid lacks alpha hydrogen, it does not produce this reaction, but acetic acid does because it contains an alpha hydrogen atom.
Ques: How can you tell the difference between alcohol and carboxylic acid? (2 Marks)
Ans: Phenol does not respond to the sodium bicarbonate test. So, it is the best approach for experimentally distinguishing carboxylic acid from alcohol. Carboxylic acid interacts with sodium bicarbonate to produce sodium acetate and a rapid effervescence of carbon dioxide in this test.
Ques: Explain which is more acidic, phenol or acetic acid. (2 Marks)
Ans: If we draw the resonating structure of acetic acid, the same negative charge sits on the oxygen atom, but if we draw the resonating structure of phenol, the negative charge moves from oxygen to carbon, resulting in a non equivalent structure. As a result, acetic acid is more acidic than phenol.
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