Principle reactions procedure and calculations of Volhard method
The Volhard method is a reaction in which the alpha hydrogen group of a carboxylic acid is replaced by a halogen. This reaction is a type of halogenation reaction. This reaction is used for the preparation of halogen derivatives of carboxylic acid. Here, we will discuss the mechanism of the Volhard method.
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What is the Volhard method reaction referred to as? The Volhard method is referred to as Hell Volhard Zelinsky Reaction. Only the carboxylic acid-containing alpha hydrogen group compounds can give this reaction. Carboxylic acids are the compounds containing the carboxyl function group.
Volhard Method Procedure
In the Volhard method procedure of halogenation, the carboxylic acids having alpha hydrogen react with chlorine or bromine in the presence of a small amount of red phosphorus to give compounds in which alpha hydrogen atoms get replaced by halogen atoms. This reaction is called the Hell Volhard-Zelinsky reaction.
The General Reaction of the Volhard Method Procedure is Shown below:
R-CH2COOH → RCHX-COOH (reaction takes place in the presence of halogen molecule, red phosphorus, and water molecule).
R-CH2COOH is a carboxylic acid
RCHX-COOH is alpha halocarboxylic acid.
In the above reaction, a halogen molecule can be chlorine or bromine.
CH3COOH + Cl2 + red.P + H2O → ClCH2COOH Cl2 + red.P + H2O→ Cl2CHCOOH Cl2 + red.P + H2O → Cl3CCOOH
CH3CH2COOH + Br2 + red.P + H2O → CH3CHBrCOOH +Br2 + red.P + H2O → CH3C(Br)2COOH
The Function of Red Phosphorus
The function of red phosphorus is to first combine with bromine to form PBr3. This then reacts with a carboxylic acid to form a corresponding acid bromide which enolizes to a larger extent than the acid. Thus, it brings out alpha bromination readily.
P4 + 6Br2 → 4 PBr3
3CH3CH2COOH + PBr3 → 3CH3CH2COBr + H3PO3
The halogen atom in monosubstituted acid can be easily replaced by a suitable atom or group to form a variety of compounds. Thus, alpha- halo acids are important synthetic intermediates.
For Example,
(1) Action with aqueous Potassium Hydroxide
CH2(Cl)- COOH + KOH (aq) → CH2 (OH)- COOH + KCl
CH2 (OH)- COOH is alpha-hydroxy acetic acid also known as glycolic acid.
(2) Reaction with Alcoholic Ammonia
CH2 (Cl)- COOH + NH3 → CH2 (NH2)- COOH + HCl
CH2 (NH2)- COOH is an alpha-amino acetic acid also known as glycine.
Glycine or alpha-amino acetic acid exists in a dipolar form (NH3+CH2COO-).
(3.) Action with Potassium Cyanide
CH2(Cl)-COOH + KCN CH2(COOH)-COOH
Condition for the Hell Volhard Zelinsky Reaction Mechanism
The Hell Volhard Zelinsky Reaction does not occur under normal conditions. It requires a little severe environment. The optimum temperature for such a reaction to takes place is 373 K. The rate of this reaction is low. Therefore, it is a slow reaction. This reaction takes place in the presence of red phosphorus. This reagent acts as a catalyst for the reaction. It combines with the halogen molecule (chlorine and bromine) and forms phosphorus trihalide. This phosphorus trihalide further halogenates the carboxylic acid and forms halogenated carboxyl derivatives.
Back Titration Method
The back titration method is a titration process used to determine the concentration of the analyte using an excess reagent. The excess amount of reagent is the known concentration. This known concentration of the excess reagent helps in the determination of the concentration of the analyte. By using this titration process we can also find the strength of the analyte.
This type of titration method is used in the following reactions:
When the acid or base component acts as an insoluble agent in the solution.
When it is hard to find out the endpoint of the solution of the analyte.
When the rate of reaction is very slow.
It is used in the determination of carboxylic acid concentration using a halogen molecule (it can be chlorine or bromine) as an excess reagent. The rate of reaction for this type of halogenation reaction is too slow. Therefore, it is hard to find out the endpoint of this reaction.
Did You Know?
The name of Hell Volhard Zelinsky represents the name of three chemists.
The carboxylic acid without alpha hydrogen does not give this reaction.
The climax part of this reaction is tautomerization.
FAQs on Volhard Method in Argentometric Titration
1. What is the Volhard method in chemistry?
The Volhard method is an argentometric back titration method used to determine halide ions (Cl-, Br-, I-) by reacting excess silver nitrate with the sample and titrating the leftover Ag+ with potassium thiocyanate. In this method:
- An excess of AgNO3 is added to precipitate silver halide (e.g., AgCl).
- The unreacted Ag+ is titrated with KSCN.
- Fe3+ acts as an indicator, forming a red complex at the endpoint.
2. What is the principle of the Volhard method?
The principle of the Volhard method is based on the precipitation of halide ions with excess AgNO3 followed by back titration of unused Ag+ with SCN- in acidic medium. The key reactions are:
- Precipitation: Ag+(aq) + Cl-(aq) → AgCl(s)
- Back titration: Ag+(aq) + SCN-(aq) → AgSCN(s)
- Endpoint indicator reaction: Fe3+(aq) + SCN-(aq) → [FeSCN]2+(aq) (reddish-brown complex)
3. Why is the Volhard method carried out in acidic medium?
The Volhard method is carried out in acidic medium (usually HNO3) to prevent precipitation of iron as Fe(OH)3 and to avoid interference from other ions. Acidic conditions:
- Keep Fe3+ soluble as the indicator.
- Prevent formation of Ag2CO3 or AgOH.
- Ensure accurate formation of the [FeSCN]2+ complex at the endpoint.
4. What is the role of ferric ion in the Volhard method?
The ferric ion (Fe3+) acts as an indicator in the Volhard titration by forming a reddish-brown complex with thiocyanate at the endpoint. When all excess Ag+ has reacted, the first excess SCN- reacts as:
- Fe3+(aq) + SCN-(aq) → [FeSCN]2+(aq)
5. What is the difference between Mohr method and Volhard method?
The main difference between the Mohr method and the Volhard method is that Mohr is a direct titration while Volhard is a back titration. Key differences include:
- Mohr method: Direct titration of Cl- with AgNO3 using K2CrO4 indicator in neutral medium.
- Volhard method: Excess AgNO3 added first, then back titrated with KSCN using Fe3+ indicator in acidic medium.
- Volhard can be used for Cl-, Br-, and I-, and is suitable for colored solutions.
6. How do you calculate chloride concentration using the Volhard method?
The chloride concentration in the Volhard method is calculated by subtracting the moles of Ag+ titrated with SCN- from the total moles of Ag+ added. Steps:
- Calculate moles of AgNO3 added: M × V.
- Calculate moles of KSCN used in back titration.
- Moles of Cl- = (moles of AgNO3 added − moles of KSCN used).
7. What are the reactions involved in the Volhard titration?
The Volhard titration reactions include precipitation, back titration, and indicator reaction. The balanced reactions are:
- Precipitation of halide: Ag+(aq) + Cl-(aq) → AgCl(s)
- Reaction with thiocyanate: Ag+(aq) + SCN-(aq) → AgSCN(s)
- Indicator reaction: Fe3+(aq) + SCN-(aq) → [FeSCN]2+(aq)
8. What are the advantages of the Volhard method?
The Volhard method offers advantages such as accuracy in acidic medium and suitability for colored solutions. Its main benefits include:
- Applicable to Cl-, Br-, and I-.
- Can be used in strongly acidic solutions.
- Provides a sharp endpoint due to formation of [FeSCN]2+ complex.
- Useful when direct titration methods are not suitable.
9. Can the Volhard method be used for bromide and iodide estimation?
Yes, the Volhard method can be used to estimate bromide (Br-) and iodide (I-) ions because they form insoluble silver halides with Ag+. The reactions are:
- Ag+(aq) + Br-(aq) → AgBr(s)
- Ag+(aq) + I-(aq) → AgI(s)
10. What is the endpoint of the Volhard titration?
The endpoint of the Volhard titration is the first permanent appearance of a reddish-brown color due to formation of the [FeSCN]2+ complex. This occurs when:
- All excess Ag+ has reacted with SCN- to form AgSCN.
- The next drop of SCN- reacts with Fe3+.
