How Does the Volhard Method Work? Principles & Application
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 Chemistry: Step-by-Step Guide
1. What is the Volhard method in analytical chemistry?
The Volhard method is a type of precipitation titration used for determining the concentration of halide ions (Cl⁻, Br⁻, I⁻) and other anions that form precipitates with silver ions. It is an indirect or back titration technique where a known excess of standard silver nitrate solution is added to the sample, and the unreacted silver nitrate is then titrated with a standard potassium thiocyanate solution.
2. What are the essential reagents required to perform the Volhard method?
The primary reagents used in the Volhard method are:
- Standard Silver Nitrate (AgNO₃) solution: Added in a known excess amount to precipitate the analyte.
- Standard Potassium or Ammonium Thiocyanate (KSCN or NH₄SCN) solution: Used as the titrant to determine the excess silver nitrate.
- Ferric Alum (FeNH₄(SO₄)₂·12H₂O) or Ferric Nitrate (Fe(NO₃)₃): Acts as the indicator.
- Concentrated Nitric Acid (HNO₃): To provide an acidic medium for the titration.
3. What are the key chemical reactions involved in the Volhard method?
The Volhard method involves two main chemical reactions. For an analysis of a chloride (Cl⁻) sample, the reactions are:
1. Precipitation of Halide: A known excess of silver ions (Ag⁺) is added to the sample, which precipitates the halide ions.
Ag⁺(aq) (excess) + X⁻(aq) → AgX(s) (where X⁻ = Cl⁻, Br⁻, I⁻)
2. Back Titration of Excess Silver: The unreacted Ag⁺ is titrated with thiocyanate ions (SCN⁻) until the endpoint.
Ag⁺(aq) (unreacted) + SCN⁻(aq) → AgSCN(s)
The amount of halide is calculated from the difference between the initial amount of AgNO₃ added and the amount that reacted with KSCN.
4. How is the endpoint detected in the Volhard method?
The endpoint is detected using a ferric ion (Fe³⁺) indicator. After all the excess silver ions (Ag⁺) have precipitated as silver thiocyanate (AgSCN), the very next drop of the thiocyanate (SCN⁻) titrant reacts with the Fe³⁺ indicator. This reaction forms a distinct, soluble, blood-red complex, [Fe(SCN)(H₂O)₅]²⁺. The appearance of this stable red colour signals that the endpoint has been reached.
5. How does the Volhard method differ from the Mohr method for halide determination?
The Volhard and Mohr methods are both precipitation titrations but differ in several key aspects:
- Titration Type: The Volhard method is a back titration, whereas the Mohr method is a direct titration.
- pH Conditions: The Volhard method is performed in a strongly acidic medium (using HNO₃), while the Mohr method requires a neutral or slightly alkaline pH (6.5-9).
- Indicator: The Volhard method uses a ferric ion (Fe³⁺) indicator, which forms a red complex. The Mohr method uses a potassium chromate (K₂CrO₄) indicator, which forms a red-brown precipitate (Ag₂CrO₄).
- Applicability: The Volhard method can determine Cl⁻, Br⁻, and I⁻, while the Mohr method is generally suitable only for Cl⁻ and Br⁻.
6. Why is the Volhard method considered a 'back titration' technique?
It is called a back titration because the analyte (the halide ion) is not titrated directly. Instead, a two-step process is used: first, a reagent (silver nitrate) is added in a precisely known excess amount to ensure the complete precipitation of the analyte. Then, the amount of the 'leftover' or unreacted reagent is determined by titrating it with a second standard solution (potassium thiocyanate). The initial amount of analyte is then calculated by subtracting the leftover amount from the total initial amount of the reagent added.
7. Why must the titration in the Volhard method be carried out in an acidic medium?
An acidic medium, provided by nitric acid (HNO₃), is crucial for two main reasons. Firstly, it prevents the precipitation of silver ions as silver hydroxide (AgOH) or silver oxide (Ag₂O), which would occur in neutral or alkaline solutions, leading to inaccurate results. Secondly, the indicator (ferric ion, Fe³⁺) would also precipitate as ferric hydroxide (Fe(OH)₃) if the solution were not acidic, preventing it from functioning correctly to signal the endpoint.
8. What is the 'Modified Volhard Method' and why is it necessary for analysing chloride ions?
The Modified Volhard Method is a necessary adjustment when determining chloride ions. This is because the precipitate formed, silver chloride (AgCl), is more soluble than the precipitate formed during the back titration, silver thiocyanate (AgSCN). Consequently, the thiocyanate titrant can react with the AgCl precipitate, displacing chloride ions (AgCl + SCN⁻ ⇌ AgSCN + Cl⁻), which results in a fading endpoint and an overestimation of the chloride concentration. The modification involves one of two techniques before back-titrating:
- Filtering the AgCl precipitate out of the solution.
- Adding an immiscible organic liquid like nitrobenzene to coat the AgCl particles, preventing them from reacting with the thiocyanate solution.
9. What are some common sources of error in the Volhard method?
Common sources of error in the Volhard method include:
- Forgetting to use the modified procedure for chloride analysis, leading to a fading endpoint and inaccurate results due to the conversion of AgCl to AgSCN.
- Performing the titration in a solution that is not sufficiently acidic, causing the precipitation of silver or ferric hydroxides.
- Inaccurate standardisation of the AgNO₃ or KSCN solutions.
- Adding too much indicator, which can consume a noticeable amount of the titrant.
- Difficulty in precisely detecting the first appearance of the permanent faint red colour at the endpoint.
