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Understanding Kolbe Reaction Mechanism

Understanding Kolbe Reaction Mechanism

Kolbe reaction (or Kolbe electrolysis) is used to prepare symmetrical alkanes from sodium or potassium salts of carboxylic acids through electrolytic decarboxylation. When carboxylic acid salts of sodium (or potassium) undergo decarboxylation in an electrolytic way, alkanes are produced at the anode.

Let us understand how the mechanism of the Kolbe reaction happens. Read on to learn more about it in detail.

What is the Kolbe Reaction?

In aqueous electrolysis, the carboxylate ion loses CO₂ at the anode, generating an alkyl radical. Two such radicals couple to form an alkane containing double the number of carbon atoms of the parent acid.

Looking at the general reaction, we see:

2 RCOONa → R–R (alkane) + 2 CO₂ + H₂

Breakdown of Kolbe Reaction Mechanism

The Kolbe Reaction Mechanism consists broadly of 3 steps.

Here is a complete understanding of its constituent steps.

Step 1: Ionisation in aqueous solution

The reaction is as follows:

RCOONa → RCOO⁻ + Na⁺

Step 2: Decarboxylation (Anodic oxidation)

At the anode, there is a loss of one electron of the carboxylate ion

The reaction is as follows:

RCOO⁻ → R· (alkyl radical) + CO₂

Step 3: Radical coupling

There is combining of two alkyl radicals to form an alkane

The reaction is as follows:

R· + R· → R–R

Because of this, at the cathode, H₂ gas is formed by H⁺ ions (from water) that are reduced.

Example

We can look at the electrolysis of sodium acetate:

T

 

Electrolysis of CH₃COONa

Details At A Glance

Property Details
Reaction type Electrolytic decarboxylation
Reactant Sodium/potassium salt of carboxylic acid
Main product Alkane (R–R)
By-products CO₂, H₂ (at cathode)
Anode reaction Oxidation of carboxylate ion
Cathode reaction Reduction of H⁺ → H₂
Key intermediate Alkyl radical (R·)

Important Notes

Thus, it is important that for the Kolbe Reaction Mechanism, we remember some crucial things.

  • Only alkanes with an even number of carbon atoms are given by the Kolbe reaction.
  • A mixture of alkanes (including cross-coupled products) is obtained if we mix two different carboxylate salts.
  • The reaction proceeds via a radical mechanism.
  • Reaction is not suitable for preparing methane (CH₄), since only symmetrical higher alkanes are formed.

Summing Up

The Kolbe Mechanism reaction involves: (i) ionisation of sodium/potassium salt of a carboxylic acid, (ii) anodic decarboxylation is done in order to generate an alkyl radical, (iii) radical coupling is done to form an alkane. Thus, it can be concluded that this reaction provides a useful method for preparing symmetrical alkanes.

Frequently Asked Questions

Q1. What is formed in the Kolbe reaction?

Symmetrical alkanes (R–R), along with CO₂ and H₂.

Q2. Why is CO₂ released in the reaction?

Because of the decarboxylation that the carboxylate ion undergoes at the anode.

Q3. Can Kolbe reactions form unsymmetrical alkanes?

Yes, if a mixture of different carboxylate salts is used, but the result is a mixture (not selective).

Q4. What type of mechanism is followed in the Kolbe reaction?

A free radical mechanism.

Q5. What is an industrial use of Kolbe electrolysis?

Kolbe electrolysis is widely used for the preparation of higher alkanes at the lab level as well as on an industrial level.

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