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Q7.18 (i):
Explain the following with an example. Kolbe’s reaction.

Solution :

Theoretical Foundation of Kolbe’s Reaction

Kolbe’s reaction (also known as the Kolbe-Schmitt reaction) is a chemical process involving the carboxylation of a phenoxide ion. When phenol is treated with sodium hydroxide, it forms sodium phenoxide, which undergoes electrophilic aromatic substitution with carbon dioxide under high pressure and temperature to produce salicylic acid as the primary product.

Step 1: Formation of Sodium Phenoxide

Phenol ($C_6H_5OH$) is a weak acid. Upon reaction with a strong base like sodium hydroxide ($NaOH$), it loses a proton to form the resonance-stabilized phenoxide ion, which exists as sodium phenoxide in the reaction mixture.

$C_6H_5OH + NaOH \rightarrow C_6H_5O^-Na^+ + H_2O$ [Per Acid-Base Neutralization]

Step 2: Electrophilic Attack by Carbon Dioxide

The phenoxide ion is highly reactive toward electrophilic aromatic substitution because the oxygen atom donates electron density into the ring through the $+M$ effect, specifically activating the ortho and para positions. Carbon dioxide ($CO_2$), acting as a weak electrophile, attacks the ortho position of the phenoxide ion.

This is performed at a temperature of approximately $398\text{ K}$ and a pressure of $4\text{--}7\text{ atm}$.

Step 3: Protonation and Product Formation

The intermediate formed is a salicylate. Subsequent acidification with a mineral acid (such as $HCl$) yields salicylic acid ($2\text{-hydroxybenzoic acid}$).

$C_6H_4(O^-Na^+)(CO_2Na) + 2HCl \rightarrow C_6H_4(OH)(COOH) + 2NaCl$

Reaction Schematic

Phenol 1. NaOH 2. CO₂, 4-7 atm 3. H₃O⁺ Salicylic Acid

Summary Table of Reaction Parameters

Parameter Specification
Substrate Phenol
Reagents $NaOH$, $CO_2$, followed by $H^+$ workup
Temperature $398\text{ K}$
Pressure $4\text{--}7\text{ atm}$
Major Product Salicylic Acid ($2\text{-hydroxybenzoic acid}$)

Final Solution: Kolbe’s reaction is the carboxylation of phenoxide ions with carbon dioxide at $398\text{ K}$ and $4\text{--}7\text{ atm}$ pressure to yield $2\text{-hydroxybenzoic acid}$ (salicylic acid) upon acidification.


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