Question

Aromatic electrophilic substitution reaction that is reversible is

• A. nitration
• B. chlorination
• C. sulphonation
• D. alkylation
• E. acylation

Hint:

If you want to remove a \(-SO_3H\) group from a benzene ring, just add dilute \(H_2SO_4\) and pass steam through the mixture!

Answer 1

The Correct Option is C

Concept: Electrophilic Aromatic Substitution (EAS) involves the replacement of a hydrogen atom on an aromatic ring with an electrophile. While most of these reactions proceed to completion (irreversible), others can be driven backward under specific conditions.

Step 1: Thermodynamics of EAS reactions.
In reactions like Nitration (A) or Halogenation (B), the bond formed between the carbon and the substituent (C-N or C-Cl) is significantly stronger than the C-H bond that was broken. The energy released makes the forward reaction highly favorable and the reverse reaction virtually impossible under standard conditions.

Step 2: The unique case of Sulphonation.
Sulphonation involves the reaction of benzene with sulfur trioxide (\(SO_3\)) in concentrated sulfuric acid. \[ C_6H_6 + H_2SO_4(fuming) \rightarrow C_6H_5SO_3H + H_2O \]
• The C-S bond is relatively weak compared to C-C or C-N bonds.
• The activation energy for the reverse reaction is low enough that it can occur.
• The reaction exists in a state of dynamic equilibrium.

Step 3: Mechanism of Reversibility (Desulphonation).
By applying Le Chatelier's Principle, we can reverse the reaction.
• Forward: Uses concentrated/fuming acid to drive the substitution.
• Reverse: Uses dilute acid and superheated steam. The high concentration of water drives the equilibrium back toward the starting hydrocarbon, releasing the \(-SO_3H\) group as \(H_2SO_4\).

Step 4: Synthetic Utility.
Because it is reversible, the sulphonic acid group is often used as a "blocking group." It can be placed on a specific position of the ring to prevent other substituents from entering there, and then easily removed once the desired synthesis is complete.