Answer
Reversibility of Electrophilic Aromatic Substitution (EAS) Reactions
EAS reactions involve the substitution of a hydrogen atom on an aromatic ring with an electrophile. Some of these reactions are reversible, while most are irreversible under normal conditions. Whether an EAS reaction is reversible depends on factors like the stability of the electrophile, reaction intermediates, and reaction conditions (e.g., heat, catalysts, etc.).
✔ Reversible EAS Reactions
1. Sulfonation (with H2SO4/SO3)
This is the most notable reversible EAS reaction.
Forward Reaction: Benzene + SO3/H2SO4 → Benzenesulfonic acid
Reverse Reaction: Heating benzenesulfonic acid with dilute H2SO4 and steam removes the sulfonic group, regenerating benzene.
Why Reversible? The sulfonation equilibrium is sensitive to temperature and water concentration. Desulfonation is favored at high temperatures and in the presence of water.
2. Friedel–Crafts Alkylation (limited reversibility)
Under certain conditions, especially with benzylic or tertiary carbocations, rearrangements or reverse alkylations (cleavage of C–C bonds) can occur.
Note: It is not fully reversible under standard lab conditions but can undergo competing dealkylation reactions at high temperatures or in acidic media.
✘ Irreversible EAS Reactions
1. Nitration
Benzene + HNO3/H2SO4 → Nitrobenzene
Why Irreversible? The nitro group is strongly electron-withdrawing and stabilizes the product, making the reverse reaction (denitration) extremely difficult and non-spontaneous under normal conditions.
2. Halogenation (Cl2/Br2 + Lewis acid)
These reactions are not reversible because the C–X bond formed is stable and strong, and removing the halogen is not favorable.
3. Friedel–Crafts Acylation
The acylated aromatic ring is stabilized by resonance and cannot regenerate the acylium ion easily. Hence, the reaction is not reversible.
🌟 Summary Table
| Reaction Type | Reversible? | Remarks |
|---|---|---|
| Sulfonation | Yes | Reversible via desulfonation with heat and water |
| Nitration | No | Nitrobenzene is too stable for reverse reaction |
| Halogenation | No | Halogen–carbon bond is stable |
| Friedel–Crafts Alkylation | Partially | Some reverse reactions can occur under harsh conditions |
| Friedel–Crafts Acylation | No | Highly stabilized acylated product |
✅ Final Verdict
The only truly reversible EAS reaction under laboratory conditions is: Sulfonation