When it comes to organic chemistry reactions, choosing the right acid can make or break the reaction. A common query among students and enthusiasts is:
“Why don’t we use sulphuric acid (H₂SO₄) when converting alcohols to alkyl iodides using potassium iodide (KI)?”
Let’s break it down step by step.
🔹 The Reaction Context
The reaction we’re talking about is the conversion of alcohols (R–OH) into alkyl iodides (R–I) using KI. The general reaction is:
R–OH+KI+acid→R–I+H2OR–OH + KI + acid → R–I + H_2O
Here, an acid is needed to activate the alcohol, making it more reactive.
🔹 Why Sulphuric Acid Doesn’t Work
Although sulphuric acid is a strong acid, it has a major drawback: it is a strong oxidizing agent.
When you add H₂SO₄ to KI, the iodide ions (I⁻) get oxidized into iodine (I₂) instead of reacting with the alcohol:
2I−+H2SO4→I2+SO2+2H2O2 I^- + H_2SO_4 → I_2 + SO_2 + 2 H_2O
Result:
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The solution turns brown due to free iodine.
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KI is destroyed, reducing the amount of iodide available.
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The alcohol fails to convert into the desired alkyl iodide.
So using sulphuric acid ruins the reaction.
🔹 The Ideal Acid Choice
Instead of sulphuric acid, chemists use phosphoric acid (H₃PO₄) or sometimes hydroiodic acid (HI).
Why phosphoric acid?
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It is a non-oxidizing acid, so it doesn’t attack iodide ions.
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It effectively protonates the alcohol, forming a better leaving group (R–OH₂⁺).
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This ensures a smooth reaction to form the alkyl iodide.
Reaction with phosphoric acid:
R–OH+KI+H3PO4→R–I+H2OR–OH + KI + H_3PO_4 → R–I + H_2O
🔹 Quick Comparison Table
| Acid Used | Effect on KI | Result |
|---|---|---|
| H₂SO₄ | Oxidizes I⁻ → I₂ | Alkyl iodide not formed |
| H₃PO₄ | Non-oxidizing | Smooth formation of R–I |
| HCl | Weak acid | Less effective |
| HI | Provides I⁻ directly | Ideal, high yield |
🔹 Conclusion
Sulphuric acid is avoided in the conversion of alcohols to alkyl iodides using KI because it oxidizes iodide ions, stopping the reaction. Phosphoric acid or HI are preferred, ensuring a high-yield formation of the desired product.
Key takeaway: Always consider both acidity and oxidizing properties when choosing an acid for substitution reactions.
