Sulfuric Acid ($H_2SO_4$)
The master of dehydration, sulfonation, and strong acid catalysis.
Sulfuric Acid ($H_2SO_4$) is one of the most widely used industrial chemicals. In organic chemistry, it serves three primary roles depending on its concentration and temperature: a powerful dehydrating agent, a strong acid catalyst, and an electrophilic sulfonating agent for aromatic rings.
1. Temperature-Dependent Dehydration of Alcohols
Alkenes vs. Ethers
When primary alcohols (like ethanol) are treated with concentrated $H_2SO_4$, the reaction pathway is strictly governed by temperature.
At high temperatures ($170^\circ C$ or $443 \ K$), the acid protonates the $-OH$ group, water leaves to form a carbocation, and a neighboring proton is removed to form an alkene (Saytzeff's Rule applies).
At a lower temperature ($140^\circ C$ or $413 \ K$) with an excess of primary alcohol, an $S_N2$ reaction dominates. An unprotonated alcohol attacks a protonated alcohol, yielding a symmetrical ether.
2. Aromatic Chemistry: Sulfonation & Nitration
Sulfonation and the Nitrating Mixture
Heating benzene with conc. $H_2SO_4$ (or fuming $H_2SO_4$ / Oleum) introduces the sulfonic acid group ($-SO_3H$). The actual electrophile is neutral Sulfur trioxide ($SO_3$).
Unique Feature: Reversibility!
Unlike most EAS reactions, sulfonation is reversible. Heating benzenesulfonic acid with dilute acid (desulfonation) yields benzene back.
In the nitration of benzene, concentrated $H_2SO_4$ is mixed with concentrated $HNO_3$. Because $H_2SO_4$ is the stronger acid, it forces $HNO_3$ to act as a base, protonating it to generate the powerful Nitronium ion electrophile ($NO_2^+$).
3. Hydration of Alkenes and Alkynes
Dilute H₂SO₄ as a Catalyst
Dilute Sulfuric acid provides the protons ($H^+$) necessary to initiate the addition of water across $\pi$ bonds.
Alkenes react with water in the presence of dilute $H_2SO_4$ to form alcohols. The proton adds to form the most stable carbocation, followed by water attack (Markovnikov's Rule).
Alkynes are less reactive toward electrophilic addition than alkenes. They require the addition of Mercuric Sulfate ($HgSO_4$) to the dilute $H_2SO_4$ to act as a catalyst. This hydration initially forms an unstable enol, which tautomerizes into a ketone (or aldehyde for ethyne).
4. Summary of Conditions and Products
| Substrate | Reagent / Condition | Action | Major Product |
|---|---|---|---|
| $1^\circ$ Alcohol | Conc. $H_2SO_4$, $\mathbf{170^\circ C}$ | Intramolecular Dehydration | Alkene |
| $1^\circ$ Alcohol | Conc. $H_2SO_4$, $\mathbf{140^\circ C}$ | Intermolecular Dehydration | Ether |
| Alkene | Dilute $H_2SO_4$ | Markovnikov Hydration | Alcohol |
| Alkyne | Dilute $H_2SO_4$ + $\mathbf{HgSO_4}$ | Kucherov Hydration | Ketone (Aldehyde if Ethyne) |
| Benzene | Fuming $H_2SO_4$ / Oleum | Electrophilic Sulfonation | Benzenesulfonic acid |
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