1. Reaction
Reaction
Conditions
- Reagents: Concentrated Nitric Acid (
) and Concentrated Sulfuric Acid ( ). - Conditions: Reflux at 25°C to 60°C (Usually exactly 50°C)
- Electrophile: Nitronium ion (
) Effect of higher temperature 60°C, multiple substitutions occur (e.g. 1,3-dinitrobenzene).
If temperature >
2. Mechanism:
Nitration: Electrophilic Substitution
- The nitronium ion is attracted to the delocalised
bond of benzene. - It eventually breaks the ring of electrons and forms a dative
bond to one of the carbon atoms of the ring. - The remaining four
electrons are spread over the remaining five carbon atoms of the ring, in a five-centre delocalised orbital, forming an intermediate carbocation. - The carbocation loses a proton, the electrons of the former
bond reform the sextet of electrons.
3. Formation of Electrophile
Formation of
Step 1: Protonation of Nitric Acid
is a strong acid, but is even stronger. - The
acts as proton donor while the is forced to act as proton acceptor. - Nitric acid has a hydroxyl group (-OH) attached to its central nitrogen. The oxygen atom in this -OH group has lone pairs of electrons.
- One of the lone pairs forms a dative bond with the
donated by . This forms protonated nitric acid (an intermediate) and leaves behind hydrogen sulfate ion (
).
Step 2: Formation of Nitronium Ion (
)
- The protonated nitric acid is highly unstable because of the positive charge on an oxygen atom.
- The bond between oxygen and nitrogen breaks, and the electrons go entirely to the oxygen.
An
molecule breaks away, leaving behind a highly reactive Nitronium Ion ( ).
Step 3: Protonation of the byproduct water
- The reaction is taking place in an extremely acidic environment.
- The neutral water molecule produced in step 2 cannot exist for long in this environment.
- It immediately acts as a base and grabs a proton form a second molecule of sulfuric acid.
This forms a hydronium ion (
) and a second hydrogen sulfate ion ( ). Importance of this step
- In reality, the reactions of all 3 steps are reversible.
- According to Le Chatelier’s Principle:
- By constantly turning the byproduct water into hydronium, the water is removed from the equilibrium of Step 2.
- This prevents the reaction from going backwards and drives the entire process forward, ensuring a large, steady supply of
electrophile for the benzene to react with.
4. Regeneration of Catalyst
Regeneration of Catalyst
produced by deprotonation of benzene combines with hydrogen sulfate ion to form sulfuric acid.
Catching Point: Dilution
- While the
is completely regenerated chemically, there is a practical issue in the lab. - Generation of electrophile produces water (
/ ) as byproduct. - As the reaction runs, the water begins to dilute the concentrated sulfuric acid.
- Eventually, the acid mixture becomes too watered down to effectively protonate the nitric acid, the reaction slows to a halt.