Convert Ethyne to Acetanilide: The Acetylation of Aniline
One of the most important transition points in Organic Chemistry is moving from an open-chain Aliphatic compound to a closed-ring Aromatic compound.
In this conversion, we start with Ethyne (Acetylene gas) and must build an aromatic ring from scratch, populate it with an amino group, and finally "protect" that amino group to form Acetanilide. Let's trace this classic 4-step JEE/NEET roadmap.
Video Tutorial: The 4-Step Synthesis
Watch Abhishek Sengar sir from CHEMCA expertly map out the reagents for cyclic polymerization, nitration, reduction, and acetylation.
Step-by-Step Conversion Roadmap
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Cyclic Polymerization (Aliphatic to Aromatic):
We pass 3 moles of Ethyne gas (CH≡CH) through a Red Hot Iron (Fe) tube at 873 K. The three molecules trimerize, folding into a stable 6-membered aromatic ring.
Intermediate A: Benzene (C6H6). -
Nitration:
We treat the Benzene with a nitrating mixture: Concentrated HNO3 + Concentrated H2SO4. The sulfuric acid helps generate the powerful Nitronium ion electrophile (NO2+).
Intermediate B: Nitrobenzene (Ph-NO2). -
Reduction (Nitro to Amino):
To convert the -NO2 group to an -NH2 group, we need a reducing agent. We use a metal with an acid, commonly Sn/HCl or Fe/HCl. (Note: Fe/HCl is preferred in industry because the FeCl2 formed gets hydrolyzed to release HCl back, so only a tiny amount of HCl is needed to initiate the reaction!)
Intermediate C: Aniline (Ph-NH2). -
Acetylation (Protection Step):
Finally, we treat Aniline with an acylating agent like Acetyl Chloride (CH3COCl) or Acetic Anhydride in the presence of a mild base like Pyridine. One hydrogen from the -NH2 group is replaced by the acetyl group (-COCH3).
Final Product: Acetanilide (Ph-NH-CO-CH3).
Fig: Notice the transition from a linear alkyne to a highly substituted aromatic ring.
The amino group (-NH2) is a very powerful activating group. If you try to react Aniline directly with Bromine water, it is so reactive that you instantly get a tri-substituted product (2,4,6-tribromoaniline).
By reacting it with Acetyl Chloride, we attach a carbonyl (C=O) group to the nitrogen. The lone pair on the nitrogen now enters into resonance with the carbonyl oxygen, making it less available to donate into the benzene ring. This "Protects" the ring, reducing its reactivity so you can successfully perform monosubstitution (getting just ortho or para products)!
Practice Questions for JEE & NEET
Ensure you grasp the specific industrial and mechanistic reasons behind these reagents!
Question 1: In Step 3, Nitrobenzene is reduced to Aniline. While Sn/HCl is a common laboratory reagent, why is Fe/HCl the strictly preferred method for large-scale industrial synthesis?
Answer: Because the reaction becomes self-sustaining and cheaper.
Reasoning:
When Iron reacts with HCl, it forms Iron(II) Chloride (FeCl2). In the aqueous reaction mixture, this FeCl2 undergoes hydrolysis to regenerate HCl.
Because the acid is continuously regenerated during the reaction, you only need to add a very tiny, catalytic amount of HCl to kickstart the entire industrial batch. This makes Fe/HCl significantly cheaper and more efficient than Sn/HCl.
Question 2: During the Acetylation step (Aniline to Acetanilide), a base like Pyridine is always added to the reaction mixture. What is the specific chemical role of Pyridine here?
Answer: To neutralize the HCl byproduct and push the reaction forward.
Reasoning:
The reaction is: Ph-NH2 + CH3COCl → Ph-NH-COCH3 + HCl.
This is an equilibrium reaction that produces Hydrochloric Acid as a byproduct. According to Le Chatelier's Principle, if the HCl builds up, the reaction might reverse. By adding Pyridine (a base), the HCl is immediately neutralized as soon as it forms, preventing the reverse reaction and driving the synthesis of Acetanilide to completion.
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