Nitration of acetophenone with nitric acid and sulfuric acid occurs
a. faster than nitration of benzene and gives mostly m-nitroacetophenone.
b. faster than nitration of benzene and gives a mixture containing mostly o- and p-nitroacetophenone.
c. slower than nitration of benzene and gives a mixture containing mostly o- and p-nitroacetophenone.
d. slower than nitration of benzene and gives mostly m-nitroacetophenone.
The correct answer and explanation is :
The correct answer is:
d. slower than nitration of benzene and gives mostly m-nitroacetophenone.
Systematic Approach to Solving This Problem:
- Identify the Reactant and the Reaction Type:
- The reactant is acetophenone (C₆H₅COCH₃), which consists of a benzene ring with a ketone (-COCH₃) substituent.
- The reaction is electrophilic aromatic substitution (EAS), where a nitro group (-NO₂) is introduced into the benzene ring using a nitrating mixture (HNO₃ + H₂SO₄).
- Determine the Effect of the Ketone (-COCH₃) on Reactivity:
- The -COCH₃ group is an electron-withdrawing group (EWG) because the carbonyl oxygen withdraws electron density via both inductive (-I effect) and resonance (-R effect).
- This deactivates the benzene ring by reducing its electron density, making it less reactive than benzene itself.
- Since the ring is deactivated, nitration occurs slower than nitration of benzene.
- Determine the Directing Effect of the Ketone (-COCH₃):
- Electron-withdrawing groups deactivate the ring and direct incoming electrophiles to the meta position because the ortho and para positions develop partial positive charges due to resonance effects, making them less favorable for electrophilic attack.
- The meta position remains the most electron-rich relative to the rest of the ring, so substitution occurs there preferentially.
- Eliminate Incorrect Answer Choices:
- Since nitration is slower, eliminate options a and b (which suggest faster nitration).
- Since nitration occurs at the meta position, eliminate option c (which suggests mostly ortho/para products).
- This confirms that option d is correct.
Conclusion:
Acetophenone undergoes nitration slower than benzene due to the electron-withdrawing nature of the -COCH₃ group. The major product is m-nitroacetophenone because the ketone group directs substitution to the meta position.