Question

Which of the following is easily nitrated using a mixture of \(HNO_3\) and \(H_2SO_4\):

• A. Toluene
• B. Fluorobenzene
• C. Chlorobenzene
• D. Nitrobenzene

Hint:

Toluene undergoes nitration faster than benzene due to its electron-donating methyl group (+I effect), increasing its reactivity in electrophilic aromatic substitution reactions.

Answer 1

The Correct Option is A

Step 1: Understanding the nitration reaction. Nitration is an electrophilic aromatic substitution reaction where a benzene derivative reacts with a mixture of concentrated nitric acid (\(HNO_3\)) and sulfuric acid (\(H_2SO_4\)) to introduce a nitro group (-NO\(_2\)) onto the aromatic ring. 

Step 2: Reactivity of the given compounds. - Toluene (\(C_6H_5CH_3\)) has a +I (electron-donating) methyl group, which increases the electron density on the benzene ring, making it highly reactive towards nitration. 
- Fluorobenzene (\(C_6H_5F\)) has a fluorine atom, which has both electron-donating and electron-withdrawing effects, but its overall influence is deactivating. 
- Chlorobenzene (\(C_6H_5Cl\)) is less reactive than benzene due to chlorine’s -I effect (electron withdrawal). 
- Nitrobenzene (\(C_6H_5NO_2\)) has a strong -I and -M effect (electron-withdrawing), making it very deactivated toward further nitration.

Conclusion. Since toluene is the most activated due to the electron-donating methyl group, it undergoes nitration most easily.

Answer 2

Nitration with a mixture of \( HNO_3 \) and \( H_2SO_4 \) is an electrophilic aromatic substitution, so the ring that is easiest to nitrate is the one with the most electron-rich benzene ring. Let's rank each compound by how its substituent affects ring electron density.

1. Toluene: The methyl group on toluene pushes electron density into the ring through the +I (electron-donating) effect. This raises the electron density at the ortho and para positions, making the ring far more attractive to the electrophilic nitronium ion (\( NO_2^+ \)) than plain benzene.
2. Fluorobenzene: Fluorine withdraws electron density inductively through the sigma bond but donates it back into the ring through resonance from its lone pairs. The inductive withdrawal dominates overall, so the ring is net deactivated compared to benzene, even though fluorine still directs to the ortho and para positions.
3. Chlorobenzene: Like fluorine, chlorine withdraws electron density inductively (-I effect) more strongly than it donates by resonance, leaving the ring deactivated and slower to react with the nitronium ion than toluene.
4. Nitrobenzene: The nitro group is strongly electron-withdrawing by both induction and resonance (-I and -M), pulling electron density away from the ring. This makes nitrobenzene the least reactive of the four toward further nitration, needing much harsher conditions.

Since the methyl group is the only substituent among the four that genuinely donates electron density into the ring, toluene's ring is the most electron-rich and reacts fastest with the nitrating mixture.

So the correct answer is Toluene.