Electrophilic Substitution in Benzene - UNSOLVED PRACTICE SET

Q7. Write the mechanism for the nitration of benzene, showing the formation of the electrophile and the sigma complex (arenium ion).

Q8. Why is AlCl₃ used as a catalyst in Friedel-Crafts reactions? What is its role in generating the electrophile?

Q9. Explain why aniline undergoes electrophilic substitution at the ortho and para positions, despite the –NH₂ group withdrawing electrons inductively.

Q10. Why does benzene not undergo Friedel-Crafts alkylation when a strong deactivating group (like –NO₂) is already present on the ring?

Q11. Your school lab prepares nitrobenzene from benzene. Why is the temperature maintained at 50-60°C and not higher? What would happen if the temperature exceeds this range?

Q12. Why is halogenation of benzene carried out in the presence of Fe or FeCl₃, whereas halogenation of alkanes requires UV light?

Q13. Discuss the mechanism of electrophilic aromatic substitution (EAS) with reference to:

(a) Nitration — formation of NO₂⁺ electrophile, mechanism, and product

(b) Sulphonation — formation of SO₃ electrophile, reversible nature, and uses

(c) Halogenation — role of Lewis acid catalyst, formation of X⁺ electrophile

(d) Friedel-Crafts alkylation and acylation — formation of carbocation and acylium ion, limitations

For each reaction, show the complete mechanism including the sigma complex intermediate.

Q14. Explain the directive influence of substituents in electrophilic aromatic substitution:

(a) Ortho-para directing groups — activating (–OH, –NH₂, –OCH₃, –R) and deactivating (–Cl, –Br)

(b) Meta-directing groups — deactivating (–NO₂, –CN, –CHO, –COOH, –SO₃H)

(c) Explanation using resonance and inductive effects

(d) Why halogens are ortho-para directing despite being deactivating

Illustrate with nitration of toluene, chlorobenzene, and nitrobenzene.

Q15. Electrophilic substitution reactions are the backbone of the dye and pharmaceutical industries in India. Discuss:

(a) How nitration and subsequent reduction of nitrobenzene produces aniline, a precursor for dyes and medicines

(b) The synthesis of paracetamol from phenol through electrophilic substitution and subsequent steps

(c) Why the Indian textile industry (Surat, Tirupur) relies heavily on azo dyes made from substituted benzene derivatives

(d) The environmental concerns of electrophilic substitution reactions (acid waste, heavy metal catalysts) and green chemistry alternatives

Q16. In the nitration of toluene (C₆H₅CH₃), three products are possible: ortho-nitrotoluene, meta-nitrotoluene, and para-nitrotoluene.

(a) Draw the structures of all three isomers.

(b) The observed product ratio is ortho : meta : para = 58 : 4 : 38. Explain why the meta product is formed in such a small amount.

(c) If 100 g of toluene is nitrated with 80% conversion, calculate the mass of each product formed.

[Given: Molar masses: Toluene = 92 g/mol, Nitrotoluene = 137 g/mol]

Q17. The Friedel-Crafts acylation of benzene with acetyl chloride (CH₃COCl) gives acetophenone.

(a) Write the balanced equation and show the mechanism.

(b) Calculate the mass of acetophenone produced from 50 g of benzene and excess acetyl chloride. (Assume 75% yield)

(c) Why is Friedel-Crafts acylation preferred over alkylation for preparing alkylbenzenes with straight chains?

[Given: Molar masses: Benzene = 78 g/mol, Acetophenone = 120 g/mol]

Q18. A dye manufacturing unit in Surat produces 500 kg of an azo dye per day. The synthesis involves diazotisation of aniline followed by coupling with phenol.

(a) Write the two-step synthesis showing the electrophilic substitution in the coupling step.

(b) If aniline costs ₹ 180 per kg and phenol costs ₹ 120 per kg, and the stoichiometry requires 1:1 molar ratio, calculate the raw material cost for producing 500 kg of dye. (Molar mass of dye = 226 g/mol)

(c) The effluent from this process contains unreacted aniline. Calculate the COD (chemical oxygen demand) contribution if 1 kg of aniline requires 3.2 kg of oxygen for complete oxidation.