Benzene Structure Resonance Aromaticity - UNSOLVED PRACTICE SET

Q7. Draw the two Kekulé structures of benzene and explain why neither structure alone represents the true structure of benzene.

Q8. Why does benzene not decolourise bromine water, despite having three double bonds? What does this indicate about its stability?

Q9. Explain why the heat of hydrogenation of benzene (-208 kJ/mol) is much less than the calculated value for cyclohexatriene (-358.4 kJ/mol).

Q10. What is Hückel's rule? Use it to explain why benzene is aromatic but cyclobutadiene is not.

Q11. Your teacher heats benzene with hydrogen in the presence of Ni catalyst. What product is formed, and why does this reaction require more vigorous conditions than hydrogenation of alkenes?

Q12. Why is benzene planar, and why does each carbon atom in benzene use sp² hybridisation?

Q13. Discuss the structure of benzene:

(a) Kekulé structures and their limitations

(b) Resonance hybrid structure — delocalisation of π electrons

(c) Evidence for resonance: bond lengths, heat of hydrogenation, and lack of addition reactions

(d) Molecular orbital picture of benzene — formation of π molecular orbitals

(e) Draw all resonance structures of benzene and explain the concept of resonance energy

Q14. Explain the concept of aromaticity in detail:

(a) Definition of aromatic, anti-aromatic, and non-aromatic compounds

(b) Hückel's rule (4n + 2 π electrons) — conditions for aromaticity

(c) Planarity, complete delocalisation, and cyclic nature as essential criteria

(d) Examples of aromatic compounds (benzene, naphthalene, cyclopentadienyl anion) and anti-aromatic compounds (cyclobutadiene, cyclopentadienyl cation)

Q15. Benzene and its derivatives are central to India's chemical industry. Discuss:

(a) Why the petrochemical complex in Gujarat (Hazira, Dahej) is a major producer of benzene and xylenes

(b) The use of benzene-derived products in India's dye and pigment industry (Azo dyes for textiles)

(c) Why benzene is classified as a carcinogen and the safety measures in Indian chemical plants

(d) The shift towards using toluene instead of benzene in some applications due to lower toxicity

Q16. The heat of hydrogenation of cyclohexene is -120 kJ/mol. The heat of hydrogenation of benzene is -208 kJ/mol.

(a) Calculate the expected heat of hydrogenation if benzene had three isolated double bonds (like cyclohexatriene).

(b) Calculate the resonance energy of benzene from this data.

(c) Explain what this resonance energy means in terms of benzene's stability and reactivity.

Q17. Consider the following compounds and determine whether they are aromatic, anti-aromatic, or non-aromatic:

(a) Benzene (C₆H₆)

(b) Cyclobutadiene (C₄H₄)

(c) Cyclopentadienyl anion (C₅H₅⁻)

(d) Cycloheptatrienyl cation (C₇H₇⁺)

(e) Cyclooctatetraene (C₈H₈)

For each compound:

(i) Count the number of π electrons

(ii) Check if it follows Hückel's rule (4n + 2)

(iii) Determine its aromatic character

Q18. A chemical plant produces benzene by catalytic reforming of naphtha. The plant processes 10,000 kg of naphtha containing 30% aromatic precursors.

(a) If the conversion efficiency to benzene is 40%, calculate the mass of benzene produced.

(b) If benzene is sold at ₹ 65 per kg, calculate the revenue from benzene production.

(c) The plant aims to reduce benzene emissions to below 1 ppm in workplace air. If the plant volume is 50,000 m³, calculate the maximum allowable mass of benzene in the air. (Density of benzene vapour at STP = 3.48 g/L)

[Given: Molar mass of benzene = 78 g/mol]