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Free questions · JEE Advanced · Chemistry

Benzene and Haloarenes: questions with solutions

3 free questions from this chapter (the full set has 30). Marking: Multi-correct: +4 all correct, +1 per correct option if none wrong, −2 if any wrong · Numerical: +4 / 0 · Matching: +3 / −1.

Question 1Level 2 · ExamSingle correct
Suppose benzene had a single fixed Kekulé structure, with three localised C=C bonds and three localised C–C single bonds that never interconvert. Treat two dibromo derivatives CHBr as different whenever no symmetry operation of that fixed structure converts one into the other. If is the number of dibromobenzenes this model predicts and is the number actually known, then is ______. Exactly one option is correct.
A(3, 3)
B(4, 3)
C(5, 3)
D(4, 2)
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Answer: B

Number the ring C1–C6 with fixed double bonds C1=C2, C3=C4, C5=C6. 1,2-Dibromo: the two Br atoms can sit on carbons joined by a double bond (C1, C2) or by a single bond (C2, C3). No symmetry of the fixed structure turns a double bond into a single bond, so these are two different compounds. 1,3-Dibromo: every 1,3 pair (e.g. C1, C3) is joined through one double and one single bond; all such pairs are equivalent, so one compound. 1,4-Dibromo: all para pairs are equivalent, so one compound. Fixed model: = 2 + 1 + 1 = 4. In reality all six C–C bonds are identical (139 pm); the two Kekulé structures are only resonance contributors to one delocalised structure, so there is a single 1,2-dibromobenzene. Known isomers: ortho, meta, para, = 3. Answer: (B). Difficulty: Hard
Question 2Level 2 · ExamOne or more correct
Which of the following statements about benzene, CH, are correct? One or more options may be correct.
AAll twelve atoms lie in one plane, and every carbon atom is hybridised.
BThe two Kekulé structures of benzene are distinct molecules in rapid equilibrium, which is why all C–C bond lengths appear equal.
CBenzene has 12 bonds, and its six electrons are delocalised over all six ring carbons.
DThe enthalpy of hydrogenation of benzene to cyclohexane is more negative than three times the enthalpy of hydrogenation of cyclohexene to cyclohexane.
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Answer: A, C

(A) Correct. Each carbon forms three bonds (two C–C, one C–H) using orbitals; the ring is a regular planar hexagon and all 12 atoms lie in one plane. (B) Incorrect. The two Kekulé structures are resonance contributors, not real molecules in equilibrium. Benzene has a single structure (the resonance hybrid); nothing interconverts. (C) Correct. bonds: 6 C–C + 6 C–H = 12. The six unhybridised p orbitals (one per carbon) overlap all round the ring, and their six electrons are delocalised above and below the plane. (D) Incorrect. The enthalpy of hydrogenation of benzene (about –208 kJ mol) is much less negative than three times that of cyclohexene (3 × –120 = –360 kJ mol). The difference (about 150 kJ mol) is the resonance (delocalisation) energy. Answer: (A), (C). Difficulty: Hard
Question 3Level 3 · AdvancedSingle correct
Consider the following statements. I. All six carbon–carbon bonds in benzene have the same length (about 139 pm), which lies between the C–C single bond length (154 pm) and the C=C double bond length (134 pm). II. The enthalpy of hydrogenation of benzene to cyclohexane is approximately three times the enthalpy of hydrogenation of cyclohexene to cyclohexane. III. In the presence of ultraviolet light, benzene adds chlorine to give hexachlorocyclohexane, CHCl. IV. Benzene readily decolourises cold, dilute, alkaline KMnO (Baeyer's reagent) at room temperature. Which of the statements are correct? Exactly one option is correct.
AI only
BI, II and III only
CI and III only
DI, III and IV only
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Answer: C

I. Correct. X-ray data show a regular hexagon with all C–C bonds 139 pm, intermediate between 154 pm and 134 pm, as expected for delocalised bonding. II. Incorrect. Cyclohexene: about –120 kJ mol; three times this is about –360 kJ mol, but benzene releases only about –208 kJ mol. The shortfall (about 150 kJ mol) measures the resonance stabilisation. III. Correct. Under UV light (a radical reaction) benzene adds three molecules of Cl to give CHCl (benzene hexachloride). This addition is the exception, needing light energy; in the dark with a Lewis acid, benzene undergoes substitution instead. IV. Incorrect. Because of aromatic stabilisation, benzene does not behave like an alkene: it does not decolourise Baeyer's reagent or bromine water under ordinary conditions. Correct: I and III only. Answer: (C). Difficulty: Very hard

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