18A.1 Bonding and stability of benzene Quiz: Pearson Edexcel Chemistry, Unit 18

The 20 questions

1. Which statement describes the delocalised model of benzene?

   • Each carbon is sp2 hybridised, and overlapping p-orbitals above and below the ring form a ring of delocalised pi electrons
   • Each carbon forms four sigma bonds and no p-orbitals are involved in the ring, so the pi system is absent altogether
   • Alternating single and double bonds are fixed between specific carbon atoms in the ring, giving three localised pi bonds
   • Six separate double bonds are localised between pairs of carbon atoms, with no overlap of orbitals between neighbouring carbons

2. Which evidence supports the delocalised model of benzene over the Kekule model?

   • All six carbon-carbon bond lengths are equal and lie between typical single and double bond lengths
   • Benzene has a lower boiling temperature than cyclohexane, which shows that its pi electrons are weakly held in the ring
   • The molecular formula is C6H6, with a fixed number of double bonds that can be counted from the molecular formula alone
   • Benzene decolourises bromine water rapidly at room temperature, showing that its double bonds are very reactive

3. Why is the pi bonding in benzene described as delocalised?

   • The pi electrons are localised on the hydrogen atoms attached to the ring, which carry the negative charge across it
   • The p-electrons are shared over all six carbon atoms rather than located between two specific carbon atoms
   • The electrons are removed from the ring by the catalyst during each reaction and then returned at the end of the step
   • The electrons move freely out of the ring into the surrounding solvent, where they are held by the solvent molecules

4. The measured enthalpy of hydrogenation of benzene is -208 kJ mol-1. Cyclohexene has an enthalpy of hydrogenation of -120 kJ mol-1. What is the delocalisation energy of benzene?

   • 152 kJ mol-1
   • 360 kJ mol-1
   • 208 kJ mol-1
   • 88 kJ mol-1

5. Which list gives the bond lengths in order from longest to shortest?

   • C-C in ethane, then C=C in ethene, then C-C in benzene
   • C-C in benzene, then C-C in ethane, then C=C in ethene
   • C=C in ethene, then C-C in benzene, then C-C in ethane
   • C-C in ethane, then C-C in benzene, then C=C in ethene

6. Why does the benzene ring have six equal carbon-carbon bond lengths?

   • Each bond alternates rapidly between single and double bonds, so the ring has an average bond length in the middle
   • The ring is planar and each carbon is attached to a fixed hydrogen atom, which fixes each bond at the same length
   • The pi electrons are delocalised over all six carbon atoms, so each bond has partial double-bond character
   • Each bond has a bond order of two with no single-bond character, which makes all six bonds identical in strength

7. Which orbitals overlap to form the delocalised pi system in benzene?

   • d-orbitals on each carbon overlapping to form a ring of sigma bonds in the plane of the molecule
   • Adjacent p-orbitals on each sp2 carbon, overlapping sideways above and below the ring
   • sp3 hybrid orbitals pointing along the ring axis, which overlap end-on around the ring
   • s-orbitals on each carbon overlapping end-on around the ring, forming one continuous band

8. How many pi electrons are delocalised in benzene?

   • 6
   • 2
   • 3
   • 12

9. How many sigma bonds are there in one molecule of benzene, C6H6?

   • 12
   • 6
   • 9
   • 15

10. What does the circle inside the hexagon in the benzene symbol represent?

    • A single pi bond shared between two carbon atoms
    • A ring of six hydrogen atoms bonded to the carbons
    • Three fixed double bonds that alternate with single bonds
    • Six delocalised pi electrons spread evenly around the ring

11. What is the approximate C-C bond length in benzene?

    • 0.134 nm
    • 0.154 nm
    • 0.120 nm
    • 0.139 nm

12. Why is benzene described as planar with bond angles of about 120 degrees?

    • The ring is planar because of hydrogen bonding between adjacent hydrogen atoms
    • All six carbon atoms are sp2 hybridised, so the bond angles around each carbon are about 120 degrees
    • All six carbon atoms are sp3 hybridised, so the bond angles are about 109.5 degrees
    • Each carbon is sp hybridised, giving linear geometry along the ring

13. A student claims that three isolated double bonds would give the same hydrogenation enthalpy as benzene. Which evaluation is best?

    • The claim is wrong, since the measured value is less exothermic than three cyclohexene units, showing delocalisation stabilisation
    • The claim is wrong, since benzene has no pi electrons to hydrogenate, so its hydrogenation enthalpy must be zero under all conditions
    • The claim is right, since the measured value always equals three times the cyclohexene value because each C=C bond behaves independently
    • The claim is right, since benzene is less stable than cyclohexene and releases more energy on hydrogenation than three isolated bonds

14. Why does benzene resist addition reactions compared with ethene?

    • Addition would disrupt the delocalised system and lose its extra stabilisation, so substitution is preferred
    • The ring is too small to accommodate two extra bromine atoms without strain, so only one bromine can add in each reaction
    • Benzene is too polar to attract a bromine molecule, so the bromine simply passes over the surface of the ring without reacting
    • Addition needs a lone pair on each carbon, which the benzene ring lacks because its electrons are shared around it

15. Which observation shows benzene is less reactive towards addition than ethene?

    • Both react at the same rate with bromine water in the dark
    • Ethene reacts with bromine only in the presence of aluminium chloride
    • Benzene decolourises bromine water faster than ethene does
    • Benzene does not decolourise bromine water at room temperature, while ethene does

16. Benzene burns in air with a smoky flame. What causes this?

    • Its high hydrogen content forms water droplets that look like smoke
    • It contains nitrogen, which forms smoke when it burns
    • Its high carbon-to-hydrogen ratio leads to incomplete combustion that produces soot
    • It is ionic, so it produces sodium chloride smoke on burning

17. Which quantity is used to deduce the delocalisation energy of benzene?

    • The sum of the bond enthalpies of all six C-H bonds
    • The difference between the boiling temperatures of benzene and cyclohexane
    • The difference between the expected and measured enthalpy changes of hydrogenation
    • The enthalpy of combustion divided by the number of carbon atoms

18. The Kekule model predicts a hydrogenation enthalpy of -360 kJ mol-1 for benzene, but the measured value is -208 kJ mol-1. What does the difference of 152 kJ mol-1 indicate?

    • Each C-H bond in benzene is weaker than in cyclohexene, so less heat is released
    • The hydrogenation is endothermic overall, so benzene absorbs heat
    • Benzene is less stable than predicted, so it reacts more readily with hydrogen
    • Extra stabilisation from delocalisation, so benzene is more stable than the Kekule form predicts

19. Why does the benzene ring attract electrophiles despite resisting addition?

    • Its hydrogen atoms carry a partial negative charge that attracts electrophiles to the ring surface, as in alkenes
    • Its carbon atoms each carry a full negative charge after bonding, which attracts positively charged electrophiles strongly
    • Its delocalised pi cloud above and below the ring has high electron density that attracts electrophiles
    • The ring has no electrons in its outer shell, so it attracts any positive ion strongly in a reaction mixture

20. X-ray diffraction data show that all six C-C bonds in benzene have identical lengths. Which model does this contradict?

    • The delocalised model with a continuous pi system
    • The model with bond angles of 120 degrees
    • The Kekule model with alternating single and double bonds
    • The model with all carbons sp2 hybridised

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