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Data about the hydrogenation of cyclohexene and of benzene are given - AQA - A-Level Chemistry - Question 6 - 2018 - Paper 2
Question 6
Data about the hydrogenation of cyclohexene and of benzene are given. H₂ + Cyclohexene → Benzene ΔH° = -120 kJ mol⁻¹ 3H₂ + Benzene → Cyclohexane ΔH° = -208 kJ mol⁻... show full transcript
Worked Solution & Example Answer:Data about the hydrogenation of cyclohexene and of benzene are given - AQA - A-Level Chemistry - Question 6 - 2018 - Paper 2
Step 1
Explain the bonding in and the shape of a benzene molecule: Compare the stability of benzene with that of the hypothetical cyclohexa-1,3,5-triene molecule.
Answer
Bonding in Benzene
Benzene is characterized by a unique bonding structure that includes six carbon atoms bonded in a hexagonal ring. Each carbon atom forms three sigma (σ) bonds: two with neighboring carbon atoms and one with a hydrogen atom. The remaining unhybridized p-orbital electrons on each carbon atom contribute to a delocalized π-bonding system, leading to resonance stabilization.
Shape of Benzene
The benzene molecule adopts a planar shape due to the sp² hybridization of its carbon atoms, which results in bond angles of approximately 120°, characteristic of trigonal planar geometry. This symmetrical arrangement allows for maximum overlap of the p-orbitals, enhancing the stability of the molecule.
Stability Comparison
The hypothetical cyclohexa-1,3,5-triene is expected to have alternating double bonds, which would suggest higher instability due to localized π-bonds and possible isomerization. The calculated enthalpy of hydrogenation for cyclohexene is -120 kJ mol⁻¹ and for benzene is -208 kJ mol⁻¹. The greater negative value for benzene's hydrogenation indicates that it is more stable than cyclohexa-1,3,5-triene, as resonance in benzene contributes to additional stability by distributing electrons over the entire molecule rather than localizing them in specific bonds.
Step 2
The enthalpy of hydrogenation of cyclohexa-1,3-diene is not exactly double that of cyclohexene. Suggest a value for the enthalpy of hydrogenation of cyclohexa-1,3-diene and justify your value.
Answer
The enthalpy of hydrogenation of cyclohexene is -120 kJ mol⁻¹, thus doubling this would yield -240 kJ mol⁻¹ for cyclohexa-1,3-diene. However, due to the presence of resonance in the diene structure, it is expected that the actual enthalpy of hydrogenation will be less negative than -240 kJ mol⁻¹. Therefore, a suggested value for the enthalpy of hydrogenation of cyclohexa-1,3-diene could be around -220 kJ mol⁻¹. This reflects that while the diene is less stable than cyclohexene, it has some degree of resonance stabilization that prevents the value from being exactly double.