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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. $ ext{C}_6 ext{H}_6 + ext{H}_2 ightarrow ext{C}_6 ext{H}_{12} ext{ΔH}^ heta = -120 ext{ k... 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
Answer
Benzene () has a unique bonding structure that can be described as having six carbon atoms arranged in a planar hexagonal ring, with each carbon atom sp² hybridized. This results in a bond angle of 120 degrees, giving it a trigonal planar arrangement. Each carbon is bonded to one hydrogen atom, contributing to the characteristic flat structure of benzene.
Furthermore, the stability of benzene is attributed to its delocalized pi electrons, which form a resonance structure. In contrast, the hypothetical cyclohexa-1,3,5-triene would not possess this extensive delocalization, making it less stable than benzene. The reaction data shows that the enthalpy change for the hydrogenation of benzene is -120 kJ mol⁻¹, indicating it has greater stability compared to the hypothetical structure, which would feature localized double bonds, leading to higher enthalpy values and less stability.
Step 2
The enthalpy of hydrogenation of cyclohexa-1,3-diene is not exactly double that of cyclohexene.
Answer
Given that the enthalpy change for the hydrogenation of cyclohexene is -120 kJ mol⁻¹, one might initially suggest that the enthalpy for cyclohexa-1,3-diene would be roughly -240 kJ mol⁻¹. However, due to its partially conjugated system, there is some stabilization from resonance that cyclohexene does not have. Therefore, a more reasonable estimate might be -210 kJ mol⁻¹. This accounts for the additional stability from resonance effects in cyclohexa-1,3-diene compared to a simple doubling of cyclohexene's enthalpy.