Thermodynamic data regarding the hydrogenation of 1-methylcyclohexene and 1,3,5-trimethylbenzene are given below:
(1) 1-methylcyclohexene+H2→methylcyclohexane\text{1-methylcyclohexene} + \text{H}_2 \rightarrow \text{methylcyclohexane}1-methylcyclohexene+H2→methylcyclohexane, ΔH⊖=−120 kJ mol−1\Delta H^\ominus = -120\text{ kJ mol}^{-1}ΔH⊖=−120 kJ mol−1
(2) 1,3,5-trimethylbenzene+3H2→1,3,5-trimethylcyclohexane\text{1,3,5-trimethylbenzene} + 3\text{H}_2 \rightarrow \text{1,3,5-trimethylcyclohexane}1,3,5-trimethylbenzene+3H2→1,3,5-trimethylcyclohexane, ΔH⊖=−210 kJ mol−1\Delta H^\ominus = -210\text{ kJ mol}^{-1}ΔH⊖=−210 kJ mol−1
Explain the bonding in and the shape of the benzene ring in a 1,3,5-trimethylbenzene molecule. Compare the stability of 1,3,5-trimethylbenzene with that of the hypothetical 1,3,5-trimethylcyclohexa-1,3,5-triene molecule. Use the data in your answer.
The enthalpy of hydrogenation of 1,3-dimethylcyclohexa-1,3-diene is not exactly double that of 1-methylcyclohexene. Suggest a value for the enthalpy of hydrogenation of 1,3-dimethylcyclohexa-1,3-diene and justify your value.