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Resonance Energy
Concepts tested here
- Resonance energy from enthalpy of hydrogenation
All Questions
2006 AIPMT-PRE 1 question
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The enthalpy of hydrogenation of cyclohexene is $-119.5$ kJ mol$^{-1}$. If resonance energy of benzene is $-150.4$ kJ mol$^{-1}$, its enthalpy of hydrogenation would be :-Cyclohexene has one $C=C$ bond; its enthalpy of hydrogenation is $-119.5$ kJ mol$^{-1}$.
If benzene were cyclohexatriene with three isolated double bonds, its calculated enthalpy of hydrogenation would be $3 \times (-119.5) = -358.5$ kJ mol$^{-1}$
Benzene is more stable than this hypothetical structure by its resonance energy, 150.4 kJ mol$^{-1}$, so less heat is released on its hydrogenation.
Resonance energy = observed $\Delta H$ $-$ calculated $\Delta H$ (taken with the sign convention of the question)
$-150.4 = -358.5 - x$
$x = -358.5 + 150.4$
$x = -208.1$ kJ mol$^{-1}$