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Question

The enthalpy of hydrogenation of cyclohexene is 119.5 kJ/mol. If resonance energy of benzene is 150.4 kJ/mol then its enthalpy of hydrogenation would be:

A
269.9 kJ/mol
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B
358.5 kJ/mol
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C
508.9 kJ/mol
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D
208.1 kJ/mol
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Solution

The correct option is D 208.1 kJ/mol
As, Enthalpy of hydrogenation of cyclohexene is 119.5 kJ/mol, i.e, Hydrogenation of one C=C is 119.5 kJ/mol.
ΔHcal for benzene is 3×119.5=357 kJ/mol
So,
ΔHobs=ΔHcal+Energy needed to disturb resonance
=(3×119.5)+150.4=208.1 kJ/mol
Thus, enthalpy of hydrogenation of benzene is 208.1 kJ/mol

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