Question

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

• A. $-208.1\, kJ\, mol ^{-1}$
• B. $-269.9\, kJ\, mol ^{-1}$
• C. $-358.5\, kJ\, mol ^{-1}$
• D. $-508.9\, kJ\, mol ^{-1}$

Answer 1

Answer: (A)

The enthalpy of hydrogenation of cyclohexene (having one double bond) is $-119.5\, kJ\, mol ^{-1}$.
Hence, theoretically enthalpy of hydrogenation of Kekule's structure of benzene should be $3 \times(-119.5) kJ\, mol ^{-1}$.
Since the resonance energy of benzene is $-150.4\, kJ\, mol ^{-1}$, the actual enthalpy of hydrogenation of benzene will be,
$=[3 \times(-119.5)]-(-150.4)$
$=-208.1\, kJ\, mol ^{-1}$.