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  • The enthalpy of hydrogenation of cyclohexene is . If resonance energy of benzene is <img alt="-150\;k

The enthalpy of hydrogenation of cyclohexene is -120\;kJ/mol. If resonance energy of benzene is -150\;kJ/mol. Enthalpy of hydrogenation of benzene (in KJ/mol) is?

Option: 1

-270


Option: 2

-210


Option: 3

-360


Option: 4

-420


Answers (1)

best_answer

Resonance Enthalpy: 

The theoretical difference in molecular energy between a resonance hybrid and the 'most stable' resonance contributor (if this resonance contributor existed as a real molecule). In other words, the stability gain by electron delocalization due to resonance versus the absence of such delocalization. The resonance energy of benzene is 36 kcal mol-1 .

 
The resonance energy is calculated as the \mathrm{\Delta H_{res} = \Delta H_{exp} - \Delta H_{cal}}

So we have given, the resonance energy of Benzene as, \mathrm{\Delta H_{res} = -150 kJ/mol} And enthalpy of hydrogenation of cyclohexene is \mathrm{-120 kJ/mol}.


 Which means that the energy to break one C=C bond is  \mathrm{-120 kJ/mol}. 

In benzene we have 3 C=C bonds so \mathrm{\Delta H_{cal} = 3\times (-120)\; kJ/mol} 

Now putting values in \mathrm{\Delta H_{res} = \Delta H_{exp} - \Delta H_{cal}} equation.

\mathrm{-150 = \Delta H _{exp} -(-360)}

\mathrm{\Delta H _{exp} = -210 kJ/mol}

Hence, option number (2) is correct
 

Posted by

Anam Khan

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