(E = activation energy)
Relation between activation energies of above reactions is :

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$E_{3} > E_{1} > E_{2}$

$E_{2} > E_{1} > E_{3}$

$E_{3} > E_{2} > E_{1}$

$E_{1} > E_{2} > E_{3}$

answer is D.

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Detailed Solution

Addition of one molecule of hydrogen produces cyclohexadiene, in which almost all of the resonance energy of benzene has been lost. Hence the activation energy of this step is much greater than that required for each succeeding step, in which the double bonds behave like their acyclic analogues. Thus we have a multi-step reaction in which the first step is ratedetermining. This means that the conditions required for the first step are more vigorous than those required for the successive steps (all steps involve the addition of one molecule of hydrogen). Because of this, it is not possible to stop the reaction proceeding to complete reduction of benzene to cyclohexane, and consequently it is not possible to isolate the intermediates.

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