The hydrogenation of conjugated diene H over a solid metal catalyst can produce…

CHEMISTRY · Multi-part · AP · Chemistry · Thermodynamics and Electrochemistry · Thermodynamics and Electrochemistry

Problem

The hydrogenation of conjugated diene H over a solid metal catalyst can produce either the 1,2-addition product J or the 1,4-addition product K. In the uncatalyzed thermal reaction the activation energies are

E_{a}(J) = 58\,\text{kJ mol}^{-1}

and

E_{a}(K) = 74\,\text{kJ mol}^{-1}

, while

\Delta G^\circ(K) - \Delta G^\circ(J) = -15\,\text{kJ mol}^{-1}

298\,\text{K}

. A newly developed heterogeneous catalyst is found to lower the activation energies to

E_{a,cat}(J) = 22\,\text{kJ mol}^{-1}

and

E_{a,cat}(K) = 24\,\text{kJ mol}^{-1}

without altering the free energies of the products. (a) Sketch, on a single qualitative energy diagram, the uncatalyzed and catalyzed potential-energy profiles for formation of J and K. Label all reactants, transition states, and products, and indicate relative energy differences accurately to scale. (b) For reactions conducted at

298\,\text{K}

under otherwise identical conditions, predict the major product in (i) the absence and (ii) the presence of the new catalyst. Support each prediction with quantitative or semi-quantitative reasoning. (c) Discuss whether the catalyst changes the thermodynamic favorability of either product and explain the mechanistic origin of any change—or lack thereof—in product distribution. (d) Design an experiment (including temperature, reaction time, and analytical method) that would allow you to determine conclusively whether kinetic or thermodynamic control governs the catalyzed reaction under standard laboratory conditions. Provide the expected observations for each possible outcome.

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