The stepwise catalytic conversion of propyne (CH3C≡CH) to propane (CH3CH2CH3) proceeds through an alkene intermediate under conditions where each hydrogen addition is a separate elementary step.
Step I CH3C≡CH+H2⟶CH3CH=CH2
Step II CH3CH=CH2+H2⟶CH3CH2CH3
Average bond energies (kJ mol−1):
C≡C 839 C=C 614 C–C 348 C–H 413 H–H 432
Assume no other bond types are present and that the intermediate contains one C=C bond.
(a) For each carbon atom in propyne, identify its hybridization and determine the percent s character.
(b) Using the bond energies provided, calculate the enthalpy change for Step I and for Step II.
(c) Construct a quantitative energy profile (describe the shape and relative positions of reactants, intermediate, and products on a potential-energy diagram) based on your calculations.
(d) Propose which step is expected to have the higher activation energy and justify your prediction using both bond energy considerations and the changes in orbital overlap that occur during each step.
(e) Derive a linear relationship between percent s character in the carbon orbital of a C–H bond and its bond energy using the data: sp (50 % s) – 553 kJ mol−1, sp2 (33 % s) – 464 kJ mol−1, sp3 (25 % s) – 413 kJ mol−1. Then use this relationship to predict the average bond energy of a hypothetical C–H bond in an sp3d planar environment (20 % s character).