Question

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d) Concisely explain why hydrogenation of the alpha olefin 1-octene is considered a reduction reaction. Using the frontier orbitals of 1-octene and hydrogen, show that Pd catalysis is necessary because the uncatalyzed concerted one-step hydrogenation transition state is very high in energy На Catalytic Pd/C

Answer 1

Addition of hydrogen to a carbon-carbon double bond is called hydrogenation. Overall effect of such an addition is the reductive removal of the double bond functional group.

1-Octene ------> Ocatne

A Substance is said to be reduced if it undergoes one of the four conditions
- Lose Oxygen
- Gain Hydrogen
- Gain Electrons
- Decrease in Oxidation State

More precisely since C=C shows unsaturation of electrons on C atom, this in removed by addition of electrons (Reduction is addition of electrons ) from H₂ : H₂ gives its electron to C=C and adds itself across this bond as protons H⁺ . So, this is considered reduction.

Hydrogenation of a double bond is thermodynamically favorable reaction because it forms a more stable product. Although overall hydrogenation reaction is exothermic, a high activation energy prevents it from taking place under normal conditions.

This restriction may be circumvented by the use of a catalyst (Pd ).

Uncatalyzed 1-step concerted reaction has very transition state energy ,

as shown HOMO of H-H and LUMO of 1-octene participate in one step concerted reaction , this step is antrafacial, suprafacial which is very high in energy and not possible for 2+2 addition reaction in normal circumstances.

Frouker arbital - octene R 8 LUMO 110MO concerted reaction ? Astrafagal- Grup

Pd catalyst uses its electron and H-H adsorbed on catalyst surface as H and H , thus as separate entity and when 1-Octene comes in contact these adds across double bond : thus Pd reduces high activation energy  to very low.