Catalytic hydrogenation on a metal surface template

1,2-Dimethylcyclohexene with H₂ over Pd gives cis-1,2-dimethylcyclohexane. In the Horiuti–Polanyi scheme (simplified here: surface atoms drawn as Pd) H₂ dissociates to surface H atoms, one H adds to give a half-hydrogenated alkyl, then the second H adds from the same face and the alkane leaves.

Mechanism step by step

H₂ and the alkene bind to the metal, and both H atoms add from the metal side: syn addition

1. H₂ adsorbs – Schematic: H₂ binds to the metal and the H–H bond breaks to give surface Pd–H
2. First H (half-hydrogenated) – The alkene lies flat on the surface by one face; Pd–H adds across C=C (four-centre, syn): H to one carbon, the other bonds to the metal
3. Second H, desorption – The second H comes from the same (metal) face: both new C–H bonds are cis

Key points

• Syn addition: both H atoms come from the metal surface, so 1,2-dimethylcyclohexene gives cis-1,2-dimethylcyclohexane.
• Heterogeneous catalysts (Pd/C, PtO₂, Raney Ni) lower the barrier; without a catalyst H₂ and alkenes do not react. Heats of hydrogenation (about −120 kJ mol⁻¹ for a disubstituted C=C) measure alkene stability.
• Horiuti–Polanyi steps are reversible: the half-hydrogenated alkyl can lose an H again, which explains H/D scrambling and some C=C migration (the accepted mechanism, simplified here).
• C=O, aromatic rings and esters are untouched under mild conditions; alkynes go to alkanes unless the catalyst is poisoned (Lindlar).

Drawn with 62 atoms, 6 arrows, 432 shapes – every one editable in MolDraw.

Sources

• Wade, Organic Chemistry, 8th ed., Pearson 2013, ch. 8 Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 23 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 15 Textbook