Catalytic Hydrogenation
Understanding the Horiuti-Polanyi mechanism, stereochemistry of syn-addition, and Lindlar's Catalyst.
1 General Reaction
Catalytic hydrogenation is the addition of molecular hydrogen ($H_2$) across a carbon-carbon double ($C=C$) or triple ($C\equiv C$) bond. Because the $H-H$ bond is incredibly strong ($436 \text{ kJ/mol}$), the reaction requires a transition metal catalyst to proceed at room temperature.
The most common catalysts are finely divided Palladium (Pd), Platinum (Pt), or Nickel (Ni). These act as heterogeneous catalysts, meaning they are in a different phase (solid) than the reactants (gas or liquid).
2 The Horiuti-Polanyi Mechanism
The widely accepted mechanism for heterogeneous hydrogenation is the Horiuti-Polanyi mechanism. The entire reaction takes place on the surface of the solid metal catalyst in four distinct steps: Adsorption of $H_2$, Adsorption of the Alkene, Migratory Insertion, and Reductive Elimination.
Step A: Dissociative Adsorption
Hydrogen gas molecules collide with the metal surface. The metal atoms break the strong $H-H$ bond and form individual Metal-Hydrogen ($M-H$) bonds. The alkene also approaches and binds to the metal via its $\pi$-electrons.
Phase 1: Adsorption on Metal Surface
Hydrogen and Alkene bind to the solid catalyst.
Step B: Migratory Insertion & Desorption
One of the adsorbed hydrogen atoms shifts onto one of the alkene's carbons (Migratory Insertion), forming a half-hydrogenated intermediate alkyl group attached to the metal. Then, the second hydrogen atom adds to the other carbon (Reductive Elimination). The fully saturated alkane loses its affinity for the metal and desorbs (floats away).
Phase 2: Syn-Addition & Desorption
Both Hydrogens add from the same face of the alkene.
Stereochemical Consequence: Syn-Addition
Because the alkene approaches a flat, solid metal surface, both hydrogen atoms are delivered to the exact same face (same side) of the double bond. This stereospecificity is called syn-addition.
3 Hydrogenation of Alkynes & Lindlar's Catalyst
Alkynes ($R-C\equiv C-R$) can also be hydrogenated. If standard $Pd/C$ or $Pt$ is used, the alkyne will be completely reduced all the way to an alkane without stopping.
To stop the reaction at the alkene stage, chemists use a "poisoned" catalyst known as Lindlar's Catalyst.
What is Lindlar's Catalyst?
It consists of Palladium deposited on Calcium Carbonate ($Pd/CaCO_3$), which is then "poisoned" (deactivated) using lead acetate and quinoline. The poison blocks the most reactive sites on the palladium.
The Result: Strictly cis-Alkenes
Because Lindlar's catalyst is a solid surface, it forces a syn-addition of the two hydrogen atoms. Since both H's are added to the same side of the triple bond, the resulting product is exclusively a cis-alkene (Z-alkene).
Knowledge Check
10 Practice MCQs on Catalytic Hydrogenation
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