Metallic palladium deposited on calcium carbonate with lead acetate and quinoline is
known as Lindlar's catalyst and use for reduction of
alkynes into cis alkenes.
The
reactant alkyne molecules and hydrogen molecules get
adsorbed at the surface of metal catalyst. It is chemical adsorption (chemisorption). In this state, the reactants lie
very close to each other and so the hydrogen atoms start forming bond with
carbon. Two hydrogen atoms are added to two triply bonded carbon atom from the
same side of π-bond and a cis or syn addition product is formed. The product
alkene now escapes away from the surface of the catalyst. Quinoline occupies the metal surface inhibiting further reduction to be
alkanes Quinoline therefore is called
catalyst poison and palladium is called deactivated catalyst or poisoned
catalyst.
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