Lindlar Catalyst

What is Lindlar's Catalyst?

A catalyst is a substance that changes or accelerates the pace of any chemical reaction without any change taking place by itself. A catalyst is usually used in smaller amounts compared with the reactants or reaction participants.

Lindlar is a heterogeneous catalyst composed of palladium formed on calcium carbonate and treated with different types of lead. A heterogeneous catalyst is a catalyst that is always in a different phase or situation (solid, liquid, or gas solution) with the reactant solution.

The term "Lindlar" was awarded after Herbert Lindlar, their founder. Using lead will be needed to deactivate the palladium at some locations. Because of the existence of lead, this is often denoted as a "poisoned catalyst." A catalyst becomes poisonous when its potency begins to decline in the presence of another chemical substance known as poison catalyst.

To poison the palladium, different compound contaminants such as lead acetate and lead oxide are used. The palladium element is normally just 5 percent of the catalyst's overall weight. The catalyst is applied to alkenes to hydrogenate alkynes.

Lindlar Catalyst Formula: Pd/CaCO3

Lindlar Catalyst Structure

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Lindlar Catalyst Properties

  • It has a Specific surface area- 150-260 m2/g

  • It consists of Impurity less than 0.5%

  • It has water content less than 5%

  • It has pH about 8

Lindlar Catalyst Preparation

It's normally prepared by lowering palladium chloride in a calcium carbonate mixture accompanied by adding lead acetate. Finally, a catalyst with a large surface area is obtained which increases the reactivity. Provided that the catalyst is used to reduce alkynes to alkenes, the introduction of quinoline prevents further reduction to alkanes. Quinoline therefore,  serves as a deactivator to improve the catalyst's selectivity.

Lindlar Reaction Mechanism

Alkyne hydrogenation to alkenes involves the presence of molecular hydrogen (H2) that lowers the alkyne to alkenes. The Hydrogen (H2) atoms are introduced to the alkenes in pairs where the alkynes ' triple bond is reduced to a double bonded alkene. 

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In addition, the further reduction to one single bond is obstructed. In fact, the reduction of alkenes to alkanes is quicker than the reduction to alkenes due to the addition of quinoline.

In the above-mentioned hydrogenation reaction, the hydrogen atom is transferred to the same side (cis) of the alkyne, resulting in cis alkenes by introducing syn (addition of two substituents on the same side of a double or triple bond resulting in a decrease in bond number). All hydrogen and alkyne are closely bound up with the catalyst's large surface where the hydrogen atoms then slowly bind into the alkyne's triple bond.

Therefore, alkyne hydrogenation becomes stereoselective and occurs by syn addition. Stereoselectivity leads to the formation of an uneven mixture of stereoisomers (isomeric molecules that have the same molecular formula but different tridimensional atom orientations in space). In addition, the reaction is exothermic.

Lindlar's Catalyst Examples

Using the Lindlar catalyst 1-phenylpropyne is reduced in this catalytic hydrogenation reaction. The alkyne is lowered to the equivalent cis alkene but not reduced to the alkane any further. If the catalyst had been Pd alone (without a poison), the alkene could not be extracted as it would be reduced easily to the equivalent alkane.

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FAQ (Frequently Asked Questions)

1. What is the Lindlar Catalyst Formula?

Lindlar catalyst: H2 / Pd / CaCO3 is used for alkyne hydrogenation to mold cis alkene as hydrogenation using H2 / Pd would directly create alkanes and therefore CaCo3 will be used as a pesticide to avoid further hydrogenation and thus form an alkene.

2. What is Pd BaSO4?

Pd / BaSO4 is called Lindlar Catalyst along with quinolone, and is used again to convert alkynes to alkenes. This reagent provides the accompanying beginning alkynes cis alkene. The reduction of terminal alkynes is not limited, terminal alkenes are produced. In the case of benzene, no interaction with Benzene.

3. What is the Product Obtained When Alkynes are Subjected to Hydrogenation in The Presence of Lindlar Catalyst?

Lindlar catalysts comprise of finely distributed palladium as the enabling catalyst formed on calcium carbonate. Lindlar catalyst hydrogenation of alkynes continues in synthetic form in a stereoselective way. We are shaped cis alkenes.