What is Lindlar Catalyst Composition Preparation Mechanism and Applications
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's Catalyst
A substance that changes or accelerates the pace of any chemical reaction without any change taking place by itself is a Catalyst.
Lindlar is a heterogeneous catalyst composed of palladium that is 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. Lindlar’s Catalyst is used for the hydrogenation of alkynes into alkenes. The Lindlar’s Catalyst is used, on a large scale, in the synthesis of Vitamin A, and also used in the synthesis of dihydro vitamin K1.
The term "Lindlar" is named after a British chemist, Herbert Lindlar, their founder.
Properties of Lindlar’s Catalyst
Lindlar’s catalyst has a specific surface area of 150-260 m2/g and consists of Impurity less than 0.5%
The water content of Lindlar’s catalyst is less than 5%, and the pH is 8.
Lindlar Catalyst Preparation
Lindlar Catalyst is prepared by lowering the palladium chloride in a calcium carbonate mixture and lead acetate is added to it. A catalyst with a large surface area is obtained and this increases the reactivity. If the catalyst is used to reduce alkynes to alkenes, the introduction of quinoline prevents further reduction to alkanes. Quinoline here serves as a deactivator to improve the catalyst's selectivity.
Lindlar catalysts, which are available for commercial purchase, are also prepared in laboratories with the reduction of palladium(II) chloride in semi-liquid calcium carbonate and the subsequent poisoning of the resulting mixture with a suitable catalyst poison. Common choices here are lead acetate, lead(II) oxide, and quinoline.
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 Catalyst Formula: Pd/CaCO3
Lindlar Catalyst Structure
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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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FAQs on Lindlar Catalyst in Selective Hydrogenation Reactions
1. What is Lindlar catalyst?
The Lindlar catalyst is a poisoned palladium catalyst used for the selective partial hydrogenation of alkynes to cis-alkenes. It consists of palladium deposited on calcium carbonate (Pd/CaCO3) and deactivated (“poisoned”) with compounds such as lead(II) acetate and quinoline.
- Used with H2(g) under mild conditions
- Stops reduction at the alkene stage
- Produces mainly cis (Z) alkenes
2. What is the composition of Lindlar catalyst?
The Lindlar catalyst is composed of palladium (Pd) supported on calcium carbonate (CaCO3) and poisoned with lead(II) acetate and quinoline.
- Pd acts as the active metal surface for hydrogen adsorption
- CaCO3 serves as the solid support
- Lead salts and quinoline reduce catalytic activity to prevent over-hydrogenation
3. What reaction does Lindlar catalyst perform?
The Lindlar catalyst performs the partial hydrogenation of alkynes to cis-alkenes using hydrogen gas. The general reaction is:
- R–C≡C–R' + H2(g) → R–CH=CH–R' (cis)
- CH3–C≡C–CH3 + H2(g) → cis-CH3–CH=CH–CH3
4. Why does Lindlar catalyst give cis-alkenes?
The Lindlar catalyst gives cis-alkenes because hydrogen adds to the alkyne via syn addition on the metal surface.
- Both hydrogen atoms are delivered from the same side
- The alkyne adsorbs flat onto the Pd surface
- This leads to formation of the Z (cis) alkene
5. What is the difference between Lindlar catalyst and Pd/C?
The main difference is that Lindlar catalyst gives cis-alkenes from alkynes, while Pd/C fully hydrogenates alkynes to alkanes.
- Lindlar catalyst: poisoned Pd, partial hydrogenation
- Pd/C: active Pd, complete hydrogenation
- With Lindlar: R–C≡C–R' + H2 → R–CH=CH–R' (cis)
- With Pd/C: R–C≡C–R' + 2H2 → R–CH2–CH2–R'
6. Can Lindlar catalyst reduce alkenes?
Under normal conditions, the Lindlar catalyst does not significantly reduce alkenes because it is deliberately deactivated.
- It is selective for alkynes over alkenes
- The poisoning limits further hydrogenation
- Prevents formation of alkanes
7. How is Lindlar catalyst prepared?
The Lindlar catalyst is prepared by depositing palladium on calcium carbonate and then poisoning it with lead salts and quinoline. The preparation steps include:
- Impregnation of CaCO3 with a Pd2+ salt solution
- Reduction to metallic Pd
- Treatment with lead(II) acetate and quinoline
8. What is an example of hydrogenation using Lindlar catalyst?
An example of hydrogenation using Lindlar catalyst is the conversion of phenylacetylene to cis-styrene. The reaction is:
- C6H5–C≡CH + H2(g) → C6H5–CH=CH2
9. Why is Lindlar catalyst called a poisoned catalyst?
The Lindlar catalyst is called a poisoned catalyst because substances like lead(II) acetate and quinoline intentionally reduce its activity.
- Poisoning blocks some active Pd sites
- Slows down hydrogenation rate
- Prevents complete reduction to alkanes
10. What is the difference between Lindlar catalyst and sodium in liquid ammonia?
The key difference is that Lindlar catalyst gives cis-alkenes, while Na/NH3(l) gives trans-alkenes from alkynes.
- Lindlar: catalytic hydrogenation, syn addition → cis (Z) alkene
- Na/NH3(l): dissolving metal reduction, anti addition → trans (E) alkene
