How Lindlar Catalyst Selectively Hydrogenates Alkynes
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
(Image will be uploaded soon)
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.
(Image will be uploaded soon)
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.
(Image will be uploaded soon)
FAQs on Lindlar Catalyst: Formula, Properties & Applications
1. What is Lindlar's Catalyst and what is its chemical composition?
Lindlar's Catalyst is a heterogeneous catalyst used for the selective hydrogenation of alkynes. It is specifically designed to stop the reaction at the alkene stage. Its composition consists of Palladium (Pd) deposited on a support of Calcium Carbonate (CaCO₃), which is then deactivated or "poisoned" with a substance like quinoline or lead acetate.
2. What is the main function of using Lindlar's Catalyst in a chemical reaction?
The primary function of Lindlar's Catalyst is the stereoselective reduction (hydrogenation) of an alkyne to a cis-alkene. Because the catalyst is "poisoned," it is active enough to break one of the pi bonds in a triple bond but not strong enough to reduce the resulting double bond of the alkene further into an alkane. This controlled reactivity is its main purpose.
3. What specific product is formed when but-2-yne is treated with Lindlar's Catalyst?
When but-2-yne (CH₃−C≡C−CH₃) is treated with hydrogen gas (H₂) in the presence of Lindlar's Catalyst, the reaction yields cis-but-2-ene. The two hydrogen atoms add to the same side of the triple bond, resulting in the 'cis' configuration where the methyl groups are on the same side of the double bond.
4. Why is Lindlar's Catalyst considered a "poisoned" catalyst?
Lindlar's Catalyst is called a "poisoned" or deactivated catalyst because its catalytic activity is intentionally reduced. The palladium catalyst is treated with a poison, such as quinoline or lead compounds. This poison blocks the most active sites on the palladium surface, reducing its efficiency. This deactivation is crucial because it prevents the catalyst from over-reducing the product, stopping the hydrogenation of an alkyne at the alkene stage instead of continuing to form an alkane.
5. How does Lindlar's Catalyst ensure the formation of a cis-alkene through syn-addition?
The formation of a cis-alkene occurs via a mechanism called syn-addition. In this process:
- Hydrogen gas (H₂) first adsorbs onto the surface of the solid palladium catalyst.
- The alkyne molecule then approaches the catalyst surface.
- Two hydrogen atoms are transferred from the catalyst surface to the same face of the alkyne's triple bond, breaking one pi bond.
Since both hydrogens add from the same side, the resulting substituents on the new double bond are in a cis configuration.
6. What are the individual roles of calcium carbonate (CaCO₃) and quinoline in Lindlar's Catalyst?
In the Lindlar's Catalyst system, each component has a specific role:
- Calcium Carbonate (CaCO₃): It acts as the support material. It provides a large surface area for the finely divided palladium particles to be deposited upon, maximizing the catalyst's exposure to the reactants.
- Quinoline: It functions as the catalyst poison. By binding to the most reactive sites of the palladium, it deactivates the catalyst just enough to prevent the reduction of the alkene product into an alkane.
7. Can Lindlar's Catalyst be used to reduce an alkene to an alkane? Explain why or why not.
No, Lindlar's Catalyst cannot effectively reduce an alkene to an alkane. The deliberate poisoning of the palladium catalyst makes it too weak to break the strong pi bond of an alkene. Its catalytic activity is precisely calibrated to be strong enough for the more reactive triple bond of an alkyne but insufficient for the double bond of an alkene, thereby ensuring the reaction stops at the cis-alkene stage.
8. How does the product from using Lindlar's Catalyst differ from using Birch Reduction (Na in liquid NH₃) on an alkyne?
The products are stereoisomers of each other due to different reaction mechanisms:
- Lindlar's Catalyst: Performs a syn-addition of hydrogen, leading to the formation of a cis-alkene.
- Birch Reduction (Sodium in liquid Ammonia): Performs an anti-addition of hydrogen. The mechanism involves a radical anion intermediate, and the two hydrogen atoms add to opposite faces of the triple bond, resulting in a trans-alkene.
9. What are some important real-world applications of Lindlar's Catalyst?
Lindlar's Catalyst is crucial in organic synthesis where precise stereochemistry is required. Key applications include:
- The synthesis of Vitamin A and other complex natural products.
- The preparation of specific pheromones that rely on the cis-geometry of a double bond for their biological activity.
- In the pharmaceutical industry for building complex molecules where retaining a specific alkene geometry is a critical step.
