Core Concepts
In this tutorial you will learn about the purpose, mechanism, and utility of Lindlar’s catalyst in organic chemistry.
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What is Lindlar’s Catalyst?
Lindlar’s catalyst is the set of reagents used to hydrogenate alkynes to alkenes without further reducing them to alkanes. Consisting of palladium deposited on a substrate, a lead cationic salt, and the dicyclic molecule called quinoline, Lindlar’s catalyst is essential in many synthetic applications which require a cis-alkene. Lindlar’s catalyst always reduces the alkyne via syn-addition. This is because the molecule contacts the surface of the palladium which is relatively large and is hence, hydrogenated on the same side.
Catalytic Properties
Lindlar’s catalyst is a poisoned catalyst, meaning that some of the catalytic activity is reduced to change the product. Lindlar’s catalyst balances the high catalytic efficiency of palladium with the deactivating properties of the lead salt and quinoline. These reagents used together allow for a very selective hydrogenation when exposed to hydrogen gas at around 1 atm. In order to maximize the surface area of incredibly expensive palladium metal, a substrate is used such as charcoal or calcium carbonate, upon which the palladium is deposited. The commonly used palladium and charcoal reagent is denoted by the name “Pd/C”, or palladium on carbon. This substance is insoluble in water and can be filtered out after the reaction is complete to be recycled and reused in other reactions.
Catalytic Hydrogenation Mechanism
Lindlar’s catalyst results in the formation of C-H bonds via exchange of hydrogen atoms at the surface of the palladium. The planar or linear nature of unsaturated carbon molecules allow them to approach the surface of the catalyst with both carbons of the double or triple bond adsorbing to the palladium simultaneously. This unique adsorption phenomena is what gives rise to the characteristic syn-addition products of catalytic hydrogenation. Although much is still unknown regarding the full process by which protons are exchanged, the most well-understood first step involves the adsorption of hydrogen atoms to the surface of the palladium. The alkyne molecule of interest binds to the palladium surface by overlapping its π orbitals with the vacant orbitals of the metal atoms, as seen by the dotted lines in step (1). Then, an adsorbed hydrogen atom will quickly form a bond with a complexed carbon, resulting in hydrogenation of a singular carbon (step 2). As the molecule diffuses away from the metal surface, another hydrogen will bond to the remaining carbon, resulting in a hydrogenated molecule which has a syn-addition on the side of the molecule which had faced the surface of the palladium. This is seen in step (3).
As seen in step (1), singular hydrogen atoms are already adsorbed to the surface of the metal. This step is performed before the carbon reactant of interest is added to the solution. Generally, this is performed by bubbling an alcohol solution containing Palladium/Carbon with hydrogen (H2) gas.
Note that using lindlar’s catalyst, the lead and quinoline helps to decrease the efficiency of this reaction and only cause alkenes to be produced.
Applications and Emerging Subjects
While Lindlar’s catalyst is often taught as the characteristic reagent for converting alkynes to alkenes, its use has recently been the subject of debate due to its contradiction to emerging green chemistry principles. Lindlar’s catalyst uses lead compounds which must be disposed of in toxic chemical waste after use. Additionally, the process of catalytic hydrogenation uses rare palladium which can only be used a limited number of times before it becomes degraded. Scientists are currently doing research to find more green catalysts to perform the same reaction.
Practice Problems
Problem 1
Draw the product of this reaction and label whether the product is in E or Z configuration.
Lindlar’s catalyst is used in the production of Vitamin A. Explain why Lindlar’s catalyst is used instead of just typical H2 with Pd/Carbon to convert 6,10,14-trimethylpentadecan-2-one into vitamin A. Is syn-addition required to get the desired product?
Answers
Problem 1
The product is shown below and it is in the Z configuration.
Problem 2
Lindlar’s catalyst converts the alkyne to an alkene as opposed to an alkane. Syn-addition is not required because one carbon on the triple bond has hydrogen attached to it. Therefore, the configuration of the added hydrogens is unimportant because a Z/E product is irrelevant.