In a blatant plug for the Reagent Guide, each Friday I profile a different reagent that is commonly encountered in Org 1/ Org 2. Version 1.2 just got released, with a host of corrections and a new page index.
One thing that’s useful to keep in mind as you learn about reagents is that there are some reagents that will only descend (or ascend) one rung of the oxidation ladder at a time. Last week, it was sodium borohydride, which reduces aldehydes and ketones but doesn’t touch esters. This week, it’s Lindlar’s catalyst, which reduces alkynes, and stops at the alkene.
What it’s used for: the Lindlar catalyst is a “poisoned” metal catalyst that performs hydrogenations of alkynes in the presence of hydrogen gas (H2). By “poisoned” we mean that it lacks the normal activity associated with palladium catalysts for reducing double bonds. This is useful because sometimes we’d like to start with an alkyne and go down one “rung” of the oxidation ladder to an alkene. But if you use normal palladium on carbon, you’ll get full reduction to the alkane.
Lindlar’s catalyst is a palladium catalyst poisoned with traces of lead and quinoline, that reduce its activity such that it can only reduce alkynes, not alkenes. It always gives the cis-alkene, in contrast to Na/NH3, which gives the trans alkenes. Lindlar’s catalyst doesn’t really have a “structure”. Like Raney nickel, it’s basically a metal that has been modified in a very particular way to provide a certain desirable set of properties. Sometimes you might see it written as Pd-CaCO3-PbO2, but it’s usually just written “Lindlar”.
Similar or the same as: There’s a whole family of poisoned catalysts that are similar. You might also see Ni–B (nickel boride), Pd-CaCO3, palladium on barium sulfate, Pd-CaCO3-quinoline and others enlisted to do the same task.
Examples: Reduction of alkynes to cis-alkenes
How it works
It is thought that the role of lead (Pb) is to reduce the amount of H2 absorbed, while quinoline helps avoid the formation of unwanted byproducts, but it’s hard to be more specific than that. A discussion of how Lindlar’s catalyst works would be a good thing to ask a surface chemist like Gerhard Ertl. You can see a procedure for making Lindlar’s catalyst here.
P.S. You can read about the chemistry of Lindlar’s catalyst and more than 80 other reagents in undergraduate organic chemistry in the “Organic Chemistry Reagent Guide”, available here as a downloadable PDF.