Lindlar catalyst - how does poisoning work?

[MattC]

When an Alkyne reacts with H2 gas over a Lindlar catalyst, the reaction stops (according to what I have read, and what my professor has said) at the alkene - it does not proceed to the alkane, whereas with a catalyist like PtO2, it will reduce the element all the way down to an alkane (at least, in some cases, as I understand it). Is the difference in the mechanism the electophilic nature of the catalyst? Is a Lindlar catalyst electrophilic enough to attract a triple bond, but not enough to lock onto a double bond? The sources I have read talk about "poisoning" the catalyst, and this confuses me. Is it a different catalyst (a "weakened" palladium?) that nevertheless acts as a catalyst should (and is not changed at all in the process), or is something happening to the catalyist itself, reducing it's ability to function? The latter is how I have heard it described, but I suspect I may have misunderstood.

 

 

Thank you in advance for any help!

[insane_alien]

When a catalyst is poisoned it usually means something has strongly bonded to the surface/active sites and therefore the reactants cannot be adsorbed and therefore the desired reaction cannot be catalysed. Poisoning materials usually include heavy metals like lead

[MattC]

I am not sure, but I think the lindlar catalyst mechanism may be something else. Otherwise, using H2 gas and a lindlar catalyst on an alkyne would result in a combination of alkenes and alkanes; supposidely it results in alkenes, which makes me think that the catalyst works well on triple bonds, but not double bonds.