Alkynes bear many similarities to alkenes, but as we have already seen, their chemistry can differ in subtle and interesting ways. Today’s post is another case in point.
Using a normal catalyst, hydrogenation of alkynes gives alkanes
The reduction of alkenes by hydrogen in the presence of a metal catalyst (“catalytic hydrogenation”) is a time-honoured reaction recognized by Sabatier’s receipt of the Nobel Prize for Chemistry in 1904. Incidentally, the products of this reaction are a part of our daily lives – modern margarine is produced from hydrogenation of vegetable oils for example [Trans-fats are an unfortunate byproduct of catalytic hydrogenation].
Bearing two carbon-carbon π bonds, alkynes may likewise be hydrogenated. Under conditions used for the hydrogenation of alkenes, both bonds are reduced, producing alkanes. [You would be reasonable to think you could prevent over-reduction simply by only using one molar equivalent of hydrogen gas; in practice, this doesn’t work very well ]
If that’s all there was to the hydrogenation of alkynes, we’d quickly have to move on. The chemistry of alkanes, is – to put it bluntly – kind of dull*, and although reduction of alkynes to alkanes certainly has its place, you won’t find many reactions in organic chemistry which begin with alkanes. If you think of functional groups like airports, with all the reactions they can perform like flights that connect them to other hubs, a reaction that leads only to alkanes is a bit like taking a one-way flight to Saskatoon, Saskatchewan. Not nowhere, mind you, but a little far from the action.*
Partial Hydrogenation of Alkynes Gives Alkenes
That’s not the whole story, of course. Imagine for a second that instead of hydrogenating both double bonds, we’d be able to stop at hydrogenating just one. This would allow us to convert alkynes into alkenes. Using the “functional groups as airports” analogy, a reaction that produces alkenes is like flying into O’Hare: as we just saw in the previous series, this will give us plenty of subsequent options in synthesis, as there is an extremely rich variety of alkene addition reactions.
As it turns out, because the second π bond of alkynes is not quite as strong as the first [approx 46 kcal/mol vs 68 cal/mol], conditions have been found that allow for the partial reduction of alkynes.
For our purposes there are two ways to do this.
Catalytic Hydrogenation Using the “Poisoned” Lindlar’s Catalyst Gives cis Alkenes
The first, catalytic hydrogenation, operates on the same principle as described above: treatment of the alkyne with hydrogen gas and a metal catalyst. The trick, however, is to modify the behavior of the catalyst such that it is powerful enough to reduce the first π bond but not reactive enough to affect the second. In other words, “poisoning” its reactivity. In practice, this is done by combining palladium on carbon with lead carbonate (PbCO3) and quinoline (an aromatic amine). The resulting mixture, known as “Lindlar’s catalyst” after its inventor, is effective for the partial reduction of alkynes.
Note the stereochemistry! Just as in conventional alkene hydrogenation, both hydrogen atoms are delivered in syn fashion to provide us with the “cis” (Z) alkene.
Reduction of Alkynes Using Sodium and Ammonia Gives Trans Alkenes
There’s another way to reduce alkynes that doesn’t involve catalytic hydrogenation. As described in this old Reagent Friday post, sodium metal in ammonia (Na/NH3) can also reduce alkynes to alkenes. This process is called dissolving metal reduction. It’s a different process than catalytic hydrogenation. In this reaction, electrons from Na metal sequentially add to the alkyne, resulting in an anion that is protonated by the NH3 solvent. An interesting facet of this reaction is, again, the stereochemistry: due to electronic repulsion, the geometry of the resulting alkene is trans [for the full mechanism see this post].
The Stereochemistry of Partial Reduction Has Important Consequences In Synthesis
So the bottom line here is that through using different reducing agents, we can obtain alkenes of different geometries. This might not seem like such a big deal at the moment, but it will have very important consequences for subsequent reactions – stay tuned. I’ve said this before and no doubt I’ll repeat the same comment: stereochemistry is one of the key testable concepts in Org 1, and the reactions of alkynes are a key component.
* Modern advances in C-H activation chemistry excepted, of course