Alkynes – 3 Patterns
Earlier, I said you could divide the reactions of alkenes into 3 big “buckets”: the carbocation pathway, the “3 membered ring” pathway, and the “concerted” pathway.
A lot of those reactions still work with alkynes. Today, let’s just go through the most straightforward reactions of alkynes.
The carbocation pathway
As with alkenes, strong acids such as H-Cl and H-Br add to alkynes in a Markovnikoff fashion – the halogen (chlorine or bromine) ends up attached to the most substituted carbon. Again, just as with alkenes, these reactions go through the formation of a carbocation, followed by trapping of the carbocation with the halide nucleophile.
If a second equivalent of the acid is added, a second Markovnikoff addition will occur, resulting in a “geminal dihalide” – that is, a carbon with two halogens attached to the same carbon.
Addition of H3O(+) also works with alkynes, and goes through a carbocation – but there’s a twist to this. More on this soon.
The “three membered ring” pathway
With alkenes, halogens like Br2, Cl2, and I2 form a 3-membered ring “halonium” ion which undergoes backside attack to give the trans product. There’s no difference between the pathway of alkenes and that of alkynes. We still get the trans product.
If a second equivalent of Br2, Cl2, or I2 is added, we do a second addition to the resulting alkene, giving a tetrahalogenated alkane.
Oxymercuration also works on alkynes, and results in Markovnikoff products, but (like H3O(+)) there’s an extra twist to this reaction too.
The “concerted” pathway.
This category probably shows the most differences between alkynes and alkenes. Of all the reactions in this category, epoxidation (mCPBA), dihydroxylation (OsO4), and cyclopropanation don’t work with alkynes.
The main reaction to think about in this category is hydrogenation. When alkynes are treated with hydrogen gas and Pd/C, the alkyne will be hydrogenated to the alkene, but it doesn’t stop there. The alkene is then hydrogenated to the alkane.
In order to get the hydrogenation to stop after a single addition, we use a “poisoned” catalyst that doesn’t hydrogenated alkenes. The most common version is “Lindlar’s catalyst”. This gives us the “cis” product.
Hydroboration also works with alkynes, but again, this will be talked about in more detail when we go through the reactions of H3O+ and oxymercuration.
Oxidative cleavage also works with alkynes, but it’s much simpler. Whether we use KMnO4 or O3, we always get carboxylic acids, no matter what.
That’s it! Next time we’ll zero in on some of the key reactions of alkynes.
Thanks for reading! James
P.S. Here’ s a summary sheet on the reactions of alkynes that isn’t on my blog