In the previous three posts on alkynes we’ve introduced some new reactions that are specific to alkynes (versus alkenes): deprotonation (and subsequent substitution), partial reduction to alkenes, and the formation of aldehydes and ketones through net “hydration”. With all the focus on the ways in which alkyne chemistry can differ from alkene chemistry, it’s helpful to be reminded of all the ways in which they are similar.
In this post we’ll go back to a key reaction mechanism pattern we observed with alkenes: the so-called, “carbocation pathway” – and explore how many of the reactions of alkenes we’re familiar with can also be used with alkynes.
The three major examples in this category are the reaction of hydrohalic acids (H-Cl, H-Br, and H-I) with alkynes. If you recall, when added to alkenes these reagents were attacked by the π bond of the alkene to give a carbocation (on the most substituted carbon, giving “Markovnikov” regioselectivity) followed by attack of halide ion on the carbocation.
Since alkynes merely differ from alkenes in the addition of a second π bond, we would expect that these reactions would also work for alkynes as well – and they do!
If we treat an alkyne with a single equivalent of H–Cl [note – we’ll just use H-Cl in all of these examples, but HBr and HI work in exactly the same way] we end up forming an alkenyl chloride. Note that the chlorine atom ends up attached to the most substituted carbon of the alkene [“Markovnikov” regioselectivity].
Note that the product here still has a π bond. You might be wondering if it’s possible to for this π bond to react with a second equivalent of H-Cl. The answer is yes. [Note – it is possible to just “stop” the reaction at this stage if we use just one equivalent, because the product (alkenyl chloride) is less reactive towards HCl than the starting alkyne]. Indeed, if we add a second equivalent of H-Cl, it adds to either side of the C-C π bond, giving us the product where two chlorine atoms are on the same carbon. By the way, we call this a “geminal” dichloride (think Latin – “gemini” = twins).
We can also get this product if we simply add two equivalents of H-Cl to the starting alkyne.
So how might this reaction work? In a very similar fashion to how H-Cl adds to alkenes.
The first step is protonation of the alkyne with H-Cl in such a manner as to give the most stable carbocation intermediate. Since carbocations are stabilized to a greater extent by electron releasing alkyl substituents than by hydrogen, the new carbocation will form at the end of the alkyne bearing the carbon substituent. In the next step, the carbocation is attacked by the chloride ion to give the alkenyl chloride.
What about the second equivalent of H-Cl ? Given the fact that the geminal dichloride is the product here, the most reasonable mechanism for its formation is merely a repeat of the steps from the first reaction (as shown). However it’s worth pointing out one interesting feature. Note that the carbocation in this case bears a chloride ion. Since carbocations are electron poor, and chlorine is quite an electronegative element, it’s interesting to point out that the electron releasing ability of the alkyl group [and the ability of chlorine to donate a lone pair to the carbocation] “win out” here over the electron-withdrawing character of chloride ion.
As mentioned above, the reactions of alkynes with HBr and HI (as well as HF, just in case you’re curious) follow the exact same pathway.
It’s probably worth tying back this post to the post on alkenes and the carbocation pathway, noting the similarities and differences between the chemistry of alkenes and alkynes. Hopefully this table will prove useful:
As with alkenes, reactions that follow this pathway proceed through a carbocation intermediate and provide the “Markovnikov” products as major. The key difference in this pathway is that hydration of alkenes gives alcohols, whereas hydration of alkynes gives carbonyl derivatives (i.e. ketones/aldehydes) after keto-enol tautomerism of the intermediate enol.
In the next post, we’ll explore the “3-membered ring” pathway with alkynes.