How does the chemistry of alkynes compare to alkenes? As we’ve seen in some previous posts, there are some significant differences, but a lot of the chemistry “rhymes”, if you will. In the series on alkenes we broke down most of the reactions into three major categories according to their mechanisms – the “carbocation”, “3-membered ring”, and “concerted” pathways, and – you guessed it – since we’ve covered the carbocation and 3-membered ring pathways for alkynes in previous posts, this post concerns the “concerted” category. Recall that reactions that proceed through a “concerted” mechanism break the C-C π bond with concomitant formation of two new single bonds to the adjacent carbons, which form on the same face (“syn” addition). As we’ll see, there isn’t actually a lot of new ground in this post that we haven’t discussed before, except – interestingly – for some reactions that are absent in alkyne chemistry.
The “Concerted” Pathway for Alkynes
So – what works, and what doesn’t?
Two major reactions in the “concerted” pathway that work for alkynes are hydrogenation and hydroboration. However, as we’ve already seen, each of these reactions comes with a twist when applied to alkynes.
With hydrogenation, treatment of an alkyne with a late metal catalyst such as Pd-C (or platinum on carbon, among others) in the presence of hydrogen leads not just to one hydrogenation, but two. The product is an alkane. It’s possible to get the reaction to stop “halfway” by using a less reactive catalyst such as “Lindlar’s catalyst” or by using nickel boride. This provides the cis alkene. Alternatively (although this doesn’t really count as a “concerted” mechanism, one can obtain the trans alkene through the use of sodium in ammonia (Na/NH3).
Hydroboration provides the anti-Markovnikov product just as it does for alkenes, although the resulting product after oxidation – the “enol” – is usually unstable relative to its constitutional isomer, the “keto” form, with which it is in equilibrium (an average stability ratio is about 5000:1 favoring the keto form) through a reaction known as “keto-enol tautomerism”.
There’s actually a third reaction that does work for alkynes, although it is rarely mentioned in this context. It is possible to form cyclopropenes through the “Simmons-Smith” reaction of alkynes with zinc-copper couple (Zn-Cu) and diiodomethane (CH2I2). Although this is an interesting result, and cyclopropenes are fun intermediates in advanced organic chemistry (and even are found in nature!) their application in introductory organic chemistry is limited and we shall speak no more of this reaction.
An even more interesting question on this topic of “concerted” reactions is “What Doesn’t Work?”
First of all, one of the more useful reactions of alkenes is their conversion to epoxides through the use of a peroxyacid like m-chloroperoxybenzoic acid (m-CPBA).
Try it on alkynes, though, and nothing happens! It just doesn’t work.
Ha! Learning organic chemistry – as you must know by now – is a process of being continually surprised by the complex phenomena that can lurk behind the most innocuous seeming questions. Two answers to this question are appropriate: one is “you don’t need to know yet”, which, to be honest, is an answer a lot of students are perfectly fine with. The second answer is that it turns out that the product of the hypothetical reaction between m-CPBA and an alkyne is a molecule called an “oxirene” – which has a very interesting property known as “antiaromaticity“. For reasons we can’t get into right now, antiaromatic molecules are particularly unstable, and in fact oxirenes have only rarely been isolated – and even then, only at very low temperatures.
Dihydroxylation with OsO4 is another useful reaction of alkenes that fails for alkynes (or at the very least, is not significant). In all my years I don’t recall seeing a single example of this reaction being effective on an alkyne, but if someone out there has, please feel free to correct me. [update – thank you to commenter Dion who mentioned an example, I’m attaching a screen shot of the page to the bottom of the post] In any case, the fact that OsO4 is not a significant reaction for alkynes is useful to keep in mind – this will become important when we start to plan out sequences of reactions (synthesis!).
In the next post let’s circle back a bit an talk about an interesting way to make alkynes – and then we’ll finally get to the really good stuff: how to design sequences of reactions.
Next Post: Alkynes Via Elimination Reactions
Update : Here’s some examples from this reference of reactions of OsO4 with alkynes. Safe to say this doesn’t appear in introductory textbooks. Thanks to commenter Dion for mentioning this. [Edit: Commenter Jon mentions that this reaction cannot be found on Reaxys.]