Last time we discussed the similarities (and differences) between the carbocation pathway for alkenes and alkynes. In this post, we’ll do the same for the “3-membered ring pathway”.
If you’ll recall from the series of posts on alkenes, alkenes react with certain electrophiles (such as halogens, among others) to give positively charged bridged intermediates. These intermediates then undergo backside attack by a nucleophile, giving products with trans stereochemistry.
So… we might also expect that alkynes, being so similar to alkenes, should also react in a similar fashion. [Of course, as someone who has studied organic chemistry for awhile could tell you, what we "expect" to happen is not always what does actually happen!].
Well, happily for us, the reaction of alkynes with electrophiles such as Cl2, Br2, and I2 does give very similar results to what is observed with alkenes. For example, treatment of an alkyne with 1 equivalent of Cl2 provides a dichlorinated alkene with the two chlorides opposite to each other. If a second equivalent of Cl2 is added, the tetrachloro derivative will form. [Note 1]
So how might this reaction work? Here’s a proposal.
Just as with alkenes, a π bond from the alkyne can act as a nucleophile, attacking Cl2 and giving rise to a bridged intermediate. In the next step, chloride ion attacks the carbon from the back face, leading to the trans product.
There is actually a very interesting observation to point out here, but I’ll leave that to the “Notes” section below as it isn”t absolutely essential for most readers’ purposes. Here’s the teaser, though: alkynes are considerably slower to react than alkenes are. [Note 2].
What’s next on this list? Well, for our purposes, halogenation of alkynes just about covers the important reactions for the 3-membered ring pathway of alkynes. Yes, oxymercuration of alkynes proceeds through the 3-membered ring pathway, but as discussed in this earlier post, this reaction is a bit of an anomaly: treatment of alkynes with mercury (II) and water [and acid] actually provides us with a carbonyl species [ketones, generally] after keto-enol tautomerism of an enol intermediate.
At this point it’s worth summarizing the key similarities and differences between the 3-membered ring pathway for alkynes and alkenes.
In the next post, we’ll compare the “Concerted” pathway for alkynes and alkenes.
[Note 1]. It’s not a problem to isolate the dichlorinated alkene here: the two electron-withdrawing chlorines make it a poorer nucleophile than the starting alkyne. This means that it’s possible to add a different electrophile to the alkene, for instance. So if you wanted to chlorinate then brominate, that would be a feasible option here (still giving the tetrahalide product).
[Note 2]. Why might the reaction of alkynes be slower than that for alkenes? After all, shouldn’t the alkyne be more “exposed” than the alkene, less sterically hindered? Well, the 3-membered ring intermediate formed from alkynes and halogens has two properties which make it more unstable than the corresponding 3-membered ring intermediate formed from alkenes. First of all, the additional double bond leads to considerably more ring strain; sp2 hybridized carbons [ideal angle 120°] constrained into a triangle [internal angle 60°] is more unstable than an sp3 hybridized carbon [ideal angle 109°] would be.
There’s a second point which doesn’t become apparent for most students until second-semester organic chemistry. The 3-membered ring intermediate formed has antiaromatic character. That is, there are 4 π electrons constrained in a conjugated ring, similar to the [never isolated] oxirene. Therefore this intermediate should be particularly high-energy and have a higher activation barrier to formation.
Note that there is some disagreement on the mechanism; it has been proposed that this reaction might proceed through nucleophilic attack on alkyne, at least for the first equivalent of Br2 [according to my March 5th ed. - Sinn, H. et. al., Montash Chem 1965, 96, 1036 ]
[Note 3] What about formation of halohydrins with, say, Cl2 and H2O, like we did with alkenes? Well, that reaction also works, but just as with oxymercuration, it’s complicated: we make a halogenated enol intermediate, which again goes through keto-enol tautomerization and forms a carbonyl. Because tautomerization is usually a 2nd semester topic, and most textbooks figure that it’s not worth going into this reaction in detail at this time , the topic is usually absent from the chapter on alkynes. For the super curious, here’s a proposal: