This "Impossible" Triple bond Actually Exists

Added: 2026-05-12

[00:00:00]This is an alkyne, perfectly linear.

[00:00:03]But, what happens when you distort a triple bond?

[00:00:07]Should the molecule fall apart, or does something unexpected happen?

[00:00:12]By the end of the video, you will see how chemists push bonds far beyond their limits without breaking them.

[00:00:19]To understand the concept of distortion, it's good to compare a triple bond with a double bond.

[00:00:26]Here you can see how a pi bond is created. Two p orbitals of the carbon atoms have a side-to-side interaction.

[00:00:33]When the double bond has a planar structure, the p orbitals have maximum overlap.

[00:00:39]The same is true for a triple bond. Here is the orbital description of the pi bonds.

[00:00:45]Two p orbitals on the y axis and two p orbitals on the z axis have side-to-side interaction to create the pi bond.

[00:00:54]For an alkyne in linear geometry, the orbitals have maximum interaction.

[00:01:00]Take this double bond as an example.

[00:01:02]Let's distort the structure. As you can see, the molecule doesn't have planar geometry, and the overlap between p orbitals decreases. So, the pi bond becomes weaker in the distorted structure.

[00:01:15]If we continue distorting in extreme conditions, the two p orbitals become orthogonal to each other. This means there's no interaction, and the pi bond is broken. This type of distortion is called twisting, but it's not the only type of distortion. Generally speaking, for pi bonds, there are three types of distortion: bending, twisting, and pyramidalization. You've just understood the concept of twisting in alkynes. Now, let's explore bending in alkynes.

[00:01:46]Here I show the interaction between two orbitals in a pi bond. In a normal triple bond, the angle between two substituents is 180°.

[00:01:55]In bending, this angle decreases, and the substituents come closer together.

[00:02:00]As a result of bending, the interaction between P orbitals decreases because the upper lobes move farther apart, which means that the pi bond is weaker in a bent structure.

[00:02:14]Pyramidalization is another type of distortion. Take this double bond as an example, which has a planar geometry with all substituents in the same plane.

[00:02:24]In pyramidalization, the carbon atom of the double bond is deviated from the conventional trigonal planar geometry, such that it resembles a tetrahedral shape.

[00:02:34]What is the effect of pyramidalization on the P orbitals of the double bond?

[00:02:39]How does it change the reactivity of alkenes?

[00:02:42]Understanding this type of distortion may be challenging. To fully understand it, I've created a video in which I explained the concept of pyramidalization in detail.

[00:02:52]I've put the link in the description.

[00:02:53]Check it out if you're interested.

[00:02:56]Now I show you what we can do with a distorted pi bond.

[00:03:00]Before jumping into synthetic application, you should understand the orbital interaction in a distorted pi bond.

[00:03:08]These are HOMO and LUMO orbitals of a pi bond. When it's distorted, the energy level of the LUMO orbital decreases and the energy level of the HOMO orbital increases. This means that the LUMO orbital in the distorted structure is more accessible for the reaction. In other words, distortion weakens the pi bond. This weakness is reflected in the lower energy of the LUMO orbital, resulting in higher reactivity, especially in cycloaddition reactions.

[00:03:38]Let's get us started with bending in alkynes. A triple bond is forced to bend when it's confined in a ring. A cyclic alkyne with an eight-membered ring is the most popular member of this family because of its stability and commercial availability. To understand the reactivity of cyclic alkynes, let's compare it with a linear alkyne.

[00:04:00]The cycloaddition is known as click reaction, which occurs between acetylene and azide.

[00:04:06]This is actually a 3 + 2 cycloaddition resulting in a five-membered ring. As you see, heat or catalyst is needed to provide the activation energy for the cycloaddition.

[00:04:18]But, the reaction can be carried out without catalyst by using cyclooctyne.

[00:04:24]In other words, the benz alkyne is reactive enough to give the final product.

[00:04:29]This is a good example of strain-promoted azide-alkyne coupling in which the strain energy of the alkyne pushed reaction forward.

[00:04:38]Calculations show that the strain energy of cyclic alkynes is affected by the size of the ring.

[00:04:45]Three to six-membered rings are very reactive and undergo side reactions such as rearrangement or dimerization. So, they aren't suitable for cycloaddition reactions.

[00:04:58]Take the indole skeleton as an example.

[00:05:01]The triple bond in this position cannot exist because the small size of the five-membered ring makes this intermediate very unstable.

[00:05:10]So, this compound is not accessible with common synthetic approaches.

[00:05:15]But, organic chemists can stabilize this reactive intermediate by using transition metals. In this strategy, the triple bond coordinates to the transition metal and can be isolated.

[00:05:27]To do this, readily available 3-bromoindole is lithiated at the two position, setting the stage for borylation.

[00:05:35]In the next step, it undergoes oxidative addition with a nickel complex in the presence of triphenylphosphine.

[00:05:43]Notice that in the reaction mixture, the cyclooctadiene is exchanged with triphenylphosphine.

[00:05:49]Next, this intermediate undergoes another ligand exchange with DCPE to produce a triple bond in the five-membered ring. It should be activated by potassium tert-pentoxide.

[00:06:01]In the activation step, tert-pentoxide coordinates to the boron complex. One of the oxygens of the pinacol ligand attached to the nickel complex and the bromine leaves the compound.

[00:06:14]This intermediate undergoes transmetallation producing the desired triple bond coordinated to the nickel center.

[00:06:22]This interesting compound has ambiphilic reactivity. It means that it reacts as both a nucleophile and an electrophile.

[00:06:30]We can use this feature in bifunctionalization.

[00:06:34]For example, in reaction with 2-pyridylzinc bromide results in nucleophilic insertion at the free position and the following electrophilic coupling with iodomethane leads to the installation of the methyl group at the two position. With this strategy, we can produce a variety of bifunctionalized indoles.

[00:06:55]Another interesting molecule that contains a benz alkyne is benzyne. This highly reactive species is derived from benzene. The common strategy for creating benzyne is the Kobayashi reaction in which a triflate is installed at the ortho position of TMS.

[00:07:12]Adding a fluoride source generates the benzyne.

[00:07:15]Notice that because of the high reactivity of benzyne, it's generated in situ and trapped in the reaction.

[00:07:22]Mechanistically, the fluoride ion coordinates to the silicon to form a negatively charged silicate.

[00:07:29]Then it undergoes dissociation leaving the negative charge on the ring.

[00:07:33]The creation of the strong silicon-fluoride bond is a driving force for the reaction.

[00:07:39]Finally, benzyne is generated by elimination of the triflate.

[00:07:44]Benzyne can be used as a two-carbon unit in cycloaddition reactions. For example, furan, as a four-carbon unit, reacts with benzyne to produce a six-membered ring with a bridge structure.

[00:07:57]Here is another interesting example. As a result of a 4 + 2 cycloaddition, a bridge-type intermediate is produced, but it's not stable.

[00:08:06]Acetone leaves the compound by moving the pi bond and breaking the sigma bonds, leaving us with derivatives of naphthalenes.

[00:08:15]In this video, I showed you how a benzalkyne can be created, why it's so reactive, and discuss its synthetic applications.

[00:08:24]If you're interested in unusual structures in chemistry, check out this video about unusual reactivity of cyclopropane. I put the link in the description.