How strong a long chain of c=c double bonds will be?

[yonos]

How strong a long chain of carbon carbon double bonds will be compared to nano tubes?
H>c=c=c=c=c=c=c=c=...=c=c=c=c=c=c=c=c=c=c=c=c<H

 

[mfb]

They are quite reactive. Here is a chain of 6000 - but they had to wrap the chains in double-walled carbon nanotubes. I don't see any discussion about the mechanical resistance.

 

[TeethWhitener]

Two CC double bonds with a common center (that is, C=C=C ) are known as cumulenes, and as @mfb pointed out, they generally are easily isomerizable to polyalkynes (C-C≡C). The parent compound of this functionality is allene, which exists in equilibrium with propyne. Cumulenes are actually less stable than conjugated alkenes (C-C=C...) where a single and double bond alternate. This occurs because the valence π orbitals in a cumulene are at right angles to one another and so do not allow much delocalization. For conjugated species, however, the π orbitals are oriented such that an electron can delocalize over a large distance, lowering the energy of the system and stabilizing it.

 

[DrDu]

Cumulenes are actually less stable than conjugated alkenes (C-C=C...) where a single and double bond alternate. This occurs because the valence π orbitals in a cumulene are at right angles to one another and so do not allow much delocalization. For conjugated species, however, the π orbitals are oriented such that an electron can delocalize over a large distance, lowering the energy of the system and stabilizing it.

I think this is more a consequence of the Peierls instability of long chains with equidistant bonds.

 

[TeethWhitener]

I think this is more a consequence of the Peierls instability of long chains with equidistant bonds.

In the long-chain limit, cumulenes buckling to polyalkynes is due to Peierls distortion, but you don’t see the same strong bifurcation in CC bond lengths in polyalkenes like you do with polyalkynes. I couldn’t say for certain why that’s the case off the top of my head. (It’s also interesting that you see isomerization in short chain cumulenes as well. I have an idea for why this occurs, but no hard evidence.)

To the OP, I’ve seen first principles calculations claiming that long cumulenes are super strong, but I doubt they’re experimentally feasible. As @DrDu pointed out, they’re unstable with respect to Peierls distortion and long alkynes undergo interchain coupling which destroys the structure.

 

[mfb]

@TeethWhitener: What about a combination of these chains plus nanotubes, as discussed in the reference I posted? Could it contribute enough to the strength to make it interesting?

 

[TeethWhitener]

@TeethWhitener: What about a combination of these chains plus nanotubes, as discussed in the reference I posted? Could it contribute enough to the strength to make it interesting?

I don’t have access to the full paper right now, but I doubt it. For One, I imagine the nanotube (already a very strong material) would swamp any effect you’d see from the carbyne. Also, as far as actually using the material in bulk for its mechanical properties, the limiting factor is always going to be the weakness of the noncovalent intertube interaction. It’s the same reason the whole “elephant on a pencil” argument for graphene’s strength is so misleading.

I tend to be quite skeptical of theorists’ claims in materials science, but maybe it’s just because I’ve seen way too much vaporware in that field.