<p>Unlike the saturated hydrocarbons in kerosene, <a href="https://en.wikipedia.org/wiki/Linear_acetylenic_carbon" rel="nofollow noreferrer">carbynes</a> are unsaturated carbon chains with alternating single and triple bonds.
Molecules containing such chains are called <a href="https://en.wikipedia.org/wiki/Polyyne" rel="nofollow noreferrer">polyynes</a>, e.g. the short <a href="https://en.wikipedia.org/wiki/Cyanopolyyne" rel="nofollow noreferrer">cyanopolyyne</a> HC<sub>5</sub>N:</p>

<p>H&minus;C&equiv;C&minus;C&equiv;C&minus;C&equiv;N</p>

<p>Those carbon atoms readily interact, and long chains (if they form; see comments) are more likely to crosslink or form cycles than to remain linear.
Tarakeshwar et al. suggest that iron clusters in "pseudocarbynes" inhibit this by bonding to some of the carbons.</p>

<p><a href="https://ui.adsabs.harvard.edu/abs/2016MNRAS.463.4175L/abstract" rel="nofollow noreferrer">Loomis et al. 2016</a> looked for cyanopolyynes in radio spectra of <a href="https://en.wikipedia.org/wiki/Taurus_Molecular_Cloud" rel="nofollow noreferrer">Taurus molecular cloud 1</a>.
They got a good HC<sub>9</sub>N signal but did not detect the HC<sub>11</sub>N lines <a href="https://ui.adsabs.harvard.edu/abs/1996ApJ...469L..65T/abstract" rel="nofollow noreferrer">Travers et al. 1996</a> observed in the lab.</p>

CC BY-SA 4.0
2019-07-10T10:02:17.993Z
While this is the answers, I would like to add that even considering alkanes, while it would sound exaggerated that longer chairs are unstable it is true that both heat and free energy of formation increase as the number of C atoms increases. Easy to see it considering just entropy: their formation from separated C atoms: is getting unlikely, being the formation of more but shorter chain statistically favoured. Plus one to the answer. This is just for completeness.
Alchimista
CC BY-SA 4.0
2019-07-10T12:58:04.533Z
Oh! (almost) no hydrogens! Now I understand much better, thanks!
uhoh
CC BY-SA 4.0
2019-07-11T09:41:04.437Z
No my comment is about hydrocarbons. CH3-(CH2)n-CH3 get intrinsically less stable as n rises. While a long chain is certainly stable in practice, its formation is nevertheless not the most favored as compared to that of shorter members.
Alchimista
CC BY-SA 4.0
2019-07-10T08:48:00.153Z
2019-07-15T17:26:08.203Z
2019-07-15T17:26:08.203Z
Mike G
<p>The Phys.org article <a href="https://phys.org/news/2019-07-interstellar-iron-isnt-plain-sight.html" rel="nofollow noreferrer">Interstellar iron isn't missing, it's just hiding in plain sight</a> links to the paywalled article <a href="https://iopscience.iop.org/article/10.3847/1538-4357/ab22b7" rel="nofollow noreferrer">On the Structure, Magnetic Properties, and Infrared Spectra of Iron Pseudocarbynes in the Interstellar Medium</a> (Tarakeshwar, Buseck and Timmes, Arizona State University), and says:</p>
<blockquote>
<p>The new work may also solve another longstanding puzzle. <strong>Carbon chains with more than nine atoms are unstable, the team explains.</strong> Yet observations have detected more complex carbon molecules in interstellar space. How nature builds these complex carbon molecules from simpler carbon molecules has been a mystery for many years.</p>
<p>Buseck explained, &quot;<strong>Longer carbon chains are stablized by the addition of iron clusters.</strong>&quot; This opens a new pathway for building more complex molecules in space, such as polyaromatic hydrocarbons, of which naphthalene is a familiar example, being the main ingredient in mothballs.</p>
<p>Said Timmes, &quot;<strong>Our work provides new insights into bridging the yawning gap between molecules containing nine or fewer carbon atoms and complex molecules</strong> such as C60 buckminsterfullerene, better known as 'buckyballs.'&quot;</p>
</blockquote>
<p><strong>Question:</strong> On Earth we don't see a big difference between hydrocarbon chains with lengths below and above 9 (think kerosene, wax...), why is there such a cutoff in stability in interstellar space? What is it about hydrocarbon chains longer than 9 atoms that makes them unstable there but not here?</p>

CC BY-SA 4.0
2019-07-10T09:12:48.530Z
Strange that they didn't put in on arXiv…
pela
CC BY-SA 4.0
2019-07-10T12:59:02.190Z
@pela ya I had checked quickly and hadn't found it; at least not with the same/similar title.
uhoh
CC BY-SA 4.0
2019-07-10T14:43:07.157Z
The first two authors are chemists who don't normally use arXiv.
Mike G
<interstellar-medium><astrochemistry>
Why do these astronomers say that "Carbon chains with more than nine atoms are unstable"?
uhoh
# Why do these astronomers say that "Carbon chains with more than nine atoms are unstable"?

(Asked by: username_0 on 2019-07-10)

The Phys.org article [Interstellar iron isn't missing, it's just hiding in plain sight](https://phys.org/news/2019-07-interstellar-iron-isnt-plain-sight.html) links to the paywalled article [On the Structure, Magnetic Properties, and Infrared Spectra of Iron Pseudocarbynes in the Interstellar Medium](https://iopscience.iop.org/article/10.3847/1538-4357/ab22b7) (Tarakeshwar, Buseck and Timmes, Arizona State University), and says:

> The new work may also solve another longstanding puzzle. **Carbon chains with more than nine atoms are unstable, the team explains.** Yet observations have detected more complex carbon molecules in interstellar space. How nature builds these complex carbon molecules from simpler carbon molecules has been a mystery for many years.
>
> Buseck explained, "**Longer carbon chains are stablized by the addition of iron clusters.**" This opens a new pathway for building more complex molecules in space, such as polyaromatic hydrocarbons, of which naphthalene is a familiar example, being the main ingredient in mothballs.
>
> Said Timmes, "**Our work provides new insights into bridging the yawning gap between molecules containing nine or fewer carbon atoms and complex molecules** such as C60 buckminsterfullerene, better known as 'buckyballs.'"

**Question:** On Earth we don't see a big difference between hydrocarbon chains with lengths below and above 9 (think kerosene, wax...), why is there such a cutoff in stability in interstellar space? What is it about hydrocarbon chains longer than 9 atoms that makes them unstable there but not here?

**Tags:** interstellar-medium, astrochemistry

> **Comments:**
> **username_1**: Strange that they didn't put in on arXiv…  
> **username_0**: @username_1 ya I had checked quickly and hadn't found it; at least not with the same/similar title.  
> **username_2**: The first two authors are chemists who don't normally use arXiv.  

---

## 1 answers

### Answer by username_2 on 2019-07-10. Score: 6 | Accepted answer

Unlike the saturated hydrocarbons in kerosene, [carbynes](https://en.wikipedia.org/wiki/Linear_acetylenic_carbon) are unsaturated carbon chains with alternating single and triple bonds.
Molecules containing such chains are called [polyynes](https://en.wikipedia.org/wiki/Polyyne), e.g. the short [cyanopolyyne](https://en.wikipedia.org/wiki/Cyanopolyyne) HC5N:

H−C≡C−C≡C−C≡N

Those carbon atoms readily interact, and long chains (if they form; see comments) are more likely to crosslink or form cycles than to remain linear.
Tarakeshwar et al. suggest that iron clusters in "pseudocarbynes" inhibit this by bonding to some of the carbons.

[Loomis et al. 2016](https://ui.adsabs.harvard.edu/abs/2016MNRAS.463.4175L/abstract) looked for cyanopolyynes in radio spectra of [Taurus molecular cloud 1](https://en.wikipedia.org/wiki/Taurus_Molecular_Cloud).
They got a good HC9N signal but did not detect the HC11N lines [Travers et al. 1996](https://ui.adsabs.harvard.edu/abs/1996ApJ...469L..65T/abstract) observed in the lab.

> **Comments:**
> **username_3**: While this is the answers, I would like to add that even considering alkanes, while it would sound exaggerated that longer chairs are unstable it is true that both heat and free energy of formation increase as the number of C atoms increases. Easy to see it considering just entropy: their formation from separated C atoms: is getting unlikely, being the formation of more but shorter chain statistically favoured. Plus one to the answer. This is just for completeness.  
> **username_0**: Oh! (almost) no hydrogens! Now I understand much better, thanks!  
> **username_3**: No my comment is about hydrocarbons. CH3-(CH2)n-CH3 get intrinsically less stable as n rises. While a long chain is certainly stable in practice, its formation is nevertheless not the most favored as compared to that of shorter members.  

---

