Texas Carbon

A common mistake made in drawing organic compounds is exceeding the valency of a carbon atom:

Mistakenly-drawn carbon valencies

Colloquially, carbon atoms with five covalent bonds are called Texas Carbons
Either because the Texas star has five points or because everything is bigger in Texas. . Formally, they are known as pentavalent carbons.

It turns out that well-formed pentavalent carbons are much more frequent than an organic chemistry class may have you believe.

#SuperAcids

Superacidic cationic pentavalent carbons can be formed in the presence of very strong acids. The simplest pentavalent carbon, methanium (), can be prepared from methane in the presence of .

Ethanium () is a protonated ethane derivative, also hosting a pentavalent carbon. Ethanium is interesting partly because it is prepared using methanium:

and partly because it easily dissociates into ethenium, which also has pentavalent carbons:

$\text{C}_2\text{H}_7^+ \to \text{C}_2\text{H}_5^+ + \text{H}_2$

In practice, pentavalent acids are formed as intermediate species and during thermal cracking Per to a DOE report, page 8. .

#Both B and C

Yamamotoa’s and Akibab’s 2004 Synthesis of Hypervalent Pentavalent Carbon and Boron Compounds describes the characteristics of larger pentavalent compounds. It appears (somewhat expectedly) that bonds to well-charged, larger-size atoms like those in Group 15 and 16 help stabilize pentavalent carbon and boron.

Pentavalent Carbon and Boron Compounds

The bond angle of the emphasized is , more than the expected for a star-shaped pentavalency, and likely explained by the distance of the oxygens from the carbon.

#Are there star-shaped pentavalencies?

Yes! One is stable methanium, which we have already seen. Another is , which is predicted to be planar and possible to prepare experimentally in the gas phase. Grande-Aztatzi et al. predict the lithium cluster derivatives to also be planar. Both of these form beautiful five-point shapes.

Star-shaped pentavalent carbon derivatives

#Beyond the stars

Pentavalent carbons are one of those ideas most of us won’t deal with very often. But I think unique patterns like these are worth knowing, at least for the sake of knowing.

If you’re interested in hypervalencies, you may enjoy hexavalent carbons and undecavalent borons.