Thread by @mike_malaska

This is hilarious!
"Because everyone knows that carbon can only have four bonds, right?"

(Well, that's kinda not true....)
[thread]

https://twitter.com/compoundchem/status/1454735298106101761

...and yes, I do find this photo hilarously nerdly. And am just gonna riff off it here.

It was drilled into my head in organic chemistry that "carbon can only have four bonds" and "Haha, you drew a pentavalent carbon! That is WRONG! Points off!"

But...it can have five bonds!

And how that works is very interesting, and maybe one of the most important things in the Universe for organic chemistry.

Carbon can have 4 sp3 hybridized orbitals around it in tetrahedral geometry. But, if you stick a proton on it, it can rearrange.

CH4 is definitely Not Happy about being protonated. But if it can't play "hot potato" with the proton and give it to something else (think vacuum, deep space), the methan carbon is stuck with it and has to deal with being CH5+. At least temporarily.

So what does it look like?

It looks like this. The carbon atom still has 4-sp3 orbitals, tetrahedral geometry. But one of the sp3 orbitals is now in a 3-center 2-electron bond with effectively side-bound H2. A little triangle with 2 electrons buzzing around in there somewhere.

(oh man that drawing looks terrible. Sorry about that y'all.)

That positively charged "pentavalent carbon" is what happens in a mass spec system in positive mode with saturated hydrocarbon molecules in that don't have anywhere else to protonate. (your TA prolly never thought of that, now did they?)

This sort of thing also happens in planetary atmospheres where methane (very common molecule in Universe) gets stuck with a proton. That happens in planetary atmospheres where there is a lot of hydrogen also. (Because of H3+, but that's a whole 'nother story....) Jupiter, etc...

CH5+ mildly boring, just runs around and transfers protons. (It is Not Happy, and will shove that proton based on proton affinities.) Here is a handy chart:

So, looking at chart, any protonated H2 (= H3+, a 3c,2e- little triangle of hydrogen nuclei), gonna shove a proton onto methane if it can bump into it, and methane gonna shove off onto ethylene if it can find it, and any N or O atom is gonna get stuck with that proton.

But there's a side story where stuff gets REALLY INTERESTING. If you have a planetary atmosphere (or galactic cloud I guess) that doesn't have a lot of H3+ and that can do a proton transfer to methane, what happens?

Where you don't have a bunch of H2, and you have Ar or N2, (think Pluto or Titan way upper atmosphere) then UV radiation ionizes those, and then they react with CH4, steal an electron (reform Ar or N2), create a spare proton (H+) and generate CH3+.
CH3+ does stuff.

CH5+ pretty boring. If you drop an electron into that system (if it recombines in Titan upper atmosphere with electrons running around). It'll make .CH3 radical. (branching ratio 0.7)
Some chemists think .CH3 radical is "exciting", but in relative terms, it's pretty boring.

In contrast, CH3+ is very exciting. If you drop an electron into that system?
Whoo-boy! stuff goes down!
0.4 branching ratio to :CH2 carbene (whoa!)
0.3 branching ratio to .:CH carbyne (whoa! whoa!)
0.3 branching ratio to nekkid :C: ready to rock-and-roll and shred (OMG!!)

What does :CH2 methylene carbene do? Yeah, they don't teach you that stuff in introductory organic chemistry. Carbene chemistry is next level powerful stuff.
Think of it as acting more like an electron-defiencient metal. C-H insertions, [1+2]cycloadditions.
Metal chemistry!

And those CH3+ reactions build serious unsaturated molecules, then go on to build even more complex organic stuff. acetylene, benzene, naphthalene. On a planetary scale, most original really complex hydrocarbon molecules may have started with CH3+. CH5+ is the boring pathway.

So the CH5+ / CH3+ story is really key to understanding organic chemistry in the universe.

Hoping y'all enjoyed this thread, and maybe it might gain you a few arguments you can use on your TA to win back some organic chemistry exam points.

(and if it makes your TA stop repeating the tired old dogma that "carbon can never have more that 4 things bonded to it" then I have attained my goal.)

And of course, some links:
en.wikipedia.org/wiki/Methanium

Details of CH5+ and CH3+ in planetary atmospheres (paywall, sorry)
Imanaka and Smith, "EUV Photochemical Production of Unsaturated Hydrocarbons: Implications to EUV Photochemistry in Titan and Jovian Planets". J. Phys. Chem. A 2009, 113, 42, 11187–11194.
pubs.acs.org/doi/pdf/10.102…