Submitted by [deleted] t3_10hguop in askscience
Ive always wondered if carbon is tetra valent why cant it form bonds with itselt and become C2 molecule?
Comments
PD_31 t1_j5b0ag0 wrote
It can. Carbon can actually form a quadruple bond with itself but the fourth bond is extremely weak because of the way the hybrid orbitals are arranged. Strength of bond depends on orbital overlap and these fourth orbitals are pointing in opposite direction (think the hydrogens in acetylene) so there's little overlap.
So C2 isn't a very stable molecule; it's also not a particularly interesting one since it's literally just two carbons.
Nightnlight1 t1_j5cak0y wrote
I know about orbitals and now I wonder why wiki is showing only 2 bounds then?
https://en.m.wikipedia.org/wiki/Diatomic_carbon
I googled this some days ago because of the comet and thought more of an radical, besides the two bonds, even though this doesn't make much sense. Or maybe they're delocalized?
Indemnity4 t1_j5hlrr1 wrote
Better image of C2 quadruple bond. The molecular orbitals look impossible to a chemist, but that's more a function of how we represent images.
Nightnlight1 t1_j5ig0xv wrote
That's super cool, thank you!
RunagateRampant t1_j5ayda2 wrote
Carbon can form bonds with other carbon molecules. This can be a single bond, double bond, or triple bond. The reason a tetrabond between two carbons isn’t possible has to do with the geometry of the electron orbitals.
C2 alone isn’t possible. But C2H4 (ethylene) exists.
Indemnity4 t1_j5hlhek wrote
> C2 alone isn’t possible
Sure it is, in the gas phase! Diatomic carbon is represented as :C=C:. To get deeper into MO, it exists in both singlet and triple forms.
What is a little bit more complicated is quadruple bonded carbon. It does appear it can have a forth order bond to another carbon atom (tetravalent), but it is in the form of a pi-bond and two sigma-bonds.
[deleted] OP t1_j5az6dr wrote
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Any-Broccoli-3911 t1_j5c0zym wrote
C2 exists. It still has only a double bond though, so it has unpaired electrons which makes it very reactive. It will try to link with any other molecules it can.
https://en.m.wikipedia.org/wiki/Diatomic_carbon
Carbon rarely do triple bonds because they are not energically favorable for carbon. It will typically prefer single or double bond.
Carbon cannot do quadruple bonds because only atoms with d electrons can do a quadruple bond, and in practice only the ones in the middle of the d block do.
Indemnity4 t1_j5hmdd9 wrote
Quadruple C2 has two pi-bonds, a strong sigma-bond and a second weak sigma-bond.
The electron in the s orbital comes out to play. It's weird.
Any-Broccoli-3911 t1_j5howpz wrote
MO theory says it's a double bond while VB theory says it's a quadruple bond. It's still controversial.
https://mappingignorance.org/2013/05/22/c2-molecule-doubly-or-quadruply-bonded/
https://en.m.wikipedia.org/wiki/Quadruple_bond
In MO theory, there are 2 pi bonds from the px and py electrons.
The s electrons and the pz electrons (z is the axis along which the carbons are) can be considered to have a sigma bond and an anti-sigma bond, but having both a bond and an anti-bond is the same as having no bond.
So the total bond order is 2.
In VB theory, they have 3 regular bonds plus a weak one.
Wikipedia also says:
"Quadruple bonds between atoms of main-group elements are unknown. Molecular orbital theory shows that there are two sets of paired electrons in the sigma system (one bonding, one antibonding), and two sets of paired electrons in a degenerate π-bonding set of orbitals. This adds up to give a bond order of 2, meaning that there exists a double bond between the two carbon atoms in a dicarbon (C2) molecule. The molecular orbital diagram of diatomic carbon would show that there are two pi bonds and no sigma bonds. However, a recent paper by S. Shaik et al. has suggested that a quadruple bond exists in diatomic carbon,[7] but this is disputed.[8]"
[deleted] OP t1_j5awlct wrote
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[deleted] OP t1_j5awunx wrote
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[deleted] OP t1_j5gcq5v wrote
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[deleted] OP t1_j5axfky wrote
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the_fungible_man t1_j5bez4r wrote
The dicarbon molecule, C₂, does exist in nature. It has been detected in stellar atmospheres, the interstellar medium, and cometary comas. The fluorescence of C₂ in the latter contributes to the green appearance of certain comets, including C/2022 E3 (ZTF) currently approaching Earth.
However as discussed in this paper (link to abstract only), the 4th inter-carbon bond does not make for a 'stronger' molecule:
>Quantum chemical calculations using the complete active space of the valence orbitals have been carried out for HnCCHn (n=0–3)...
>The bond dissociation energies and the force constants suggest that C₂ has a weaker C−C bond than acetylene, C₂H₂.