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I have fond that similar question has been asked already (why don't carbons use sp3 instead of sp2 orbitals to make double bonds?), but I don't think it was formulated in a manner that was touching the concept I am interested in.

I would like to know is it reasonable to consider double and triple bonds as result of overlapping solely sp3 hybridised orbitals. I imagine that alternative view on carbon-carbon double bond in ethene could be that two pairs of sp3 hybridised orbitals from each carbon overlap - giving four molecular orbitals, two of which would be bonding and fully occupied while other two would be empty. Moreover, geometry of such constructed double bond would be correct as all four hydrogen atoms bonded by remaining four sp3 orbitals from the two carbons would be coplanar (together with these two carbon atoms as well) and length of the bond would also be shorter than for single bond. I believe that similar case could be made for carbon-carbon triple bond.

Below I am presenting you a simple figure I have made to graphically demonstrate the idea.

enter image description here

I do recall seeing this concept is some textbook being presented as legitimate thing, I do not remember where it was, unfortunately.

Could you please explain it to me if this concept is reasonable idea, and if not - then why not? It would also be great to know how does energetic levels in such instance compare to the system with multiple bonds created by overlaps including hybridised and nonhybridized orbitals. (The one thing that appears in my mind is that in commonly accepted version of multiple bonds there is sigma bond that has electron density directly between nuclei making this arrangement more favourable. However, there are systems like two electron three centre bonds, as in B2H6, in which three centres are hold in similar arrangement to discussed above)

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    $\begingroup$ The theory of hybridization is a method developed by Pauling to explain the geometry of the molecules and ions. It has an advantage : it works. But it has never been proved that hybridization exists. And there are serious chemists who have shown that using non hybridized atoms orbitals provide the same result as with hybridization. The only trouble is that it is much more difficult to follow the calculations done for minimizing the energy without hybridization. So to go back to your question, you may of course combine your sp3 orbitales to make simple, double, and triple bonds. It will work $\endgroup$
    – Maurice
    Mar 1 '20 at 15:22
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    $\begingroup$ I continue. Using sp3 orbitals to make simple, double and triple bonds does work. The distance between carbon atoms in simple, double and triple bonds is exactly what coud be calculated using 1, 2 or 3 sp3 orbitals as you did it. The only trouble is that it is difficult to use this technique to explain mesomerism and the conjugation of double bonds. $\endgroup$
    – Maurice
    Mar 1 '20 at 15:27
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    $\begingroup$ @Maurice Nobody has ever claimed that hybridisation exists. The sp, sp2, sp3 are useful mathematical approximation for orbitals to be used to form bonds via LCAO instead of plain hydrogenlike AOs. If you use them for a bond where they are not applicable, you surely won´t get correct bond lenghts etc., will you? $\endgroup$
    – Karl
    Mar 1 '20 at 18:47
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    $\begingroup$ chemistry.stackexchange.com/questions/32407/… $\endgroup$
    – Mithoron
    Mar 1 '20 at 22:03
  • $\begingroup$ Dear @Maurice thank you for your comments that are answering my question. I see a lot of value in them. Indeed, I have found some examples of using only non-hybridised s and p orbitals for explaining bonding in simple alkane molecules. $\endgroup$
    – Fractal
    Mar 13 '20 at 16:09