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Topic: Orbital Diagram: Allene. Help Needed.  (Read 17260 times)

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Offline zeldajae

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Orbital Diagram: Allene. Help Needed.
« on: June 04, 2011, 05:44:35 PM »
Hello guys, I have a general question that I am confused. I know how to draw the orbital diagrams but I have a question when connecting the pi-bonds in the molecule for "Allene".

Here is the Orbital Diagram for the molecule Allene:



I have no problem drawing it. I have got the exact same answer, it was quite easy, since I am learning O-chem on my own for the summer before I take my course on O-chem at my university this fall.

So, back on topic, the two carbon atoms on the left and right are supposed to have hybrid orbitals sp2, and the middle carbon is supposed to have hybrid orbital sp. This leaves the two carbon atoms, left and right, with one un-hybrid P-ordbital, while the middle carbon is left with two un-hybrid P-orbital.

Now, here is the confusing part that is getting me, when connecting the pi-bonds..

Since the left and right carbon has sp2 orbitals, that leaves these two carbons with a Pz orbital.
The middle carbon has sp orbital, that leaves the middle carbon with the Py, and Pz orbitals.

Here are my questions based on what I have mentioned above:
1. Should the left carbon have the P-orbital oriented towards the z-coordinates?
2. If so, then how would you connect the pi-bonds, since you will be left with three Pz orbitals, and one Py orbital?

Help would be MUCH appreciated! Thanks.

Offline azmanam

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Re: Orbital Diagram: Allene. Help Needed.
« Reply #1 on: June 04, 2011, 09:37:08 PM »
The carbon on the left in your image is rotated such that the unhybridized p orbital is pz.  As noted, the middle carbon has unhybridized py and pz.  The carbon on the right is rotated such that the unhybridized p orbital is py.

Thus, the central carbon bonds to the carbon on the right through the py orbitals, and the central carbon bonds to the carbon on the left through the pz orbitals.

Note that the remaining sp2 orbitals on the carbon on the left are oriented horizontally and the hydrogen atoms are sticking into and pointing out of the page.  The sp2 orbitals on the carbon on the right are oriented vertically and the hydrogen atoms are in the page.  This is a necessity of the orbital overlaps between the three carbon atoms.
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Offline zeldajae

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Re: Orbital Diagram: Allene. Help Needed.
« Reply #2 on: June 05, 2011, 01:32:04 PM »
I understand.

As you said that the carbon on the right is rotated such that the unhybridized p orbital is py, but why would that be the case?

Because as I recall, when creating an hybridized orbital for the carbon on the left and right, you would be left with the the hybrid orbitals Px and Py orbitals with the hybrid 2s orbital also, as shown in the below image:


Offline azmanam

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Re: Orbital Diagram: Allene. Help Needed.
« Reply #3 on: June 05, 2011, 06:58:06 PM »
When the 2s and the 2 2p orbitals hybridize to form 3 sp2 orbitals, you are correct that one unhybridized p orbital remains.  It's only pz if you're looking at the hybridized atom such that the p orbital happens to align with the z-axis.  If you rotate the hybridized atom 90 degrees, it is now aligned with the y-axis.  Rotate another 90 degrees, and it's aligned with the x-axis.  Doesn't change the hybridization or identity of the atom - it's still sp2.

So when the allene forms, the atoms are aligned such that the orbitals overlap with the maximal amount of overlap.  Because the central atom has one pz and one py unhybridized orbital, the other two carbon atoms must be rotated such that one of the unhybridized p orbitals aligns with the central pz orbital and the other unhybridized p orbital aligns with the central py orbital.

Essentially, the axis in which the orbital lies is relative to your observational vantage point.
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Offline zeldajae

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Re: Orbital Diagram: Allene. Help Needed.
« Reply #4 on: June 06, 2011, 01:31:56 PM »
Ahh. Now I see what you meant by:

Quote
Note that the remaining sp2 orbitals on the carbon on the left are oriented horizontally and the hydrogen atoms are sticking into and pointing out of the page.

Thank you for your *delete me*! :)

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