Cyclooctatetraene.. why is it not planar??

[kponenation]

No closed shell and 2 unpaired electrons in each of 2 non-bonding orbitals! Molecules with unpaired electrons are typically unstable and reactive.
Therefore a planar form of COT should not be aromatic.
Because no stability is gained by becoming planar it assumes a tub shape.


from http://www.uea.ac.uk/~c286/aromaticnotes.htm

I still don't understand this 100%. So whenever you have any unpaired electrons you have a non-planar?

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[sixkiller]

No closed shell and 2 unpaired electrons in each of 2 non-bonding orbitals! Molecules with unpaired electrons are typically unstable and reactive.
Therefore a planar form of COT should not be aromatic.
Because no stability is gained by becoming planar it assumes a tub shape.


from http://www.uea.ac.uk/~c286/aromaticnotes.htm

I still don't understand this 100%. So whenever you have any unpaired electrons you have a non-planar?

If the molecule were planar, then yes, by definition it would be antiaromatic.

In reality, as you just mentioned, it is shaped like a tub.

Molecules with 4n pi electrons that are cyclic and conjugated are DESTABILIZED by pi electron delocalization. In other words, they are destabilized by resonance.

COT adopts a non planar conformation to prevent p orbital overlap and thus prevents resonance (also escapes anti aromaticity).

 

[joonkimdds]

4n pi + conjugated = not planar, correct?

 

[sixkiller]

4n pi + conjugated = not planar, correct?

Not always, cyclobutadiene is planar and conjugated.

Hence cyclobutadiene is a true antiaromatic compound.

The rectangular shape helps to minimize p orbital overlap.

 

[kponenation]

How can you tell if a compound is planar or non-planar?
I guess that's what I want to know 🙂
Any tips?

 

[sixkiller]

How can you tell if a compound is planar or non-planar?
I guess that's what I want to know 🙂
Any tips?

Problems usually state to assume that all drawn structures are planar.

Sometimes, the problem doesn't state it, then you don't really know...should you interpret it simply as drawn, or it's real conformation. Such questions are poor and need to be more specific.

 

[kponenation]

Problems usually state to assume that all drawn structures are planar.

Sometimes, the problem doesn't state it, then you don't really know...should you interpret it simply as drawn, or it's real conformation. Such questions are poor and need to be more specific.

So I guess nothing like that on the DAT. Right? At least I hope...

 

[Dannelid]

How can you tell if a compound is planar or non-planar?
I guess that's what I want to know 🙂
Any tips?

I think for our purposes, cyclooctatetraene (the one mentioned at the beginning of the thread) is really the only fully conjugated ring that you need to know that is NOT planar. There's prolly more but for the DAT thats all you need to know. All other fully conjugated rings that they will throw at you will most likely be planar.

 

[loveoforganic]

Kind of dumb they'd have you know just one special case, seeing as how there really aren't many molecules at all that are actually antiaromatic. Even cyclobutadiene isn't antiaromatic

 

[sixkiller]

Kind of dumb they'd have you know just one special case, seeing as how there really aren't many molecules at all that are actually antiaromatic. Even cyclobutadiene isn't antiaromatic

5th Edition of McMurry says it is....

Wikipedia says no...