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### Course: Organic chemistry>Unit 9

Lesson 3: Aromatic stability

# Aromatic stability II

How the geometry of a molecule plays into aromaticity. Why cyclooctatetraene is not aromatic like benzene. Created by Jay.

## Want to join the conversation?

• Why did he rotate the cyclooctatetraene when he inscribed it into the Frost circle? If he had just inscribed it as it was on the left (like a stop sign) instead of rotating it, then it would have 4 bonding and 4 antibonding molecular oribitals, and no nonbonding ones (instead of the 3 bonding, 2 non, and 3 anti). So, why and how did he know to rotate it?
• From what I learned in class, in order for it to work you have to have a single point be touching the bottom. You can't just rotate it.
• i think i'm missing something here, but how does Jay know that 8 carbons in a ring could give a
"tub" conformation?
• It's because SP2 hybridized atoms like to have their bonds at a 120 degree angle. The strain this tendency puts on the atoms, forces it to go in a tub conformation.
• Why does a 6-carbon ring not have "angular strain", but an 8-carbon ring does? And what about 10-, 12-... carbon rings?
• We are talking about planar conjugated polyenes, all of which have sp² carbons with bond angles of 120°.
Benzene has a 6-carbon ring. It is a regular hexagon with internal angles of 120°. This exactly matches the sp² bond angles, so there is no "angular strain".
The internal angles of regular polygons
= 180°×(n-2)/n, where n is the number of sides. Thus, for 8-, 10-, and 12-membered rings, the internal angles are 135°, 144°, and 150°. The planar rings with all cis double bonds become impossibly strained as the ring size gets larger.
This does not mean that these molecules cannot exist. They just pucker in order to relieve the strain. Just as cyclohexane puckers into a chair and a boat form, cyclooctatetraene exists in a chair and a boat form, but it is not aromatic because it is not planar.
Compounds of this type are called annulenes. Benzene is [6]annulene. Annulenes with 14 or more carbons can accommodate trans double bonds in the ring and still be planar, so [14]annulene and [18]annulene are aromatic.
• At , he says that cyclotetratraene with the two +1 charged carbons is aromatic. I would disagree. Wouldn't this not be aromatic since there is not a continuous set of overlapping p-orbitals? Correct me if I'm wrong.
• The positive charges represent empty p orbitals. They are still overlapping the filled p orbitals.
• Which are more stable : aromatic or anti aromatic compounds ? And why ?
• Aromatic compounds are more stable than antiaromatic compounds because they have all of their pi electrons in low-energy bonding molecular orbitals.
• Hey guys, it seems that there were no videos related to conjugated systems or molecular orbitals, where can I read on these topics?
(1 vote)
• at he says there are 8 MOs, but then proceeds to draw 3 bonding orbitals and 3 anti bonding orbitals. 3+3=6, where are the other 2?
• There’s two on the dotted line, don’t you see them? Orbitals that have the same energy are called nonbonding orbitals and don’t contribute to the bonding in the molecule
• If a carbon has a positive charge on it with also bonded to two other carbon atoms, then what will be its hybridization state?
(1 vote)
• A positively charged carbon would normally have three other atoms bonded to it still, so I'm going to answer the question as if you have a positively charged carbon atom that is bonded to two other carbon atoms and one hydrogen atom. Then, you have 3 groups: one sigma bond to hydrogen, and two sigma bonds to the carbons. 3 groups need 3 hybrid orbitals, so we add 3 atomic orbitals (s+p+p) to get 3 hybrid sp2 hybrid orbitals. That is, carbocations are sp2 hybridized.
• Why is it that cyclooctatetraene is nonplanar but the oxidized version of it (I suppose it would be cyclooctatriene) is planar? He implies in the video that it is because the cation has all sp2 hybridized Carbons, but aren't all of the carbons in cyclooctatetraene be sp2 hybridized as well?
(1 vote)
• The interior angles in a regular octagon are 135 °.
The normal sp² bond angle is 120 °.
If the ring were planar, every bond angle would be strained by 15 °.
Cyclooctatetraene (COT) could be planar if there were some stabilization energy to compensate.
A planar COT ring would have alternating double and single bonds like benzene, but it has 8 π electrons.
This makes it antiaromatic (unstable).
With antiaromaticity and ring strain, COT will do everything it can to avoid being planar.

The COT dication contains 6 π electrons.
This makes it aromatic (stable).
The stabilization energy compensates for the strain, so the COT dication is planar.
• What is the MO Theory ? (At )
(1 vote)
• We have 2 major bonding models, valence bond (VB) theory and molecular orbital (MO) theory.

You likely already know of VB theory even if you haven't heard of it, Lewis structures, hybridisation etc. come from there.

The biggest different is in MO theory electrons are in orbitals spread over the whole molecule, whereas in VB theory electrons are in pairs shared between only 2 atoms.

I don't think there are any Khan videos specifically on MO theory to expand on this more though sorry.