[orgo814]

Why would your example of 2 methyl groups make it lose its aromaticity?

[orgo814]

Well with the example of two methyl groups there's a complete octet so the lone electrons can't delocalize to that point so I can see how that can break the aromaticity.. As it breaks the conjugation as well. But with the carbonyl the electrons in the C=O bond can move up to the oxygen atom as the lone electrons go into the pi* orbital so I don't see how there's any barriers here

[orgopete]

I tried to give a choice. If you draw the three resonance structures with six pi-electrons, the carbonyl group is not involved. I suggested replacing it with two methyl groups to show how you can still have six pi-electrons in the ring, but because the dimethyl carbon is not involved, it would not be aromatic, by the rules.

If you choose to include the carbonyl group, as is being suggested, the oxygen is exo to the ring, and if the oxygen's electrons are included, it will now have eight pi-electrons. This should be antiaromatic. Exo atoms are not used in aromatic systems (except to create them, e.g., dimethylidene cyclopentadiene).

[orgo814]

I'm starting to catch on now. So since we are using the oxygens electrons (forming the carbonyl from the OH in phenol as electrons become delocated in the ring) we are utilizing 8 pi electrons which makes it antiaromatic?

[orgopete]

I'm starting to catch on now. So since we are using the oxygens electrons (forming the carbonyl from the OH in phenol as electrons become delocated in the ring) we are utilizing 8 pi electrons which makes it antiaromatic?

No. The oxygen is not IN the ring. Even though it may be used in writing resonance structures, it is not part of an aromatic ring. It is not in the ring. When the electrons form a double bond, the carbon is no longer part of an aromatic ring because you cannot use it to write a resonance structure using the six electrons present in the ring. That is a requirement for aromaticity. The carbon can be used in an aromatic ring only if you do not write it as a carbonyl group. That was the point I was attempting to make by suggesting the carbonyl group was like a dimethyl carbon. You would know the electrons of a dimethyl carbon cannot participate in the six electrons of the cyclohexadiene anion. It fails to include every atom in the resonance structures.

[orgo814]

So for aromaticity only the electrons originally in the ring can resonate. Once the double bond is formed the carbon isn't part of an aromatic ring because electrons can delocalize out of the ring to the oxygen (or because there's a full octet on that carbon so to avoid 5 bond character). Thanks for your patience I do not know why it's been so hard for me to wrap my head around this one