[Snowflake]

http://tinypic.com/r/2zodliu/9

Hi, I have a test coming up, was really wondering why the upper resonance forms are correct, as opposed to what I drew under it (sorry for my paint-skills).

I would really appreciate help in this matter.
Thank you so much

[Alwin Kristen]

I see no reason why phenoxide wouldn't delocalize to benzene ring. What is wrong at your picture is the direction of arrows. Arrow goes from negative charge to single bond, and from double bond to above a carbon.

[Snowflake]

I think that's what I did, it's just hard to tell because of how bad I am at drawing on the computer.
So you would say that there are around 8 resonance forms for this molecule? That's a lot! It must be extra stable...

[Alwin Kristen]

well... those delocalizations doesn't happen at the same time. Methoxy and phenoxides oxide compete with each other. Both are ortho - para directing groups. The strongest group dominates and I would say, in this case, it is the phenoxides oxide. If someone knows better, please correct.

Benzene itself is very stable compound. Extra electrons on benzene doesn't make it more stable, because additional electrons occupy the destructive molecule orbitals and thus weakens the structure. Look for Hückel's rule and Hückel's method to find more about it.

[Snowflake]

ftp://Thank you so much for the quick responses!
What I meant by stable is that if a molecule has a lot contributing resonance structures, the hybrid is much more stabilized (compared to a similar molecule with less contributing resonance structures).
In this example, the professor showed only these 4 resonance contributors, and claimed that this makes it the strongest base (which means the least stable molecule) out of the four: (in order of increasing stability, or from strongest to weakest base)
1.
2.
3.
4.
In each he showed 4 resonance contributors, always electrons delocalizing from the OH side, except for the molecule I was asking about...
Sorry for the long post, would really appreciate some help

Edit: I now also see the mistake in the direction of the arrows in my first post, that was silly of me. Thank you.
Here's the correct version:
http://tinypic.com/r/2dklhyp/9

[Snowflake]

Anyone?

[Alwin Kristen]

Ok... The reason why 1st structure is most basic is that a methoxy group donates electrons to the ring. Maybe your prof just wanted to highlight that. Also methyl group interacts with ring trough sigma conjugation and donates a littlebit electrons to the ring. Bromine attracts electrons from the ring with induction, but donates electrons with conjuction from bromines p orbitals. Ketone attracts electrons from the ring and no donation at all. Do you see a trend here?

So... Why pKa value goes up in order from 4th to 1st ? I assume you know that these are conjugation bases of phenol derivatives and pKa means the pH where a phenol derivative is in equilibrium with its conjugation base.