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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: orgo814 on June 28, 2015, 12:35:59 AM

Title: Phenol resonance structures
Post by: orgo814 on June 28, 2015, 12:35:59 AM: My book says that when phenol is deprotonated at the oxygen, it loses aromaticity in its resonance structures. How is this so? The lone pair electrons in the ring still gives 6 electrons total which satisfy Huckels Rule. I'm not seeing how it loses its aromaticity
Title: Re: Phenol resonance structures
Post by: discodermolide on June 28, 2015, 05:59:23 AM: Move the negative charge in towards the C-O bond, then draw the resonance structures.
Title: Re: Phenol resonance structures
Post by: orgo814 on June 29, 2015, 07:33:04 PM: I did but even with negative charge in the ring it's still 6 electrons
Title: Re: Phenol resonance structures
Post by: discodermolide on June 29, 2015, 07:51:02 PM: Yes but that is not an aromatic system. You don't have 6 Π electrons
Title: Re: Phenol resonance structures
Post by: orgo814 on June 29, 2015, 11:24:08 PM: So the lone pair in and of itself in the ring breaks aromaticity. I have seen examples where a lone pair was in a ring and it was still considered aromatic
Title: Re: Phenol resonance structures
Post by: discodermolide on June 30, 2015, 12:35:03 AM: Its not a lone pair but a negative charge.
Sure in furan and pyrrole the lone pair contributes to the aromaticity. But that is not this case:
Title: Re: Phenol resonance structures
Post by: orgo814 on June 30, 2015, 06:13:27 PM: How is that not a lone pair? For a negative charge wouldn't 2 electrons be necessary as in a lone pair to make formal charge -1?
Title: Re: Phenol resonance structures
Post by: discodermolide on June 30, 2015, 08:56:02 PM: It may be a pair of electrons but they do not contribute to any aromaticity. How do you envisage that this could be aromatic?
Title: Re: Phenol resonance structures
Post by: orgo814 on July 01, 2015, 12:26:48 AM: By the fact it's conjugated still (still a pair of electrons in a P orbital), planar, and satisfies Huckels Rule since the number of electrons did not change.
Title: Re: Phenol resonance structures
Post by: discodermolide on July 01, 2015, 01:12:11 AM: These are resonance structures, where do you see the extended pi system? Putting the negative charge into an extended pi system into that resonance structure would require that the carbon of the C=O would have partial 5 bond character. That can't be.
Title: Re: Phenol resonance structures
Post by: Dan on July 01, 2015, 04:33:15 AM: I visualize something like this:
Title: Re: Phenol resonance structures
Post by: spirochete on July 01, 2015, 07:09:50 PM: It's a common misconception that resonance is the best explanation for phenol's acidity. Dan showed in his final resonance structure that only one set of fairly disfavored resonance structures containing a carbocation actually maintain the aromatic system once the lone pair from oxygen has been delocalized into the ring.

Induction is a better argument, keeping in mind that increasing s character on an atom makes it more electronegative. None the less if you are taking a not very advanced Sophomore organic class you'd be wise to stick to the resonance argument. Your teacher may mark you wrong if you fail to invoke resonance.

Phenol and the enol of acetone have very similar pKa's, 10.0 for phenol and 10.9 for the enol. This is more consistent with an inductive argument because the inductive effect is known to fall off rapidly with increasing distance through bonds.

Here's a blog post by a grumpy chemist discussing this: http://blog.chembark.com/2007/03/10/i-judge-people-by-their-grammar-and-knowledge-of-phenol/
Title: Re: Phenol resonance structures
Post by: orgo814 on July 02, 2015, 04:33:37 PM: I am still very confused. My question is on HOW the resonance structures break aromaticity. It Still satisfies the 4N+2 rule with the lone pair electrons
Title: Re: Phenol resonance structures
Post by: spirochete on July 04, 2015, 12:06:17 AM: Quote from: discodermolide on July 01, 2015, 01:12:11 AM
Putting the negative charge into an extended pi system into that resonance structure would require that the carbon of the C=O would have partial 5 bond character. That can't be.

I'm not sure how to answer the question better than disco already did here.
Title: Re: Phenol resonance structures
Post by: orgopete on July 04, 2015, 02:41:35 AM: Let us apply a simple test for aromaticity. Replace the carbonyl group with bonds to two methyl groups. Now you will still have six pi-electrons in a generally planar structure. Will this structure meet the rules for aromaticity? The answer should be it does not. This would explain the statement in the textbook.

If one were to include the electrons of the carbonyl group in the calculation, then the total would be 8 pi-electrons, also not aromatic.

I had not wanted to jump in here as I had earlier made the same argument as noted by spirochete. I would say that even though the textbook may say it breaks aromaticity, it may not necessarily be so. The pKa of pyrrole is also increased compared to pyrrolidine. If you carefully analyze the structure, you will find the anion (H-N) electrons are orthogonal to the pi-electrons of the pyrrole ring. That is, the electron withdrawing properties of the sp2-carbons can increase the acidity of the OH or NH-group without invoking resonance structures. I grant that resonance structures can be written and this explanation predominates in organic chemistry.
Title: Re: Phenol resonance structures
Post by: orgo814 on July 04, 2015, 11:19:56 AM: Why would your example of 2 methyl groups make it lose its aromaticity?
Title: Re: Phenol resonance structures
Post by: orgo814 on July 04, 2015, 11:56:56 AM: 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
Title: Re: Phenol resonance structures
Post by: orgopete on July 04, 2015, 04:43:22 PM: 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).
Title: Re: Phenol resonance structures
Post by: orgo814 on July 04, 2015, 06:20:45 PM: 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?
Title: Re: Phenol resonance structures
Post by: orgopete on July 04, 2015, 11:00:28 PM: Quote from: butlerw2 on July 04, 2015, 06:20:45 PM
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.
Title: Re: Phenol resonance structures
Post by: orgo814 on July 04, 2015, 11:36:56 PM: 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