[orgo814]

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

[discodermolide]

Move the negative charge in towards the C-O bond, then draw the resonance structures.

[orgo814]

I did but even with negative charge in the ring it's still 6 electrons

[discodermolide]

Yes but that is not an aromatic system. You don't have 6 Π electrons

[orgo814]

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

[discodermolide]

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:

[orgo814]

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?

[discodermolide]

It may be a pair of electrons but they do not contribute to any aromaticity. How do you envisage that this could be aromatic?

[orgo814]

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.

[discodermolide]

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.
 

[Dan]

I visualize something like this:

[spirochete]

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/

[orgo814]

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

[spirochete]

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.

[orgopete]

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.