Acidity of Phenols

Concept: Concept: Donating vs Withdrawing Groups

Video Transcript

Hey guys, in this set of videos, I want to talk about predicting acidity for phenols.
Guys, phenols are alcohols, but they're substantially more acidic than a regular alcohol due to the resonance effect. Remember that you had these effects in your acid/base chapter way back in the day that told you when something was going to be a better acid. It said that if you're able to stabilize the conjugate base, then your acid would be more acidic.
Well, think about phenol. Phenol, after it gives up its proton becomes phenoxide. Phenoxide is a negative charge. Negative charges aren't that happy. But phenoxide can resonate. Notice that it can make resonance structures inside the ring, so we'd be able to resonate the negative charge to here, here and here.
What that means is that normally the pKa of an alcohol is about 16, same as water pretty much, but the pKa of phenol is closer to 10. That's because this resonance stabilized conjugate base that you can make. It's more stable and therefore the phenol is going to be more willing to give up its proton.
It turns out that we need to also understand how electron withdrawing groups and electron donating groups play into this situation as well. In this example, when I have D, that just stands for ED group, an electron donating group. When I have W, that's an electron withdrawing group.
In general, we can say that if you're pushing electrons into the ring, do you think that's going to make it more acidic or less acidic, if you're a donating group? That's going to make it less acidic because of the fact that you're destabilizing the conjugate base. The conjugate base already has a full negative charge. Do you think it wants more electrons being jammed up into that benzene? No. Whereas, what do you think about electron withdrawing groups? Absolutely. That's going to make it more acidic. The more electron withdrawing groups we have, the better. That's going to pull more electron density out of the ring and it's going to stabilize that conjugate base. Got it?
Really quick let's just do a quick example already. Go ahead and look at these four phenols and tell me which one you think is going to be the most acidic phenol.

Concept: Example: Identify the most acidic phenol


Concept: Concept: O,P-positions vs. Meta-Positions

Video Transcript

Guys, it turns out that not all positions are created equal. It turns out that some positions are going to have more effect on acidity than others. In fact, the meta position is going to have a much lesser effect on acidity than the ortho and para positions, meaning that whatever type of group is on there, it just matters less if it's on the meta and it matters more if it's on an ortho or para.
Why is that? Think about the resonance structures that occur when you make a phenoxide. Remember that I told you guys that where would that negative charge resonate to? It would resonate to the top, to the side, and to the side. Notice what are those positions called respective to the O? Those are the ortho, para positions. That means the negative charge will rest directly on ortho, para positions and it will never rest on a meta position.
With that logic, what that means is that we know that a donator is going to make it less acidic because it's going to destabilize the negative. And we know that a withdrawing group is going to make it more acidic. But, if those same exact groups are in the meta position – so these are meta and these are ortho, I'm looking at ortho versus meta. If I have a donating group in the meta position, it is going to be a little less acidic, but only slightly less acidic than normal.
Why? Because this donating effect doesn't really matter that much because it's on the meta position, the negative charge never goes there. Same thing goes with the withdrawing group. The withdrawing group, you think this is great. It's going to make it really acidic, but it's only slightly more acidic if you rest on the meta position. Why? Because, once again, the meta position doesn't really matter because the negative charge never actually sits on it.
In terms of acidity, that brings us to the following acronym. If you're looking for an acidic phenol, what you're going to be looking for is a WOP. What a WOP stands for is a withdrawing group in the ortho and para positions. You don't care about the meta positions. The meta positions aren't helpful for us. We're looking mostly at those having withdrawing groups in the ortho and para positions.
That being said, why don't we take a stab at this question and tell me which one you think is going to be the most acidic phenol. 

Concept: Example 1: Identify the most acidic phenol


Concept: Example 2: Identify the most acidic phenol


Concept: Example 3: Identify the most acidic phenol


Problem: Rank the following phenols in order of increasing acidity.