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.