Organic acid-base mechanisms

0:00- [Voiceover] Drawing acid-base reactions is really an important skill when you're doing organic chemistry mechanisms. So let's look at an acid-base reaction. On the left, acetic acid is gonna function as our Bronsted-Lowry acid. It's gonna be a proton donor. On the right we have sodium hydroxide.

0:16And hydroxide is going to accept a proton. It is going to be a Bronsted-Lowry base. So when you're drawing the mechanism, you used curved arrows to show the flow of electrons. And these two electrons on hydroxide, let's say, are the two that are going to grab the acidic proton on acetic acid.

0:35So there is my curved arrow. Only the proton, so these two electrons are left behind on this oxygen. So let's draw the products, the products, I should say, of this acid-based reaction. So on the left we would have our carbon double bonded to our oxygen. And then now this oxygen we have three lone pairs of electrons around it which gives this oxygen a negative one formal charge.

0:58We'd also have a sodium cation here, so we could think about that. Write an ionic bond. And then what do you get if you add an H plus to OH minus? You would get H2O or water. So let me go ahead and draw water in here. And I'll put in my lone pairs of electrons.

1:15Let's follow our electrons along so the two electrons, right, this lone pair right here on the hydroxide anion picked up this proton. So let's say those two electrons in magenta are these two electrons, and this was the proton that they picked up. And then we also need to follow the electrons, the electrons in here, I'll make them blue.

1:36So these electrons in blue come off onto the oxygen. So let's say those electrons in blue are right here, which gives the oxygen a negative one formal charge. So this is an acid-based reaction. And we can even identify conjugate acid-based pairs here. So on the left, right, on the left this was acetic acid.

1:55This was our Bronsted-Lowry acid. What is the conjugate base to acetic acid? Well, that would be over here, right. Just take away a proton, and this would be the conjugate base. Let me identify this as being the conjugate base. This is the acetate anion, right.

2:11So this is our conjugate base. For hydroxide, hydroxide on the left side functioned as a base, right. So the conjugate acid must be on the right side. So if you add a proton to OH minus, you get H2O. So water is the conjugate, oops. I'm writing conjugate base here, but it's really the conjugate acid, right.

2:31So we've identified our conjugate acid-base pairs. All right, the biggest mistake that I see when students are drawing acid-base mechanisms is they mess up their curved arrows. So the biggest mistake I see, and I'll do this in red so it'll remind you not to do it, is they show this proton right here moving to the hydroxide anion.

2:55And that is incorrect, all right. That's a very common mistake, because curved arrows show the movement of electrons, right. And that's not, this is not what's happening here. These two electrons up here in magenta are the ones that are taking that acetic proton.

3:09So this is incorrect. Don't draw your acid-base mechanisms like this. Let's do one more acid-base mechanism for some extra practice here. So on the left we have acetone and on the right we have the hydronium ion, H3O plus. So the hydronium ion is gonna function as our Bronsted-Lowry acid.

3:30It's going to donate a proton to acetone, which is going to be our Bronsted-Lowry base. Remember when you're drawing an acid-base mechanism, your curved arrows show the movement of electrons. So if acetone functions as our base, a lone pair of electrons on this oxygen could take this proton right here and leave these electrons behind on this oxygen.

3:53So let's show the results of our acid-base mechanism. So on the left, right, the lone pair on the left of the oxygen didn't do anything. The lone pair on the right of the oxygen picked up a proton, formed a bond, and so we get this with a plus one formal charge on the oxygen.

4:10We'd also form water here, so H2O, let me draw that in and show our lone pairs of electrons. And let's follow our electrons again. So the electrons in magenta right here on the oxygen picked up this proton forming this bond, right, so this bond right here, the two electrons in magenta.

4:32And then the electrons in blue here move off on to the oxygen to add another lone pair of electrons onto that oxygen, giving us water. So identifying our conjugate acid-base pairs again, on the left hydronium H3O plus is functioning as our Bronsted-Lowry acid, right.

4:52And you take a proton away from that, and you're left with a conjugate base. So on the right would be water, which is our conjugate base. And on the left, acetone is functioning as a Bronsted-Lowry base. So on the right, this right here must be the conjugate acid.

5:09So this is the conjugate acid on the right. We've identified our conjugate acid-base pairs. And we've shown the movement of electrons using curved arrows. So practice your acid-base mechanisms because they really are extremely important. And you have to be able to do them fairly quickly when you're writing an organic chemistry mechanism.