Most professors will give you their own list of pKa values to memorize. Most of the time, those values aren’t extremely useful (due to professors not giving much thought to it).
Here is a list of the MINIMUM number of pKa values to memorize. By the time you get to your exam, you should at least know these values.
The 12 pKa values you want to memorize (because they are important!).
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So now that we kind of understand the definition of Pekka and how it has to do with some molecules being better acids and some molecules being worse acids, we're gonna have to go over a PK a values now. Okay, now, I don't like to give you guys a lot of stuff to memorize Inorganic chemistry. I like for you guys to understand it versus memorize it. But it turns out the PK a values or just one of those things, that it's just maybe just better for you to memorize it if you can understand it. Great. But even if you don't understand it, I think it's worth your time just to memorize them because they're going to continue to help you in or go one in or go to their that important. So let's go ahead and get started. So here I've given you a basic spectrum off some common molecules that you're gonna need to know for your test for or go to etcetera, M cat, whatever you're taking. And I'm starting off with the best. I mean, with the worst acids. Okay, not worse. Worst. And then I'm gonna go all the way to the best at the end. Are you guys cool with that? So I'm gonna start with worst. And if I'm starting off with worse, that means these we're gonna have the highest or the lowest values. What do you think, thes? We're gonna have the highest values because they're the worst acids. Remember these. We're gonna start off with, like, the shittiest assets. Okay, so let's go ahead and start off with basically SP three hybridized C h. Okay, I know that sounds really complicated, but all that is is that's just another way for me to say Al Cane. Okay, remember that. Al Keynes R S p three hybridized because you basically have ah carbon with maybe you with another carbon on it. And a bunch of age is okay. And what that means is, that has four groups. And if it's four groups, then it's sp three hybridized. Okay, well, these falcons are not acidic at all. Think about it. It's because they're not reactive. Remember that I told you guys that what makes molecules reactive dye polls charges strained double bonds. Alcon doesn't have any of that, so it doesn't want to be an acid at all. So that means that it's gonna have the worst PK of around 50. Okay, so it really doesn't get worse than in Al Qaeda. And Al Kane is the worst acid. Okay, then from there, we start to get a little bit better. So let's talk about SP two s p two hybridized ch. Sounds complicated, but that's just another way of saying Al Keen. Okay, remember what Al Keen is? It's a double bond. Okay, so that would be an h coming off of a double bond. All right, Now, is this gonna be a little bit more reactive than a single bond? Yes. Because remember, I told you guys that double bonds or sources of reactivity, so there's gonna be a little bit better, and it turns out that this one is gonna have a PK of 44. Okay, so it still sucks a lot, but at least it's a little bit better than the Alcan. Okay, so we're just gonna keep going in order. We're just gonna keep going up. So now we have a means, Okay. Ah. Means are just any time that you have an h coming off directly of a nitrogen with a single bond. Okay. And any time you have in a mean, that's going to be a peek A of 38. Okay, so it's still really high. Remember that? I said 16 is your middle point, right? So we're still really far away from that. These air still bad acids, but they're getting a little bit better. Then we get to H two. H two is a really, um, is a really important molecule that you're gonna need for some reactions. Um, it's just die atomic, hydrogen. It doesn't really. You don't need a name for it. This one's gonna have a PK of 35. Okay, this one, Honestly, there's not a lot to understand. It's just to memorize it. You just need to know that h two s a p k of 35. That's gonna help you a lot. Then we get all the way down toe SP hybrid. I c h s p would be an l kind. Okay, because remember, that s p means that it's it only has two groups or to bond sites. So that means it would literally be a triple bond with ah hydrogen coming off of it. Okay, Now that triple one is even more reactive. So it's gonna be a little bit better at donating protons. This one has a PK of 25. So I made a really big jump. That one's getting a lot better. Okay, so if you could group these together, I would recommend. Always know that you're Al Keynes Al Canes. And you're all kinds are 50 44 25. I'm going to grill you guys on that all semester long. Been really remember sp two. Remember recipe? That was 44 25. I'm gonna ask you these values over and over again because they're that important. Okay, then we've got our Alfa hydrogen. Okay. What the heck is an Alfa hydrogen? Okay, an Alfa hydrogen is any h that's coming directly off of a carbon, Neil off of the carbon next to carbon deal. So here I have a carbon. Neil. This Carbonell could be anything. It could be an Alka hide. Let's say it has an age there this carbon. If that one has an h on it, that h is gonna have a peek A of 20. Okay, now it turns out that you're not really gonna understand why this has a why this has such a low p k. Because normally what would be the PKK of a C H bond? That's SP three hybridized. Okay, what would that normally be? It would usually be 50. But now what did I just tell you? Instead, the PKK is actually gonna be closer to 20. That's a huge, huge jump. So why is it so much more stable if it's on that Alfa Carbon? And the reason has to do with a phenomenon called Todd Memorization. Okay, Now it turns out that Todd memorization is something that were you that I might teach you in or go one, but we really explore it in or go to. Okay, so this is one of those rare times that I have to teach you about something that you're not gonna understand right now, and you just have to accept it. You have to be like I mean, I hate bringing that stuff that you can't understand yet, but in this case, just take my word for it. Todd, memorization is this thing that makes the carbon next to a carbon eel much more acidic. Okay? It only has to do with the carbon. That's why I have a star There has to do with the carbon next to carbon. Neil, if it has an h on it. Cool. So then we get to some easy ones. So we have our away h, which is alcohol and water. These both have about the same peak A. They both have a PK of about 16. Okay, so if you just remember I told you as water has a ph of 16 that's the middle point. So this is like, the neutral point right here, okay? It's not really neutral. It just means that I compare all the other acids to it. So remember that I said all the ones before it, these would all be the bad acids. So that means that over here, I'm going to start getting into good acids. Is that cool? Another thing to keep in mind. Water actually has a more specific PK. Your professor might want you to know it. It's 15.7. Okay, But that's so close to 16 that I always just round it. I always just say water has a p k f 16 cool. So far. Now we're going to start getting into the good assets. Okay, So what happens if I have a nitrogen with a positive charge? Okay, A nitrogen with a positive charge and at least one h on it, actually. So you might wanna add that age. Just remember, Okay, if you have a nitrogen with a positive and at least one h on it, that is going to have a peak of around 10. Okay, Now, you could imagine this is a pretty good acid, because look, it has a full positive. Remember that I told you guys that if you have formal charges, that makes something really reactive. So this has a full positives. That means that that's very electro Filic. That's gonna be very much of good acid. Okay, so let's keep going, then. We have all your carb oxalic acids, your carb oxalic acids. Just write that here. I mean, they have acid in the name, right and crab oxalic acids are about five. Okay. And you would know that because we actually just calculated the PKK of a car. Looks like ass. Remember, acetic acid? That's the type of carbolic acid. And that one was remember, it was 4.755 So we just rounded up to five. Is that cool? Awesome. So now we're in the homestretch now, what if we have an o positive with one h on it with at least one age? Okay, so you might wanna also add that h What that is is that now we have an O positive. That's very, very It's a very, very good acid That's gonna be around negative too. Okay, so if you ever see now we're getting into the negative numbers. That means these were going to be strong acids, OK? And then finally have h ex h ex is just going to be your strong acids. Remember that. I told you, remember, your strong 63 of your strong six are hcl, hbr and H I Okay, these air all gonna have very, very, very, very acidic PKK's and all. I'm not gonna make you memorize those PKK's because most often, professors don't care about those PKK's as much because they're all negative. So I'm just going to group them together and just say they're all negative. Okay, if you want to memorize them, if your professors really picky, maybe he'll make you memorize Negative 11 and negative seven and stuff like that. But most of the time you're fine. As long as you know, it's just negative. It just means it's really, really acidic. Is that cool? So I know that was a ton of information, but thankfully, I'm gonna give you lots of practice so that by the end of this page, you're gonna be feeling more confident.
Clutch Student: “Johnny, my professor said I don’t have to memorize any pKa values for the exam”.
Me: It doesn’t matter. You’ll need to understand these values for the rest of Organic Chem I and II (even Orgo III in some schools). By not memorizing this easy list, you are doing yourself a huge disservice. Just believe me and go with the flow!
Identify all of the relevant pKa values for the indicated protons. Rank them in order of increasing acidity.
Identifying pKa values
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Alright, guys, let's get into the answer again. I'm hoping that you tried this on your own because it's great practice. So what we're gonna do is we're gonna go one letter at a time and just say what All the P k s r of those atoms and then what we're gonna do is we're gonna follow the rest of the directions and the question which are to rank them in order of increasing acidity, meaning that we start off with worst acid. So what would be the peak? A of this hydrogen right here? The hydrogen that's attached to the O. Well, there's a few things that might come to mind. There were actually two functional groups that we learned there. PK PK is off. That had o. H. One was an alcohol R o h. Remember that R O H, we said, is about 16. And then we also had car box cilic acid carb oxalic acid with C O. H. And we see that that was a little bit stronger oven acid. It has a PK of five. So this just goes back to functional groups. Is molecule a that proton? Is it an alcohol or is a carbon cilic acid. Well, this alcohol, this O H is two carbons away from a carbon eel as you'll notice this is a carbon you'll hear right. That's a carbon carbon. So I have another carbon and then I haven't Oh, okay. So that means that this is not a c o. H. Because this would have required that the O was directly attached to the carbon carbon. And, as you can see, does that red carbon in the middle. That's not making it directly attached. So going in once the inter functional groups, this is an alcohol. This is not a car oxalic acid, so this would have a peek a of around 16. Let's go on To be B is excellent practice because this H is in a very special situation. It is an n that's attached to an H. Remember that there were two PK. PK is we learned for NH. We learned that an end attached to at least one h has a PK of 38. That's what we call in a mean. But if that end has an H, and it also has a positive charge on it, that's no longer the mean, that's called in ammonium or an ammonium derivative and has a PKK closer to 10. So which one of these is it? Is it the amine, the neutral nitrogen? Or is it the ammonium? The positively charged You guys got this? This is actually an ammonium derivative, so this would have a peek A of right around 10. Okay, awesome. So I hope you guys are two for two. So far, Let's keep going. So this age here, a little bit tricky because of where it's placed. But remember, guys, I told you, there's this very special location right next to the carbon Neil called the Alfa Carbon. Okay, so the Alfa Carbon is unusually acidic. Any hydrogen that comes directly off of an Alfa carbon, meaning that it's directly next to a carbon eel is really acidic. And this one has ah Peca of what, 20 around 20. Okay, now you might be wondering, Johnny isn't the other side also Alfa? Sure it is. This is also Alfa. But notice that this Alfa Carbon doesn't have any hydrogen on it. It has an O. H. And the O. H is just that has a PK of 16 because That's just a NALC a hall. It's not special. And Alfa Carbon is Onley special if it has a hydrogen directly coming off of it, which is going to give it a unique Peca of 20 as opposed to much higher for other types of carbons. Okay, which we're gonna look at now. So let's go on to D de is just a ch okay, but the PK of the CH is going to depend on its hybridization. So what is the hybridization of the carbon it's attached to Is the hybridization sp three sp two or SP? Which one is it? Recall that s p three ch is gonna have the highest P K. It's gonna be roughly 50. So it's one of the worst assets possible. It's terrible. Terrible acid recall. The SP two ch was equal to what you guys remember 44. You could just look above we wrote it down. An S P. C. H was equal to much better. Is a much better acid. Much stronger acid of 25. Okay, so which of these is it? How do you tell the hybridization of that carbon guys, You look at the group's Remember, we've done this before. So you look at the group's this blue carbon Has, uh, Adam here and Adam here and Adam here and then It also has another age here, So that counts is four groups because that's four different atoms or four Adams total coming off of the blue carbon. That means this is s p three. And it has a peek A of about 50 and then finally coming to the last proton here. This proton is also a ch. So can I just assume because there's a carbon here Can I assume that they have the same PK? No, I have to look at the hybridization again. Right? So the hybridization of this carbon here would be What? How many groups does it have? Well, guys, it only has two groups because the green carbon has how many things coming off of it. Well, it has this carbon here, and it has this hydrogen here that's it has two atoms that councils to groups. That's S p. That means it has a Peca of 25. Got it. Cool. Awesome, guys. So, um, that's it for the p K. Is now you have to move on to the next step, which is just to rank thes So which of these is gonna be the absolute worst acid? And that's a no brainer. It has to be deep. D is the worst with the Peca of 50. So then which one comes next? Well, by the all PKs that I have written down, it looks like then it's gonna be E is gonna be a little bit better or actually, significantly better at 25. Let's keep going. What's getting more and more acidic? It looks like it's C. C has that Alfa Proton. Soc has a peak of about 20. Then what do we have? Well, now it's between A and B A. Comes next with a PK, since it's an alcohol of 16. Okay, the alcohol always have a PK of 16. It doesn't matter whether it's Alfa or whatever. What have you? The acidity comes from the O. H Bond, not from the Alfa position of the carbon deal. That doesn't really affect the acidity. Okay, so just because it's O. H, that is gonna be a PK of about 16 and then finally we have our most acidic, which has a PK about 10 and that's your ammonium. And guys, just to point this out, you could have actually used your intuition to know that was going to be one of the most acidic. Because notice what's special about B is that it has a positive charge, right? We know that positive charges are usually acidic. It means that they're electro files, right? So they're Proton donors. Okay, So because this had a positive charge, Even without knowing my Pekka is, I could have probably predicted that B was gonna be my best acid. But now we've confirmed it through PK A values. Alright, Cool, guys. So hopefully that made sense. Now let's move on to the next set of practice problems where we're gonna do the same thing. Just with a little bit more tricky, smaller molecules.
Pop quiz:If I were to ask you what the overall pKa for this molecule is, what pKa would you say? How do we figure that out?
The overall pKa of a molecule is equal to the pKa of the most acidic hydrogen. A chain is only as strong as its weakest link; if there is a proton that can easily dissociate, that will be the one that a base will choose to remove.
So the answer is approximately 10.
Rank the following organic compounds in the order of increasing pKa.
a < e < d < c < b
d < e < a < c < b
d < a < e < c < b
b < c < e < a < d
Error: In the video I actually made a mistake when I was ordering these values! Since the question is asking to order in terms of increasing pKa, just reverse the order that I gave you. Everything else is correct though!
Remember that the pKa of all these molecules is equal to the pKa of their most acidic hydrogen.
Rank the following compounds in the order of increasing acidity.