Practice: Rank the following organic compounds in the order of increasing pKa.
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|Calculating Equilibrium Constants Using pKa|
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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.
Concept #1: The 12 pKa values you want to memorize (because they are important!).
Now that we kind of understand the definition of pKa and how it has to do with some molecules being better acids and some molecules being worse acids, we're going to have to go over our pKa values now. I don't like to give you guys a lot of stuff to memorize in organic chemistry; I like for you guys to understand it versus memorize it. But it turns out that pKa values are just one of those things that maybe it's 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 Orgo 1 and in Orgo 2. They're that important. So let's go ahead and get started.
Here I've given you a basic spectrum of some common molecules that you're going to need to know for your test, for Orgo 2, etc., MCAT, whatever you're taking. I'm starting off with the worst acids. Not worse, worst. Then I'm going to go all the way to the best at the end. Are you guys cool with that? I'm going to start with the worst, and if I'm starting off with the worst, that means these are going to have – the highest or the lowest values, what do you think? These are going to have the highest values because they're the worst acids. Remember, I'm going to start off with like the shittiest acids, okay?
So let's go ahead and start off with basically sp3 hybridized C-H. I know that sounds really complicated, but all that is, is that's just another way for me to say alkane. Remember that alkanes are sp3 hybridized because you basically have a carbon, maybe with another carbon on it, and a bunch of H’s. What that means is it has four groups, and if it's four groups, then it's sp3 hybridized. Well, these alkanes are not acidic at all. Think about it: it's because they're not reactive. Remember that I told you guys, what makes molecules reactive? Dipoles, charges, strain, double bonds. Alkane doesn't have any of that, so it doesn't want to be an acid at all. So that means that it's going to have the worst pKa of around 50. So it really doesn't get worse than an alkane. An alkane is the worst acid.
Then from there, these start to get a little bit better. So let's talk about sp2. sp2 hybridized C-H sounds complicated, but that's just another way of saying alkene. Remember what an alkene is? It's a double bond. So that would be an H coming off of a double bond. Now, is this going to be a little bit more reactive than a single bond? Yes, because remember I told you guys that double bonds are sources of reactivity, so this is going to be a little bit better. And it turns out that this one is going to have a pKa of 44. So it still
sucks a lot, but at least it's a little bit better than the alkane.
Now we're just going to keep going in order. We're just going to keep going up. Now we have amines. Amines are just any time that you have an H coming off directly of a nitrogen with a single bond. Any time you have an amine, that's going to be a pKa of 38. 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 are still bad acids. But they're getting a little bit better.
Then we get to H2. H2 is a really important molecule that you're going to need for some reactions. It's just diatomic hydrogen. You don't need a name for it. This one's going to have a pKa of 35. This one, honestly, there's not a lot to understand; it's just to memorize it. You just need to know that H2 has a pKa of 35. That's going to help you a lot.
Then we get all the way down to sp hybridized C-H. sp would be an alkyne. Because remember that “sp” means that it only has two groups, or two bond sites, so that means it would literally be a triple bond with a hydrogen coming off of it. Now, that triple bond is even more reactive, so it's going to be a little bit better at donating protons. This one has a pKa of 25. So I made a really big jump. That one's getting a lot better.
If you could group these together, I would recommend always know that your alkanes, alkenes, and your alkynes are 50, 44, and 25. I'm going to grill you guys on that all semester long. I'm going to be like “Remember sp2? Remember sp? That one's 44, 25?” I'm going to ask you these values over and over again, because they're that important. Then we've got our α hydrogens. What the heck is an α hydrogen? An α hydrogen is any H that's coming directly off of the carbon next to a carbonyl. So here I have a carbonyl. This carbonyl could be anything. It could be an aldehyde, let's say. It has an H there. This carbon, if that one has an H on it, that H is going to have a pKa of 20.
Now, it turns out that you're not really going to understand why this has such a low pKa, because normally, what would be the pKa of a C-H bond that's sp3 hybridized? What would that normally be? It would usually be 50. But now what did I just tell you? Instead, the pKa is actually going to be closer to 20. That's a huge, huge jump. So why is it so much more stable if it's on that α carbon? The reason has to do with a phenomenon called tautomerization. It turns out that tautomerization is something that I might teach you in Orgo 1, but we’ll really explore it in Orgo 2. So this is one of those rare times that I have to teach you about something that you're not going to understand right now and you just have to accept it. I mean, I hate bringing up stuff that you can't understand yet, but in this case, just take my word for it. Tautomerization is this thing that makes the carbon next to a carbonyl much more acidic. It only has to do with the carbon – that's why I have a star there – it has to do with the carbon next to a carbonyl if it has an H on it. Cool.
So then we get to some easy ones. So we have ROH, which is alcohol, and water. These both have about the same pKa. They both have a pKa of about 16. So if you just remember, I told you guys water has a pKa of 16; that's the middle point. This is like the neutral point right here. It's not really neutral; it just means that I compare all the other acids to it. 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 pKa. Your professor might want you to know it. It's 15.7. But that's so close to 16 that I always just round it. I always just say water has a pKa of 16. Cool so far?
Now we're going to start getting into the good acids. So, what happens if I have a nitrogen with a positive charge? A nitrogen with a positive charge and at least 1 H on it, actually. You might want to add that H just so you remember. If you have a nitrogen with a positive and at least 1 H on it, that is going to have a pKa of around 10. Now, you can 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 positive, so that means that that's very electrophilic. That's going to be very much a good acid.
Let's keep going. Then we have all your carboxylic acids. Your carboxylic acids– I'm just going to write that here– I mean, they have acid in the name, right? And carboxylic acids are about 5. You would know that because we actually just calculated the pKa of a carboxylic acid. Remember acetic acid? That's a type of carboxylic acid, and that one, remember, it was 4.75, 5. So we just round it up to 5. Is that cool?
Awesome. Now we're on the home stretch. Now what if we have an O+ with 1 H on it, with at least 1 H? You might want to also add that H. What that is, is that now we have an O+ that's a very, very good acid. That's going to be around -2. So now we're getting into the negative numbers; that means these are going to be strong acids.
And then finally, we have HX. HX is just going to be your strong acids. Remember that I told you, remember your Strong 6? Three of your Strong 6 are HCl, HBr, and HI. These are all going to have very, very acidic pKas. I'm not going to make you memorize those pKas, because most often, professors don't care about those pKas as much because they're all negative. So I'm just going to group them together and just say they're all negative. If you want to memorize them, if your professor is really picky, maybe he'll make you memorize -11 and -7 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? I know that was a ton of information, but thankfully, I'm going to give you lots of practice so that by the end of this page, you're going to 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.
Example #1: Identify all of the relevant pKa values for the indicated protons. Rank them in order of increasing acidity.
All right 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 going to do is we're going to go one letter at a time and just say what all the pKa's are of those atoms and then what we're going to do is we're going to follow the rest of the directions in the question which are to rank them in order of increasing acidity, meaning that we start off with the worst acid. So what would be the pKa of this hydrogen right here the hydrogen that's attached to the O? Well just a few things that might come to mind: there were actually two functional groups that we learned their pKa’s of that had all OH was an alcohol ROH. Remember that ROH we said was about 16 and then we also have carboxylic acid. Carboxylic acid with COOH and we see that that was a little bit stronger of an acid. It has a pKa of 5. So this just goes back to functional groups is molecule way that proton is it an alcohol or is a carboxylic acid? Well, this alcohol this OH is two carbons away from a carbonyl. As you'll notice this is a carbonyl here. Right, that's a carbonyl carbon. So I have another carbon and then I haven't O. So that means that this is not a COOH because this would have required that the O is directly attached to the carbonyl carbon. As you can see there's that red carbon in the middle that's not making it surgically attached. So going in once again to the functional groups this is an alcohol, this is not a carboxylic acid. So this would have a pKa of around 16.
Let's go on to B. B is excellent practice because this H is in a very special situation. It is an N that's attached to an H. Now remember that there were two pKa’s we learned for NH. We learned that in an N attached to at least one H has a pKa of 38. That's what we call in amine. But if that n has an h and it also has a positive charge on it, it that's no longer to mean that's called an ammonium or an ammonium
derivative and has a pKa closer to 10. So which one of these is it? Is at the amine the neutral nitrogen or is it the ammonia the positively charged? You guys got this. This is actually an ammonium derivative so this would have a pKa of right around 10.
Okay, awesome. So I hope you guys are two for two so far. Let's keep going. So this H here a little bit tricky because of where its placed. But remember guys, actually there's this very special location right next to the carbonyl called the α carbon. So the α carbon is unusually acidic. Any hydrogen that comes directly off of an α carbon, meaning that is directly next to a carbonyl is really acidic. And this one has a pKa of what? 20. Around 20. Now you might be wondering, “Johnny isn't the other side also α?” Sure it is. This is also α. But notice that this α carbon doesn't have any hydrogen's on it. It has an OH and the OH just that has a pKa of 16 because that's just an alcohol. It's not special. An α carbon is only special if it has a hydrogen directly coming off of it which is going to give it a unique pKa of 20 as opposed to much higher for other types of carbons which we're going to look at now.
So let's go into D. D is just a CH but the pKa of the CH is going to depend on its hybridization. So what is the hybridization of the carbon that it's attached to? Is the hybridization sp3 sp2 or sp3? Which one is it? Recall that sp3 CH is going to have the highest pKa. It's going to be roughly 50. So it's one of the worst acids possible—terrible, terrible acid. Recall that sp2 CH was equal to what? Do you guys remember? 44. You can just look above we wrote it down. And sp CH was equal to much better is a much better acid much stronger acid of 25. So which of these is it? How do you tell the hybridization of that carbon? Guys, you look at the groups. Remember? We've done this before. So you look at the groups. This blue carbon has a an atom here and atom here and atom here and then it also has another H here. That counts as four groups because that's four different atoms or four atoms total coming off of the blue carbon. That means this is sp3 and has a pKa of about 50.
And then finally coming to the last proton here. This proton is also a CH. So can I just assume, because this is a carbon here. Can I assume that they have the same pKa? No, I have to look at the hybridization again. 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. What has this carbon here and it has this hydrogen here. That's it. It has two atoms that counts as two groups. That's sp that means it has a pKa of 25. Got it? Cool! Awesome, guys.
So that's it for the pKa. Now you have to move on to the next step which is just to rank these. So which of these is going to be the absolute worst acid? And that's a no brainer. It has to be D. D is the worst with the pKa of 50. So then which one comes next? Well by the all pKa's that I have written down, it looks like then it's going to be E. E is going to 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 α proton so C has a pKa of about 20.
Then what do we have? Well now it's between A and B. A comes next with a pKa, since it's an alcohol, of 16. The alcohol always has a pKaf 16. Tt doesn't matter whether it's α or whatever, what have you, the acidity comes from the OH bond not from the α position of the carbonyl. That doesn't really affect the acidity. So just because it's OH that is going to be a pKa of about 16. And then finally we have our most acidic which has a pKa of about 10 and that's your ammonium.
And guys, just to point this out, you could have actually use your intuition to know that was going to be one of the most acidic because notice what’s special B is that it has a positive charge, right? We know that positive charges are usually acidic. It means that they're electrophiles right so they're proton donors. So because this had a positive charge, even without knowing my pKa’s I could have probably predicted that B was going to be my best acid. But now we've confirmed it through pKa values.
Alright, cool guys. So hopefully that made sense. Now let's move on to the next set of practice problems where we're going to 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.
Practice: Rank the following organic compounds in the order of increasing pKa.
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.
Practice: Rank the following compounds in the order of increasing acidity.
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