pKa: Study with Video Lessons, Practice Problems & Examples

Understanding pKa values is crucial in organic chemistry, as they indicate the strength of acids. Alkanes have the highest pKa (~50), making them the weakest acids, while alkynes are stronger with a pKa of 25. Alpha hydrogens near carbonyls have a significantly lower pKa of 20 due to tautomerization. Water and alcohols have a pKa around 16, serving as a neutral reference. Strong acids, like carboxylic acids (pKa ~5) and HCl (pKa <0), are vital for various reactions. Memorizing these values aids in predicting reactivity and understanding acid-base behavior.

The 12 pKa values you want to memorize (because they are important!).

So 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. Okay? 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 it's 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 with Orgo 1 and in Orgo. They're that important. So let's go ahead and get started. So 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, etcetera, MCAT, whatever you're taking. And, I'm starting off with the worst acids. Not worst, worst. And then I'm going to go all the way to the best at the end. Are you guys cool with that? So 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, these are I'm going to start off with, like, the acids. Okay? So let's go ahead and start off with, basically, sp3 hybridized CH. Okay? 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 with maybe another carbon on it and a bunch of H's. And what that means is it has 4 groups. And if it's 4 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 that 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 hybridized CH. 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. Okay? So it's still high, but at least it's a little bit better than the alkane. So we're just going to keep going in order. We're just going to keep going up. So now we have amines. Anytime that you have an H coming off directly of a nitrogen with a single bond. And anytime you have an amine, that's going to have 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. 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 CH. sp would be an alkyne. Because remember that sp means that it only has 2 groups or 2 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. Okay? So 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. You're going to be like, remember sp2? Remember sp? It was 44, 25. I'm going to ask you these values over and over again because they're that important. Then we've got our alpha hydrogens. What the heck is an alpha hydrogen? An alpha hydrogen is any H that's coming directly off of a carbonyl, 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, where 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 CH bond that's sp3 hybridized? It would usually be 50. But now, instead, the pKa is actually going to be closer to 20. That's a huge jump. So why is it so much more stable if it's on that alpha carbon? And the reason has to do with a phenomenon called tautomerization. Now it turns out that tautomerization is something that we're going to explore more in Orgo 2. This is one of those rare times that I have to teach you something that you're not going to understand right now. And you just have to accept it. You have to 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 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. If you just remember I told you as water has a pKa of 16, that's the middle point. So 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. 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 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? If you have a nitrogen with a positive and at least one H on it, that is going to have a pKa of around 10. 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 positive. So that means that that's a very good acid. Then you have all your carboxylic acids. I mean, they have acid in the name. Right? And carboxylic acids are about 5. And you would know that because we just calculated the pKa of a carboxylic acid. Remember, acetic acid, that's a type of carboxylic acid. And that one was remember? It was 4.75. So we just round it up to 5. Is that cool? Awesome. So now we're on the home stretch. So now what if we have an O Positive with 1 H on it? What that is is that now we have an O Positive that's a very good acid. That's going to be around negative 2. So if you see 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. So 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 acidic pKas. And all in, 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's really picky, maybe he'll make you memorize negative 11 and negative 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 acidic. Is that cool? So 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 comfortable.

Identifying pKa values

All right, guys. So 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 direction 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, there are a few things that might come to mind. There are actually 2 functional groups that we learned their pKa's of that had OH. 1 was an alcohol, ROH. Remember that ROH we said is about 16. And then we also had carboxylic acid. Carboxylic acid was COOH and we see that that one's a little bit stronger of an acid. It has a pKa of 5. So this just goes back to functional groups. Is molecule A, that proton, is it an alcohol or is it carboxylic acid?

Rank the following organic compounds in the order of increasing pKa.

• A. a < e < d < c < b
• B. d < e < a < c < b
• C. d < a < e < c < b
• D. b < c < e < a < d

Rank the following compounds in the order of increasing acidity.

• A. c < d < a < b
• B. d < a < b < c
• C. b < a < d < c

Here’s what students ask on this topic:

What is the definition of pKa in organic chemistry?

pKa is a measure of the acidity of a molecule, specifically the negative logarithm of the acid dissociation constant (Ka). It indicates how easily a molecule donates a proton (H⁺). A lower pKa value means a stronger acid, as it more readily donates protons. Conversely, a higher pKa value indicates a weaker acid. Understanding pKa values helps predict the reactivity and stability of molecules in various chemical reactions.

Why is it important to memorize pKa values in organic chemistry?

Memorizing pKa values is crucial because they help predict the behavior of acids and bases in chemical reactions. Knowing these values allows you to determine which molecules will act as acids or bases, understand reaction mechanisms, and predict the outcome of reactions. This knowledge is essential for success in organic chemistry courses, exams like the MCAT, and practical applications in laboratory settings.

What are the pKa values of common functional groups like alkanes, alkenes, and alkynes?

Common functional groups have distinct pKa values: Alkanes have a pKa of around 50, making them very weak acids. Alkenes have a pKa of approximately 44, slightly more acidic than alkanes. Alkynes are more acidic with a pKa of about 25. These values are important to remember as they help in understanding the relative acidity and reactivity of these functional groups in organic reactions.

How does tautomerization affect the pKa of alpha hydrogens near carbonyl groups?

Tautomerization significantly lowers the pKa of alpha hydrogens near carbonyl groups. Normally, an sp³ hybridized C-H bond has a pKa around 50. However, the pKa drops to about 20 for alpha hydrogens due to the stabilization provided by tautomerization. This process involves the shifting of a hydrogen atom and a double bond, creating an enol form that stabilizes the conjugate base, making the alpha hydrogen more acidic.

What is the pKa of water and why is it considered a neutral reference point?

Water has a pKa of approximately 15.7, often rounded to 16 for simplicity. It is considered a neutral reference point because it lies in the middle of the pKa scale, with values lower than 16 indicating stronger acids and values higher indicating weaker acids. This reference helps in comparing the acidity of other molecules and understanding their behavior in aqueous solutions.