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 Hs.
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 alpha hydrogens.
What the heck is an alpha hydrogen?
An alpha 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 alpha 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 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.
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.
Let's go ahead and do an example.
In this first one, I want you guys to identify all the relevant pKa values for the indicated
protons.
What that means is basically go one by one and identify, on my little list, open book,
identify what the pKas would be.
I'm going to give you guys the opportunity to pause the video now, try to do all five
of these, and then come back, and then I'm going to tell you what they are.
All right, hopefully that wasn't too bad.
Let's just go in alphabetical order.
They're all kind of tricky, so let's go ahead and do A first.
A, notice it was a hydrogen attached to an O+.
Remember that O+ actually is up here.
So this is not just an alcohol.
An alcohol would be if it was neutral, but since it's an O+, that means that it's going
to be around -2.
So that's very acidic.
Let's go on to B. B is a single bond, nitrogen H.
This is an amine.
Remember that I said amines are not very good acids?
This would have a pKa of around 38.
Let's go to C. C is a C-H bond.
But does that tell us enough?
Actually, no.
That only tells us that we have some kind of hydrocarbon, but it doesn't tell me what
the hybridization is, so I have to look at this carbon and I have to say, is that sp,
sp2, or sp3?
Which one do you think it is?
That's going to change the pKA completely.
This is sp3.
The reason is because that carbon has four groups coming off of it, right?
So then that means that the pKa of this would be roughly 50.
So this one's really, really bad, because that H does not want to pop off.
Now let's go ahead and do D. D basically is an H that is also going to be attached to
an sp3 hybridized carbon, because notice that there's also an H sticking off of there.
So this one's going to be sp3 hybridized, and it's also going to be roughly 50.
Now we just got two that are 50s, and I told you to rank them, so obviously we might have
a little problem with that.
I'm going to go back to that in a second and explain what the difference is.
But they're both around 50.
Then finally, we have this H here, and this H is directly attached to a triple bond.
A triple bond, remember, is sp hybridized C-H, so that means that this one's going to
be around 25.
So there you have it.
What we have here is a very strong acid over here with -2, and then some really bad ones.
So what I want to do is I want to put them ranking in increasing acidity, so I'm going
to go ahead and start off with the worst ones, which are C and D. Let's just group them together
for now until we can tell the difference, so I'm going to say C and D are the worst.
Then what's better?
Well, from there you'd get B, because that's 38.
From there, you would get E, because that's 25.
And lastly, you would get A, which is the best, the most acidic one, because of the
fact that it is -2.
Does that make sense so far?
Cool.
Now what I want to do is just talk about basically the difference between these two.
We had the C-H of 50, and then we also had the C-H of 50 down here.
Remember that I told you that any time you have a C-H bond that's sp3 hybridized, that
should be 50.
But then remember that I told you there was actually one exception, and that might mess
everything up.
That one exception was if that C-H is directly next to a carbonyl.
Do you remember me saying that?
So it turns out that one of these is next to a carbonyl and one of them is not.
This one is not, so this one is not alpha.
That means it really is going to be 50.
But over here, this one here is actually on an alpha carbon.
Because remember that I said that this is your carbonyl, and any carbon that's right
next to your carbonyl is alpha.
So this one is alpha, and this one is alpha.
Alpha just means it's the first carbon next to the carbonyl.
What that means is that this is not just going to be a normal sp3 hybridized carbon.
It's not going to be a normal 50.
It's actually going to be closer to 20.
Does that make sense?
I was kind of letting you guys think through it.
Maybe you thought “Oh, that's sp3,” and it was.
According to the sp3 rule, it should've been 50.
But according to the alpha hydrogen rule, it should be actually closer to 20.
So does that mess our scale up?
It actually does a little bit.
What that means is that the new scale is going to be that D is the worst, and then B is still
just as bad, and then it goes also to E. But then it turns out that C is actually going
to be better than E, so C is going to go here, and then finally A. And if you wanted to see
those pKas in order, it would be 50, 38, 25, 20, and then -2.
All right, so that one was a little bit tricky, but I hope that you guys understood how to
get those.
We're just basically looking for patterns.
It's almost like looking for functional groups.
We're just looking for different types of hydrogens on this big molecule.
Now let's try this next one: rank the following compounds in order of increasing pKa.
For this one, what we're going to find is that we're going to have several hydrogens
that you could consider.
Let me for example give you the first one.
A has actually two different types of hydrogens that I could look at.
It has hydrogen there, and all of these hydrogens are the same.
Let's just say all these blue hydrogens are the same.
But then it also has a red hydrogen here and a red hydrogen here that's a little bit different.
And obviously, there would be 2 hydrogens on each of these.
I'll just draw them all in.
All those blue ones have the same pKa.
The red ones have a slightly different pKa.
So what do you guys think?
What's the pKa of the blue ones?
The pKa is 50, because those are all sp3 hybridized.
Those are all alkanes.
Is that cool?
And there's no carbonyls, so I don't have to worry about alpha hydrogens.
But how about the red ones?
The red ones are coming off of a double bond, so that's sp2, so that means the red ones
should be 44.
So if I were to say what's the pKa of this molecule, what would your answer be?
It would be 44.
The reason is because you always go with the most acidic proton.
So your pKa is always going to equal the most acidic proton.
Does that make sense?
What I want you guys to do here is look at the most acidic protons for each of these,
but remember that you have to look at hydrogens.
It only exists if it has a hydrogen on it.
So go ahead and figure out what the pKa values for the rest of these are, and then we'll
order them like we did for the last question.
So go ahead and pause the video.
All right, so B was actually really easy.
B is just a carboxylic acid, so this one would be 5.
Is that cool?
C was really easy.
Obviously it had Hs here, blah blah blah, but those Hs sucked.
Those are the sp3 hydrogens, and they are 50.
So they suck, so I would go instead with the alcohol, which is 16.
Then D kind of sucks, because D is an ether.
Remember ROR, “roar,” ether?
Does ether have a pKa?
I never taught you the pKa of ether, because there doesn't have hydrogens on it.
If you think about it, if there was an H here, that would be a different story.
But there isn't an H there.
All you have is Hs here all around.
I could draw Hs on all those carbons.
So what types of bonds are those?
There's nothing special about them.
Those are just sp3 hybridized C-H bonds.
So what is the pKa of that ether?
It's actually just the same as an alkane.
It would be 50.
Because there's nothing special about those hydrogens; they just happen to be attached
to sp3hybridized carbons.
Just because there's an O there, that doesn't change anything.
It would only change it if the H was directly attached to the O. Does that make sense?
So we're looking for what is the H attached to?
So then finally, we have this one over here, and this one has a triple bond.
Remember that triple bonds had a pKa of 25, right?
The problem is that does that triple bond have any Hs sticking directly off of it?
No.
See, I'm going to keep tricking you with this.
It doesn't.
All it has is a CH3 here, and then it has 2 Hs over here.
So what would be the pKa of those Hs?
Well, if you think about it, both of these carbons here are sp3 hybridized.
They both have four bond sites.
See – one, two, three four.
Does that make sense?
And if I did the same thing with the other one, it would also be four.
So all of these blue hydrogens, even though there's a triple bond there – let me just
erase, erase – they all have the pKa of 50.
So is that a good pKa?
No.
That means that my most acidic is actually going to be the hydrogens here on the nitrogen.
Because remember that I said that if you have hydrogen directly attached to nitrogen, that's
going to have a pKa of 38.
Does that make sense?
So now I have my pKas.
I have 44, 5, 16, 50, and 38.
Now I just have to rank them in increasing order.
The obviously worst is going to be D, then it's going to be A, then it's going to be
E, then it's going to be C, and then lastly, it's going to be B. And if you wanted to see
those pKas in order, it would be 50, 44, 38, 16, and 5.
All right, so you can see with these kinds of questions, there's two challenges.
One, figure out which is the most acidic proton.
Two, actually remember what the pKa is.
Now, some professors actually are going to give you a pKa table, but I would still suggest,
just for your own sake, for your own knowledge, know these few values.
It's not that bad.
You're going to work with them so much, eventually you have to know them anyway.
All right?
Now what I want you guys to do is pause the video again and try to do this entire problem
on your own.
Rank them again.
And I'm just telling you, I'm going to trick you again.
I'm just telling you right now, I'm going to trick you.
You're going to think that some are pKas that don't have them.
So remember to look at the H, not at the functional group.
Look, where is the H?
So go ahead and get started on that.
All right, so let's go ahead and do this one.
What my pKas would be is that this one, A, was similar to the other one.
This one would be 38, because of my amine.
Then this one over here had an ether component, which I told you there's nothing special about
it, and it had a carbonyl.
Remember that the carbonyl can have alpha hydrogens, and remember that alpha hydrogens
are more acidic, so this one would actually have a pKa of 20.
And actually, all 4 hydrogens that are coming directly off of that carbonyl on the alpha
carbon are all going to have the same pKa.
Then we have C. C is an alcohol, and an alcohol is more acidic than a double bond, so this
one's going to be 16.
And then finally, I had probably the most tricky one of all, which I'm actually going
to remove myself from the screen so that you guys can see it.
It's a little bit creepy, I'm not on the screen anymore.
But let's go ahead and look at it.
D is N with a positive charge.
What would the pKa of N with a positive charge be?
Around 10.
Does that N have any hydrogens on it?
No.
All it has is CH3 everywhere.
So is that N acidic?
No, it's not.
The only thing that's acidic is these hydrogens on the carbons.
And what's the hybridization of those C-H bonds?
All of these have the same hybridization, which is sp3.
So it turns out that this does not have a pKa of 10; it actually has a pKa of 50.
And I know you guys hate me now, because that was super tricky, but that was just to reinforce
that you have to look at the hydrogen, not the functional group.
So if we were to order these, what it would be is that D is the worst, then followed by
A, then followed by B, and then C is the best.
And I'm just going to come back so you guys can see me.
So that would be the order, and if you want to see those pKas in order, it would be 50,
then it would be 38, then it would be 20, and then it would be 16.
Cool?
All right, guys, so hopefully that was good practice with pKas.
Let me know if you have any questions.
We're going to continue to use them, so you'll get more practice, don't worry.
Let's go on to the next topic.