Practice: Which of the following compound(s) cannot be characterized as a Lewis acid?
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|pKa||25 mins||0 completed|
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|Give the conjugate acid for each compound below|
|Lewis Acids and Bases|
One of the most challenging aspects of this chapter will be understanding and differentiating between different definitions that all sound kind of similar. We’re gonna move slowly through this, step-by-step.
Concept #1: The Lewis definition of acids and bases.
So the next few videos are going to be dedicated completely to acid-base chemistry. And what I’m going to do is I’m just going to guide you through the easiest stuff first which are just the definitions. Remember that in Gen Chem there are a lot of different definitions of acids and bases so we have to remember what those were. And then I’m going to take you through the complex stuff like basically on chemical equilibrium, pKa’s and eventually what you're going to learn to do is predict the direction of a reaction in acid-base equilibrium. And that’s kind of cool and that’s also going to be kind of the ultimate goal of the section: is to teach you how to predict that equilibrium. So let’s get started with the easy stuff, okay?
So first of all, before we get started you need to know your strong six. This is something from Gen Chem that—I usually tell you you don’t need to learn everything from Gen Chem as we go to Orgo but this is one thing you’re never really allowed to forget. If I tell you that you’re reacting with HCl and you don’t know that that’s a strong acid, like come on, you’re slacking real hard. Alright? You should remember your strong acids. Okay? Other than that, everything else I’m going to teach you here today.
So what I want to do is I’m going to start off with the most general definitions of acids and bases and then go to more specific ones. Okay? And it turns out that the most general definition is going to be the Lewis definition. Okay? Now I do want to make one adjustment here. Notice that, in your PDF it looks fine, but in my PDF it got messed up. So I’m just going to write “Base”, “Base”. We’re comparing acids and bases, right? Cool. So I hope that didn’t confuse you too much. Alright.
So the general definition, the most general definition of acids and bases is just going to be the same as what we already learned in electrophiles and nucleophiles. Okay? So basically what that means is remember that I said that nucleophiles have extra electrons—it’s a negative, and then electrophiles are missing electrons. So it’s the same exact thing. What we’re going to say is that an acid is going to be an electron pair acceptor. Okay? So if one of these were to be an electron pair acceptor, which one would that be? Would it be the nucleophile or the electrophile? Well, think about it. The name electrophile means electron-lover. So this one would be the electrophile. Does that make sense? So every time I say electrophile that also means I’m talking about a Lewis acid. Okay? The same exact thing, just a different way to say it. Then a Lewis base is going to be an electron pair donor. Which of these is the donor of electrons? The nucleophile because the nucleophile has extra electrons. So any time I say nucleophile now you know that I’m talking about a Lewis base. Okay?
And now that we’ve linked these two things together now you can even predict what the reactions are going to look like based on what I taught you guys about electrophiles and nucleophiles reacting together.
The Lewis definition is the most general of the definitions that applies to all cases. It’s actually just another name for what we learned earlier as Nucleophiles and Electrophiles.
Concept #2: The Bronsted-Lowry definition of acids and bases.
Now what I want to talk about is the more specific one which is Brønsted Lowry which only has to do with protons. Okay? So the Brønsted Lowry definition only has to do with protons and what it says is that—maybe this is the one you might remember from Gen Chem—what it says is that Brønsted Lowry acid is going to be a proton donor. You’re going to give away protons. And then a Brønsted Lowry base is a proton acceptor. Okay? Now many at a time, most of the time, most of the time these are going to be the same. Most of the time your Lewis Acid is also going to be a Brønsted Lowry acid. Okay? So I would say more than 90% of the time they’re the same. But sometimes they’re different. And what that means is that sometimes one of these things is going to be true. Maybe it’s an electron-pair acceptor but the other one is not going to be true, that which is a proton donor. Okay. So that’s what we have to learn today.
The Brønsted-Lowry definition is a more specific definition that only applies to some acids and bases. In this definition, electrons don’t matter- we only look at protons.
Concept #3: Understanding the difference between Lewis and Bronsted-Lowry using 6 examples.
The easiest way to learn that is just to look at some examples so what I want to do first is just go through these six examples and then I'm going to show you the difference between Bronsted Lowry and Lewis, OK? So, let's look at A, first of all A is water and water I already told you guys was an exception what that means is that it can really do anything at once depending on what the other reagent is, so remember that it has electrons here it has a lone pair there a lone pair there and it also has protons so what that means is that it can act as a Lewis acid meaning it can be an electron pair accepter, OK? Because it can give way proton make way for an electron pair, it can also be an electron pair donor meaning it can give away one of its long pairs it can also be a Bronsted Lowry acid meaning it can give away a proton and it can also be a Bronsted Lowry base mean that it can accept a proton so it can do all of these things so just think whenever you see water all of the above it could do whatever, now we're going to go to a more specific reagent that doesn't do all of these things in fact it's only going to do one of them, OK? This is Boron and if you guys remember from my talk on I told you guys what octet rule, how many electrons does Boron want to have in its octet? It wants to have only 6, OK? 6 electrons, OK? And what that means is that it actually prefers to have 3 bonds and 6 electrons it's happy the way it is but what that means is that it's always going to have one empty orbital and that orbital is a P orbital so Boron always has an empty P orbital the reason I'm teaching you this is because it seems kind of specific but this comes up a lot in organic chemistry 1 so I want you guys to remember that Boron's kind of special It always was that empty P orbital there's actually another atom that's very similar and that's the one right under it in the periodic table and that's aluminum, aluminum also has an empty P orbital and then 3 bond, is that cool so far? That empty orbital turns out since it's empty it can accept electrons really really well, OK? But it's not a good proton donor the reason is because if it gave away a proton it would break its octet and then it would only have 4 electrons, OK? So, what that means is that is this going to be an electron pair acceptor? Yes, if I say electron pair which of these four is that? Look at my definitions above which one is the electron pair accepter Lewis acid so it turns out that BH3 is a really good Lewis acid, OK? But now let's see if it also Bronsted Lowry acid which means that it could proton donor? No, it sucks it actually is terrible so it is not a Bronsted Lowry acid but it is a Lewis so isn't that interesting? So, this is an example where the two definitions don't wind up exactly, OK? Now let's talk about the base part really quick is it a good Lewis base? Meaning does it give away electrons? Obviously not it doesn't have any electrons so no and then is it good at accepting protons? Also, No, OK? So, what that means is that it's only one of these things which is that it's a Lewis acid, OK? Now I want you to hold on to that but just keep in mind that this is different from water, how water was a good Lewis acid and a Bronsted Lowry which is like I said about 90 percent of the molecules we encounter have both of them agreeing with each other but then there are some special molecules like these guys that only do one of them, alright? So now let's look at these other ones, this is a molecule C it is a molecule that I've introduced to you guys before it's called Pyridine, OK? And it has a lone pair here, OK? So, is it going to be a good electron pair acceptor? No if it accepted another lone pair that would break its octet, right? So, no it's not a good electron pair acceptor, is it a good electron pair donor? Yeah because it has a free lone pair just hanging out here so it is a good election pair donor it's a good nucleophile or we'd say a Lewis base, OK? Is it a good Bronsted acid meaning that is can donate protons? No, OK? Is it Bronsted case meaning that it could accept protons really well? Actually, it can so this would be both one of those examples where it's both a Lewis and a Bronsted base, OK? So, I just want you guys to keep that in mind, these agree with each other because it can donate electrons but it can also accept a proton, OK? Now let's look at this next one so I'm going to go through these a little quicker but basically now I have basically lone pairs on this O and what I want to know is that could that oxygen there be a Lewis acid? Could it be an electron pair acceptor? Actually, it could if it gave away this proton if it gives away the proton then it could accept two electrons so it actually is a Lewis acid, is it a good Lewis base meaning that it's good at giving away electrons? Not really, OK? Is it a good Bronsted acid mean that it's good to giving away protons? Actually, yeah, we just said that it can give away this proton so it's a Bronsted, is it a good Bronsted base meaning that it can accept protons? No not really, OK? Now you wondering Johnny How did you know this is going to be a good acid and give away that Proton? Well think about the functional group this is called carboxylic acid, OK? if you forgot that remember that basically COOH is carboxylic acid so what that means is that it has a very acidic H so it's easy to give that H away and if it's giving the H away then what it's doing, it's going to accept a lone pair, OK? Now let's talk about this next one, a double bond is a double bond a good electron pair acceptor? So, let's say I have an electron pair and now it's going to try to go into that double bond, no that would be terrible that would break the octets of 2 carbons, OK? Is it a good electron pair donor, OK? And actually yes, it is because remember that I said that these electrons can donate to something else I can move those electrons to electrophile so that's actually a really good Lewis base, OK? Is it a Bronsted acid meaning that it can donate protons? No not at all it's not an acid at all. Is it a good Bronsted base meaning that it's easy for it to accept protons? And actually, no it's not this is not a good example of a Bronsted base because once again I would be basically breaking an octet to accept a proton, OK? So, this is going to be one of those examples where this is going to be mostly a Lewis base and it's not going to be a Bronsted base very much, OK? It's going to act more like a Lewis base because it's going to be really good at giving away the electrons, OK? But it's not very good and its normal state it's not good at accepting protons, OK? So then finally have this last one which is just an Alkane, OK? An Alkane this one is unreactive, it doesn't have anything to react, remember that we talked about reactivities before and we talked about how you need a double bond you need a dipole, you need you know a charge, some strain none of that so this is just not going to be anything it's not going to be good at donating protons and it's also not going to be good at accepting them just it's not going to do anything because it's unreactive, OK? So, in all these cases I was looking for reactive site, all of these had a reactive say except this last one.
Let’s watch a few examples so you can get the hang of the differences between these definitions:
Heads up: By definition, any Brønsted-Lowry acid is also a Lewis acid. This is because if you are able to donate a proton, you are always able to accept an electron in return!
Concept #4: Using a diagram to understand the difference between Lewis and Bronsted-Lowry definitions.
What I want you guys to visualize is this little circle chart up here and I want to show you guys the difference between basically Bronsted Lowry and Louis inside of this, so basically water would be an example of a Bronsted Lowry acid, it could be a Bronsted Lowry acid, OK? Meaning that it's good at giving away protons, OK? And that means that it's also considered a Lewis acid because notice that Lewis is the more general definition so every single type of acid can also be considered Lewis, OK? Whereas BH3 which is my second example would be one that is only a Lewis definition it is not going to be Bronsted Lowry, OK? And the reason I mean obviously the reason is because it's outside of the Bronsted Lowry circle but another reason is because I explained to you, is this a good proton donor? Is BH3 a good protein donor? No because it would break its octet if it donated a proton does that kind make sense? So, there can be some Lewis acids and some would Lewis bases like BH3 that are not going to also be that are not going to be Bronsted Lowry and that's important for you guys to know in terms of concepts your professor could ask you that, he could say are all Lewis acids Bronsted Lowry acids? And the answer is no that would be false you just look at this chart and say BH3 is not Bronsted Lowry but now if he reversed it and he said all Bronsted Lowry acids are also Lewis acids that's actually true because Bronsted Lowry acids are in the smaller circles so that's the one that's more specific it still fits in the general, alright? So just want you guys to know that.
When determining how acids and bases react together, it important to understand the concepts of conjugates and Ke.
Concept #5: Equilibrium constant and conjugates.
Now what I finally want to do is end off talking about equilibrium. And this is just a simple pattern that we’re going to use throughout all these reactions. And all it says is this: a base will always attack an acid, so notice I’m already telling you what’s going to happen, a base (negative) will attack an acid (positive) to produce conjugates—scary word, remember that from Gen Chem? Conjugates in the following chemical pattern.
So this is the way it works. Basically you’re always going to get your stronger base and your stronger acids, okay, reacting together to make a weaker base and a weaker acid. Okay? So you always go from stronger to weaker. It just makes sense. You’re never going to go from weaker to stronger. That doesn’t make any sense. Okay. And then what we’re going to do is everything that’s on the right side of the arrow, okay, everything that’s after reaction we’re going to call that a conjugate. Okay? The conjugate means that’s what happens after it reacted. Okay? So basically and then everything beforehand, we call a regular base and a regular acid. Okay? So what that means is that my base is always going to turn into a conjugate acid and my acid is always going to turn into a conjugate base. I know that’s a little bit confusing and a lot of people mix that up. Okay. But that’s just something that you have to remember. Your base is going to turn into a weaker acid and then your acid is going to turn into a weaker base—they’re the ones that are on the other side.
And then also just that the Ke has also to do with equilibrium constant and all that’s saying is that it’s always going to go to the right, basically Ke/1 means that it’s going to the right if your conjugates are weaker. Okay? If your conjugates are weaker then it’s going to the right. If your conjugates are stronger then it would to the left because it’s going the other way. Does that make sense?
Now I know that last part was a little bit rushed but I think we’re going to do tons of practice with this so by the end of today you’re not going to have a problem with conjugates. Okay? So if you have any questions, let me know, but if not let’s move on to the next topic.
Even though we still have no idea what we are doing, we should be able to use patterns of nucleophiles and electrophiles, coupled with what we learned about how to draw arrows for mechanisms, to predict our first acid-base reactions.
Practice: Which of the following compound(s) cannot be characterized as a Lewis acid?
Practice: Which of the following compounds is most likely to have come from the stronger acid?
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