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Nucleophilicity vs. basicity

Nucleophilicity vs. Basicity . The difference between what it means to be a nucleophile and a base. Created by Sal Khan.

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Video transcript

What I want to do in this video is differentiate between the ideas of nucleophilicity or how strong of a nucleophile something is, and basicity. The difference is at one level subtle, but it's actually a very big difference. And I'll show you why it's kind of confusing the first time you learn it. When we studied Sn2 reactions, you have a nucleophile that has an extra electron right here. It has a negative charge. And maybe you have a methyl carbon. Let me draw it. Maybe you have a hydrogen coming out. You have a hydrogen behind it. You have a hydrogen up top. Then you have a leaving group right over there. In an Sn2 reaction, the nucleophile will give this electron to the carbon. The carbon has a partial positive charge. Let me draw that. The leaving group has a partial negative charge because it tends to be or will be more electronegative. So this electron is given to this carbon right when the carbon gets that, or simultaneously with it, this electronegative leaving group is able to completely take this electron away from the carbon. Then after you are done, it looks like this. We have our methyl carbon so the hydrogen is in the back, hydrogen in the front, hydrogen on top. The leaving group has left. It had this electron right there, but now it also took that magenta electron so it now has a negative charge and the nucleophile has given this electron right over here and so now it is bonded to the carbon. The whole reason I did this is because this is acting as a nucleophile. It loves nucleuses. It's giving away its extra electron, but it is also acting as a Lewis base. This is a bit of a refresher. A Lewis base, which is really the most general, or I guess it covers the most examples of what it means to be a base. a Lewis base means you are an electron donor. That's exactly what's happening here. This nucleophile is donating an electron to the carbon. So, it's acting like a Lewis base. So for the first time you see that, you're like, well, why did chemists even go through the pain of defining something like a nucleophile? Why don't they just call it a base? Why are there two different concepts of nucleophilicity and basicity? The difference is that nucleophilicity is a kinetic concept, which means how good is it at reacting? How fast is it at reacting? How little extra energy does it need to react? When something has good nucleophilicity, it is good it reacting. It doesn't tell you anything about how stable or unstable the reactants before and after are, It just tells you they're good at reacting with each other. Basicity is a thermodynamic concept. It's telling you how stable the reactants or the products are. It tells you how badly something would like to react. For example, we saw the situation of fluorine. Let's think about this. We saw the situation-- actually, I should say fluoride, so fluoride looks like this. Seven valence electrons for fluorine and then it swiped one extra electron away. You get fluoride. So fluoride is reasonably basic. It is more basic than iodide. But in a protic solution-- let me write it here. But less nucleophilic in protic solution. And a protic solution, once again, has hydrogen protons around. And the reason why this is, is fluoride, it wants to bond with a carbon or something else more badly, or maybe even a hydrogen proton. It wants to bond with it more badly than an iodide anion. If it did, it actually will be a stronger bond than the iodide anion will form, that the fluoride anion is actually less stable in this form than the iodide is. If it were to be able to get a proton or give its electron away, it will be happier, but it's less nucleophilic. It's less good at reacting in a protic solution. The whole reason it's less nucleophilic is because there are other things that are keeping it from reacting. We saw in the video on what makes a good nucleophile, and in the case of fluoride, it's because it's a very small atom. It's actually a very small ion so it's very closely held. The electron cloud is very tight, and so what it allows is the hydrogens from the water to form a very tight shell around. These all have partial positive charges so they're attracted to the negative anion. They form a very tight shell protecting the fluoride anion, which makes it harder for it to react in a protic solution, so it doesn't react as well. If it was able to react, it actually will form a stronger bond than the iodide anion. So that's the big difference, just so we see the difference in trends. So basicity, it does not matter what your actual solvent is. It is a thermodynamic property of the molecule or the atom of the anion. So if you looked at pure basicity, the strongest base you see-- and I'll just write hydroxide here. It's normally something like sodium hydroxide or potassium hydroxide, but when you dissolve it in something like water the sodium and the hydroxide separates, and it's really the hydroxide that acting as a base, something that wants to donate electrons. So hydroxide is a much stronger base than fluoride, which is a stronger base than chloride, which is a stronger base than bromide, which is a stronger base than iodide. Now, if you were to look at nucleophilicity just to see the difference, we saw that what the solvent is actually matters because the solvent will affect how good something is at reacting. So in nucleophilicity, there's a difference between a protic solvent and an aprotic solvent. In a protic solvent, the thing that has the best nucleophilicity is actually iodide because it's not hindered by these hydrogen bonds as much. It doesn't have a tight shell. It has this big molecular cloud, and some people think it also has kind of a softness. It has this polarizability where that cloud can be pulled towards the carbon and do what it needs to do. So in this case, iodide is a better nucleophile, let me just say, than hydroxide, which is a better nucleophile than fluorine. Now, in an aprotic solution, where all of a sudden the interactions with the solvent are not going to be as significant, then things change. In this situation, basicity matters. So in an aprotic solution, basicity and nucleophilicity correlate. I'll put an asterisk here because there's also one other aspect of nucleophilicty that I haven't talked about yet, but I'll talk about it in a second. In this type of a situation, hydroxide will be better at reacting than fluoride, which would be better at reacting than iodide. And the whole reason why in both situations hydroxide is-- I mean, even when it can interact with the solvent, it's still a pretty good nucleophile, because if you think about hydroxide, and I have to think about this a lot, it has an extra electron. If you think about it, you could imagine it's water that took away-- let me draw it this way. You can imagine it's water where a proton left or where an electron was taken from a proton, so normally, you'd have two pairs and now you have a third pair right here. This oxygen has one, two, three, four, five, six, seven valence electrons, one more than neutral oxygen, so it has a negative charge. It already has an extra electron that gives this negative charge, but oxygen is also more electronegative than hydrogen, so it's also able to get this guy involved a little bit anyway. It's a very basic molecule. So even when it might be interfered a little bit by a protic environment like water, it's still a better nucleophile than something like fluoride. If you take the solvent out of the picture, it's a super strong base. It's also going to be a very, very good nucleophile. Now, the last aspect of nucleophilicity, remember, nucleophilicity is how good something reacts. Now, let's imagine we have something here. We have two hydroxide molecules, right? Let's say that this one is just a straight-up hydroxide. And let's say this one over here has all sorts of things off of it. Let's say it has this big chain of stuff. I don't know which one. Now if you were to look at these two molecules, if you were to try to guess which one is going to be a better nucleophile, you should just remember: nucleophilicity is how good something reacts, how good is it getting in there and making a reaction happen. This thing has this big molecule all around it. It might actually make it very hard, if you go back to this circumstance up here, it might make it very hard for it to get in there. We've talked about steric hindrance from the point of view of the carbon, but we haven't really talked about it from the point of view the nucleophile. In this nucleophile right here, it might be hard for this extra electron right here to actually get to the target nucleus. It will be hindered. While in this situation, it will be much easier, even though the group that's reacting, this oxygen that has a negative charge, this extra electron, is on some level fairly, fairly equivalent. But this one right here is a much smaller molecule. It'll be less hindered, easier to get in. So this'll be a better nucleophile. And that's why I didn't want to make the strong statement that in an aprotic solution, basicity and nucleophilicity are completely correlated, because nucleophilicity still has that other element of how hindered is it. Is it in an environment or is it part of a molecule that will keep it from reacting even though it might be a very strong base? If it actually forms a bond, it'll be very strong. The big thing to remember is that they're just two fundamentally different concepts and that's why there are two different terms for them. Nucleophilicity, how good is it at reacting, saying nothing about how good the resulting bond is. Basicity is how good is the bond? How badly does it want to react, but it doesn't say how good is it at reacting itself.