Nucleophiles and Basicity - Online Tutor, Practice Problems & Exam Prep

Understanding the difference between basicity and nucleophilicity.

Now, I want to go into nucleophiles because remember that I said we have to define nucleophiles more than just saying negative is strong and neutral is weak. So I want to remind you guys of the difference between a nucleophile and a base because that actually is going to matter for this section. Okay? This section has a lot to do with conceptual questions that once again you could get in this chapter. What a nucleophile is, if you remember, had to do with the Lewis definition of acids and bases. So I'm just going to put here nucleophile is the Lewis definition. Wow, Johnny can't spell, Lewis. Okay? And what that means is that it's a good electron donor. Okay? So remember that basically if you can donate electrons easily, that's a good nucleophile. Okay? What's a good base? Well, a base is the Bronsted-Lowry definition. Okay? Remember what the Bronsted-Lowry definition is? That you're a good proton acceptor. Okay? Now, a lot of times, a good electron donor is also going to be a good proton acceptor. So a lot of times, these things are the same. Nucleophilicity and basicity have a lot of crossover. But there are going to be some instances where one of the things gets better and the other one doesn't, or even the other one gets worse. It might get better at donating electrons, but worse at pulling off a proton. And I'm going to show you how. So this is the way we determine the rules. The first rule is actually that generalization that I told you guys earlier, which is just that if you have a negative charge, that's always going to be a stronger nucleophile than neutral. So that's when I said basically strong versus weak. Okay? So you guys already knew the first rule just from me telling you that. But there's actually two more rules that you guys need to be aware of. Okay? So the second rule is that the bulkier the substrate, if you have a very bulky nucleophile, that's going to make it more basic and less nucleophilic. Okay? So what am I saying there? Okay? What I'm saying is that if you have a really bulky negatively charged compound, let's say. That means that it's going to be worse at donating electrons. Why? Because it's going to have a more difficult time approaching electrophiles because now it's going to be so bulky. So it's actually going to be worse at donating electrons. But it's actually going to be better at pulling off protons. Why? Because protons typically are at the edges of molecules. So it's easy for it to access a proton, but it's hard for it to donate electrons. Does that kind of make sense? This is going to come into play later when we talk about elimination reactions and bases that favor elimination. Because remember, elimination is about the base, not the nucleophile. It's about pulling off a proton. Alright? And then finally, this is our last rule that you need to know and then we'll be done with nucleophiles, which is that basicity and nucleophilicity almost always go in the same direction. So as you can see, as I go toward less electronegative, my bases, I mean, nucleophiles get stronger. Okay? And then also as I go up in the periodic table, my bases and my nucleophiles get stronger. That has to do with the size effect. Remember that? So basically, as you go up, you're going to be better at donating electrons because you're smaller, so you don't like them as much. Okay? That's kind of the point. And here I have a little drawing to show that. But it turns out that there is going to be an exception to the rule. And the exception Okay. I made it naked. Okay? I'm just going to put here they're naked. Okay? Pretty scandalous. There's nothing around them shielding them or whatever. Okay? But then, if you have a protic solvent, what did I say about protic solvents? Well, protic solvents, if you guys need to be reminded, are solvents that can hydrogen bond. If you can hydrogen bond, these are solvents that are typically attracted to charges, so they're attracted to positive charges and negative charges. So what they're going to do is they're going to do something called solvating. They're going to surround that negative charge. So here I've drawn a picture of water, which can hydrogen bond, it's protic, solvating fluoride and solvating iodine. Fluoride and iodine, okay? And what we find let me just move out of the way here for a second. Is that when you have a smaller anion like fluoride, the protic molecules are able to surround it better and able to more tightly solvate it. So what that means is that it's going to be a worse electron donor because it's so covered up. It's really solvated. That's the word for it. Solvated just means it's covered in all these water molecules. Okay? Whereas an iodide is so much bigger that it's going to be more loosely solvated. It's going to be more difficult for all the water to cover all the spots. It has a lot more surface area. So it's actually going to be a better electron donor even though it's a worse nucleophile. Okay? So it turns out that in a protic solvent, iodide is actually going to be your best nucleophile. Okay? So this trend is reversed as you can see in a protic solvent, this trend is reversed, but in an aprotic solvent, the trend is the way it was at the beginning, which is just that F is the best nucleophile and I is the worst. Okay? So this is going to be the one thing that you guys have to remember in terms of concept because you could see this kind of question all the time on all kinds of exams, all kinds of test banks where professors will ask what's the best nucleophile in a protic solvent? What's the worst nucleophile in an aprotic solvent? So you need to have these trends memorized like the back of your hand to answer those kinds of conceptual questions. Now, does it matter so much for mechanisms? Not usually. Usually, like I said, mechanisms aren't determined by the solvents necessarily, but you should still know it because it's going to give you a better understanding of the content of this chapter. Alright? So I hope that made sense. Let me know if I can explain it any better. Make sure to ask questions. This is something that typically a lot of students find is a little bit confusing. I hope that my little drawing here this is actually like a new drawing I just made for you guys. I hope that it will help you guys kind of relate to what I'm talking about a little bit better. Okay? So let's go ahead and move on to the next)section.

Here’s what students ask on this topic:

What is the difference between a nucleophile and a base?

A nucleophile is defined by the Lewis definition as a good electron donor, meaning it can easily donate electrons to form a bond. A base, on the other hand, is defined by the Brønsted-Lowry definition as a good proton acceptor, meaning it can accept a proton (H⁺). While many nucleophiles are also good bases, the two properties are not always aligned. For example, a bulky nucleophile may be less effective at donating electrons but still be a strong base because it can easily access and pull off protons.

How does the bulkiness of a nucleophile affect its nucleophilicity and basicity?

The bulkiness of a nucleophile affects its nucleophilicity and basicity differently. A bulkier nucleophile is less nucleophilic because its large size makes it difficult to approach and donate electrons to an electrophile. However, it is more basic because its size allows it to easily access and pull off protons, which are typically located at the edges of molecules. This distinction is important in reactions like elimination, where the base's ability to remove a proton is crucial.

Why are smaller anions like fluoride less nucleophilic in protic solvents?

In protic solvents, smaller anions like fluoride (F^-) are less nucleophilic because they are more heavily solvated. Protic solvents can hydrogen bond and surround the anion, effectively 'covering' it with solvent molecules. This solvation makes it harder for the anion to donate electrons, thus reducing its nucleophilicity. Larger anions like iodide (I^-) are less solvated due to their size, making them better nucleophiles in protic solvents.

How do nucleophilicity and basicity trends change in protic vs. aprotic solvents?

In protic solvents, nucleophilicity decreases with decreasing anion size because smaller anions are more solvated. For example, iodide (I^-) is a better nucleophile than fluoride (F^-) in protic solvents. In aprotic solvents, the trend is reversed: nucleophilicity increases with decreasing anion size because there is no solvation effect. Thus, fluoride (F^-) is a better nucleophile than iodide (I^-) in aprotic solvents. Basicity trends generally follow nucleophilicity trends but are less affected by solvent type.

What are the key rules for determining nucleophilicity and basicity?

The key rules for determining nucleophilicity and basicity are: 1) Negatively charged nucleophiles are stronger than neutral ones. 2) Bulkier nucleophiles are less nucleophilic but more basic. 3) Basicity and nucleophilicity generally align, with both increasing as you move up the periodic table and toward less electronegative elements. Additionally, in protic solvents, smaller anions are more solvated and thus less nucleophilic, while in aprotic solvents, the trend is reversed.