Nucleophiles and Basicity

Now I want to go into nucleophile because remember that I said we have to define nucleophile more than just saying negative is strong and neutral is weak, so I want to remind you guys of what's the difference between a nucleophile and a base because that actually is going to matter for the section, OK? This section has a lot to do with conceptual questions that once again you could get in this chapter and what a nucleophile is if you remember the had to do with the Lewis definition of acids and bases, OK? So IÕm just going to put here nucleophile is the Lewis definition.... Wow OK Johnny can't spell, LEWIS OK? And what that means is that it's a good electron donator, OK? So remember that basically if you can donate electrons easily that's a good nucleophile, OK? What's a good base? Well base is the Bronsted Lowry definition, OK? Remember what the Bronsted Lowry definition is? That you're a good proton acceptor, OK? Now a lot of times a good electron donator 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 guys 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 stronger nucleophile than neutral so that's what I said basically strong versus weak, OK? So you guys already knew the first rule just form me telling you guys that but there's actually two more rules that you guys need to be aware of, OK? So the second rule is that the bulkier of the substrate, OK? If you have a very bulky nucleophile that's going to make it more basic and less nucleophilic, OK? So what am I saying there? What I'm saying is that if you have a really bulky negatively charged compound let's say, OK? That means that it's going to be worse at donating electrons, why? Because going to a more difficult time approaching electrophiles because now it's going to be so bulky, OK? So it's actually 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 electrophile, 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 nucleophile 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 basicsim and nucleophiles get stronger, OK? And then also as I go up my periodic table my bases and my nucleophile 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, OK? 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 comes with protic solvents, so as you can see in an aprotic solvent this is what the nucleophile look like, they just look naked, OK? I'm going to put here they're naked, OK? Pretty scandalous, there's nothing around them shielding them or whatever, OK? 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 solvent 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, OK? They're going to surround that negative charge so here I've drawn a picture of water which can hydrogen bond its protic solvating fluorine and solvating iodide, fluoride and iodide and we find.....Let me just move out of the way here for a second is that when you have a smaller anion like fluorine or 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, OK? It's really solvated that's the word for it, OK? Solvated just means it's covered in all these water molecules, OK? Whereas an iodide is so much bigger that it's going to be more loosely solvate, 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 better electron donor even though it's a worse nucleophile, OK? So it turns out that in a protic solvent iodide is actually going to be your best nucleophile, OK? So this trend is reversed as you guys can see in a protic solvent this trend is reversed but in an aprotic solve it the trend is the way it was at the beginning which is just that F is the best nucleophile and I is the worst, OK? So this is going to be the one thing that you guys have to remember in terms of concept because you could get.... I see this kind of question all time and in all kinds of exams all kind 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 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 concept 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 feels a little bit confusing, I hope that my little drawing here this is actually 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 better, OK? So let's go ahead and move on to the next topic.