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Organic Chemistry
Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.
7. Substitution Reactions
Good Leaving Groups
The favorability of many reactions depends on the ability of the leaving group to be stable after it gains a negative charge. Sounds familiar, yes?
That’s because this is the same exact way we determine conjugate base stability.
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How to use the factors affecting acidity to predict leaving group ability.
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as I previously mentioned, substitution reactions would not be possible without leaving groups. Why? Because remember that the Electra files do not have an empty orbital. So what that means is, in order to make a bond, I'm always gonna have to break a bond and I can't break upon unless something's gonna leave. Okay, so what that means is that it's going to be important in this chapter that we know exactly how to recognize when something's a good leaving group when something's a bad leaving group. Okay, let's go ahead and get started and learn more about leaving groups. Okay? So, as I said, leaving groups are what break upon to make it reactive. And in general, we can use the same rules that we learn from acids and bases to predict when something is going to make a good leaving group. We just use the rules that would make something a good cognac based. Okay, because what a conjugate base all it is is it's something that's stable after it gains electrons. Okay. And that's exactly what a leaving group isas. Well, after accepting an electron pair, is it gonna be stable or not? Okay, so how do we determine if a leaving group is good or bad? We use the same exact factors that affect acidity that we used for the acid based chapter. Okay, so remember that we had a few factors that affected how stable comic. It's where after they left, the one that we're going to use most often is actually just the element effect. Okay, because it turns out there aren't that many good leaving groups, and we can usually use the element effect Thio describe all of them. All right, so I know it's been a long time, but what were the trends that we use for the element effect? We used election negativity. We said that as something goes to the right towards flooring, it's gonna be more election negative, so it's gonna like to have a negative charge more. It'll be more stable, but also we use the size trend. We said that as a convoy gets bigger, the easier it is for it. Thio, except more electrons because it doesn't care. I use the word is very squishy, Adam. It's got tons of electrons everywhere, so if it gains two extra electrons, it's not really going to care Okay. So, ideally, the best leaving group would be the one that's furthest to the right and for this down. So that would be around down here around iodine. Okay, Now, you can't get much lower than I dying because then you start getting into, like, these weird transition metals that are radioactive and probably give you cancer. Okay, but iodine is a good is a good place, toe place, toe end in terms of the trend. Okay, so this doesn't explain all leaving groups, because remember, we have other types of factors affecting acidity as well. Remember that there was the inductive effect. There was the resonance effect. All that stuff still applies. But in general, the element effect does a really good job explaining that really common leaving groups. So what I want you guys to do is go through this exercise. I've given you pairs of molecules. All you have to do is compare the two different pairs and see which one would have the better leaving group. Okay? And go ahead and just try to use the factors affecting city to figure that out and then I'll explain each one to you. So go for it.
We use factors affecting acidity to determine which leaving groups will be most stable after gaining a lone pair.
- Recall that the element effect consists of two trends:
Predict which of the following pairs of electrophiles possesses the best leaving group.
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example
Predict the best leaving group
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All right, so if you were paying attention, this first one is pretty easy. Okay, We know that basically believing group is probably gonna be the oxygen or the nitrogen for a Why? Because I do have CH threes president. Okay, But just letting you know carbon is never gonna be a good leaving group. Why? Because think about it. Carbon on the periodic table is pretty far to the left. Okay, So what that means is it's gonna be a terrible leaving group. It does not like to have a negative charge. So never think of carbon or hydrogen. Hydrogen is even worse. Hydrogen all the way over there. They do not like to have electrons, so please never make those leaving group. But what I do have is I have an oxygen and have a nitrogen. What effect could I use to figure out which ones the better leaving group. The element effect? I could just say Okay, which one's more? Electra Negative. Which one's bigger? They're both actually roughly the same size because they're on the same row. But one is more Electra negative. And that's Oh, okay. So what that means is that the right answer? for a is that the O. H. Is gonna be a better leaving group than the NHS. All right, so that's the end of that question. Go ahead and try to use that same logic for B. Go for it.
Predict which of the following pairs of electrophiles possesses the best leaving group.
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example
Predict the best leaving group
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all right. So be it was actually even easier than a Why? Because for B, I don't have any good leaving groups. All I have a ch three on both sides. So this would once again part of the French be a super shitty leaving group. Okay? It would be terrible if it picked up. Electrons would have to be a C. Negatives. That just doesn't really happen. Okay, then. Whereas the other compound has a chlorine. Okay. And obviously, if chlorine picks up electrons, that's gonna be a lot more stable. Why? Because the element effect. Once again, chlorine, let me erase this little chlorine is here. And then carbon is here. So chlorine has an advantage both with it's much more Electra negative. And it's also bigger. So it's just all around a much, much better leaving group than Carbon could ever dream of being alright, magnitudes of order. Better. All right, so let's go on to see
Predict which of the following pairs of electrophiles possesses the best leaving group.
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example
Predict the best leaving group
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I'm making these too easy for you. All right, so for this one, once again, we could use the element effect, and hopefully you picked iodine. I mean, I gave you that one right at the beginning. I told you that iodine is pretty much one of the best leaving groups I could ever have. They both in terms of Elektra negativity. Actually, flooring is gonna be more Electra negative than Iowa. Okay, but the size effect is going to take over. And iodine is just gonna be so much bigger than flooring that it's going to be a better leaving groups in the flooring. Well, because the iodine will tolerate those electrons better. Okay, cool. So that's it for C last one D and then we'll be done with this page.
Predict which of the following pairs of electrophiles possesses the best leaving group.
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example
Predict the best leaving group
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so I had to throw one tricky one in there. I hope this one wasn't didn't freak out too much. Basically, this one was a challenge. Because I hope you noticed you actually could not use the element effects for this one. Because the element that's leaving is the same for both of them. They're both oxygen. Okay, so what effect could I use? And it turns out that this is gonna be one example where I would use the resonance effect. Okay, If you think about what these air gonna look like after they leave, let's actually draw them. Okay, So sorry if you didn't think about that, But let's say that the O. H. Leaves let's say that this weird thing leaves, okay. The O H would just look like this O h negative. Trying to avoid my head. Perfect. Okay, this other one would look like O negative, but that's attached to s dull. Bon. Oh, dull. Bon O C h three. Okay. Does any of these have an advantage? Well, the O. H. Negative is what we call localized. Localized is a really bad thing. That means that the charge can only be in one spot it would just be right here That's not very stable. Whereas this negative charge could resonate on this other compound that I could do is I could take these electrons and move them here so I would get a negative charge of the top. And then I could also get a negative charge at the bottom. So this one is what we call. Remember? The word was D localized. When you can resonate, that's d localized. It means that the electron, the negative charge, can spread everywhere. So it turns out that because of the crazy amount of residents that this molecule can have, this is gonna be a much better leaving group than always Negative. In fact, this is actually gonna be one of our good leaving groups later on. When we talk about leaving groups in more depth, this is This is a group called a sulfa. Nay, Esther. Okay. A Sultanate, Esther. Like I said, I'm gonna go more in detail later. But just so you know, even though 00 negative is kind of not that great of a leaving group, because there's so much resonance, it becomes an amazing leaving group. Alright, It's cool with that. So I'm just trying to show you how, really all the factors affecting acidity are fair game. But this is one where use residents to figure that out. All right, so now let's simplify things. I've given you the general rule, but let's simplify things because you have a lot of mechanisms toe learn, okay? And a lot of stuff you have to memorize. So I'm just gonna simplify everything we just talked about and summarize it in one line. Are you guys cool with that? This is the line, okay. Due to their high electro negativity Alcohol. Hey, wides will be the primary leaving group for this chapter. All right? Didn't that simplify things? So what I'm trying to say here is I'm really trying to take your entire thought process out of it. What I'm saying is this now you know how to find out if something's a good leaving group or a badly in group. If it's better or worse. But I'm just going to simplify things and tell you this. Al Kyohei Allied's 90% of time are gonna be you're leaving group. Okay, so if you see an Al Kyohei lied, just imagine that is my leaving group. If you see something else, ignore it for now, okay? Because we're gonna be dealing without your Haley's. Basically, for this entire chapter. Okay, at the very end, then I'll go ahead and add a few other leaving groups. Right now. Let's just focus on alcohol highlights, because that's going to simplify things a lot. Bueno. Cool. All right, so let's move on.
Due to their high electronegativity, alkyl halides will be the primary leaving groups for this chapter.
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