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.  

Concept: How to use the factors affecting acidity to predict leaving group ability.  

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Video Transcript

As I previously mentioned, substitution reactions would not be possible without leaving groups. Why? Because remember that the electrophiles do not have an empty orbital. So what that means is in order to make a bond, I'm always going to have to break a bond and I can't break a bond unless something's going to leave.
So what that means is it's going to be important in this chapter that we know exactly how to recognize when something's a good leaving group and when something's a bad leaving group. So let's go ahead and get started and learn more about leaving groups.
As I said, leaving groups are what break a bond to make it reactive. And in general, we can use the same rules that we learned from acids and bases to predict when something's going to make a good leaving group, we just use the rules that would make something a good conjugate base. Because a conjugate base, all it is is it's something that's stable after it gains electrons. That's exactly what a leaving group is as well. After accepting an electron pair is it going to be stable or not?
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/base chapter. So remember that we had a few factors that affected how stable conjugates were after they left, the one that we're going to use most often is actually just the element effect because it turns out there aren't that many good leaving groups and we can usually use the element effect to describe all of them.
I know it's been a long time but what were the trends that we used for the element effect? We used electronegativity. We said that as something goes to the right, towards fluorine, it's going to be more electronegative so it's going to like to have a negative charge more. It will be more stable.
But also we use the size trend. We said that as a conjugate gets bigger, the easier it is for it to accept more electrons because it doesn't care. I used the word it's a very squishy atom. It's got tons of electrons everywhere, so if it gains two extra electrons, it's not really going to care.
So ideally, the best leaving group would be the one that's furthest to the right and furthest down, so that would be around down here around iodine. Now you can't get much lower than iodine because then you start getting into these weird transition metals that are radioactive and probably give you cancer. But iodine is a good place to end in terms of the trends.
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 of that stuff still applies, but in general, the element effect does a really good job explaining the 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. And go ahead and just try to use the factors affecting acidity 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: 

Example: Predict which of the following pairs of electrophiles possesses the best leaving group.

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Video Transcript

All right, so if you were paying attention, this first one was pretty easy. We know that basically the leaving group is probably going to be the oxygen or the nitrogen for (a). Why? Because I do have CH3's present, but just letting you know, carbon is never going to be a good leaving group. Why? Because think about it, carbon, on the periodic table, is pretty far to the left so what that means is it's going to 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 is all the way over there. They do not like to have electrons, so please never make those a leaving group.
But what I do have is I have an oxygen and I have a nitrogen. What effect could I use to figure out which one's the better leaving group? The element effect. I could just say which one is more electronegative, which one's bigger. They're both actually roughly the same size because they're on the same row, but one is more electronegative and that's O. What that means is that the right answer for (a) is that the OH is going to be a better leaving group than the NH2. That's the end of that question. Go ahead and try to use that same logic for (b). Go for it. 

Example: Predict which of the following pairs of electrophiles possesses the best leaving group.

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Example: Predict which of the following pairs of electrophiles possesses the best leaving group.

1m

Example: Predict which of the following pairs of electrophiles possesses the best leaving group.

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Due to their high electronegativity, alkyl halides will be the primary leaving groups for this chapter.