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Curly arrow conventions in organic chemistry

Transkrypcja filmu video (w języku angielskim)
Sal: What I want to do in this video is talk a little bit about the curly arrow conventions used in organic chemistry and the slight variations I use in many of the videos here on Khan Academy. There's two types of curly arrows you will see. You will see a curly full arrow like this, a curly full arrow like this. And I make sure to draw it curly, you will always see the curly like this. And you will see a curly half arrow that looks like this, curly half arrow or fish hook arrow. The convention is a full arrow or a typical arrow that you're used to seeing, this is talking about the movement of pairs, of electron pairs. Movement of pairs is the convention. I'll show you in a second that I do a slight variation of that, and I do that because it helps me account for electrons, and it helps me at least visualize or conceptualize how things are, or essentially how things are happening, a little bit better. The general convention is that this is movement of pairs and this is movement of electron by itself. Electron, electron not part, electron by itself, maybe I'll write it this way. By itself. The full arrow is what you're going to see through most of organic chemistry. This is the one that you're going to see most typically, the movement of pairs. The movement of electrons by itself, this is going to show up more in free radical reactions, which we do do, but this is later on, and most of organic chemistry is going to be dealing with the movement of pairs. What I've drawn over here is a curly arrow showing the same thing happening. I'm showing you the slight variation that I do. I do it because it helps me, once again, account for the electrons, and it helps me conceptualize what is going on. The typical way that this type of mechanism will be shown, we'll say you have this electron pair on this oxygen, and this electron pair, sometimes we will say, and you will learn about this reaction in not too long, is going to the carbon, or I guess you could say it's attacking the carbon right over here. The reason why this I find a little bit less intuitive is that the whole pair is not going to the carbon, that the oxygen is still going to maintain half of this pair and it's going to form a bond. Essentially one end of this pair is going to end up at the carbon, one end of this pair is going to end up at the oxygen, and they are going to form a bond. The way I draw it, still drawing the full arrow. Here I'm still talking about pairs but I'm talking about the movement of an electron as part of a pair. That's kind of the slight non-conventional thing that I do with the full arrow. Movement, movement of electron, electron as part of pair. I'll often times draw the back of the arrow from that electron, but It's important to recognize that electron is not moving by itself, it's just ending up on one side of a bond, it is moving as part of a pair. Another way to think of it is this electron is going to be on the other side of the bond. I also want to be clear again. When I talk about electrons on either side of bonds, I like to think about that because it helps me do it for accounting purposes. We know that these covalent bonds, this one electron just doesn't sit on one side of a bond and the other electron doesn't just sit on the other side of the bond. In fact everything we do in organic chemistry isn't anywhere near as clean as the way we draw it, but I do this to remind myself that there are two electrons here, and when you have a bond there is some probability that one of the electrons is closer to the hydrogen and there's some probability that that electron is closer to the carbon, and so you can kind of imagine that there are electrons on either sides of the bond. The actual reality is that there's a blur over them and depending on which molecule is more electronegative the probability blur is a little bit more weighted on one side or another, but of course we like to clean things up with these formalisms right over here. This is kind of the example when you have this attacking pair, why I like to think of the full arrow as the movement of an electron as part of a pair. And this breaking bond over here is another example. In the typical convention you have this bond here. Remember a bond is made up of two, this covalent bond right over here is made up of two electrons. If they wanted to show this bond breaking and both of these electrons going to this bromine, the convention is to go from the middle of the bond to the bromine. That I've never found that intuitive because here, once again, bromine already essentially had part of the bond, it was already on one end of the bond. I like to visualize that it's getting the other electron that it wasn't, it's now getting both electrons. One part of the bond was already closer to the bromine, now it's getting the other, it's the other part of the bond. We're going to use full arrows for these mechanisms, just as we would typically use full arrows, but I'll often conceptualize it as the movement of an electron as part of a pair, as opposed to the entire pair, but the full arrows are still used the way it would be conventionally used. Later on when we do free radical reactions we're going to talk about an electron moving by itself. Notice this electron right over here, it's moving or it's doing something and it's not part of a pair, it's by itself so we use the fish hook arrows. Right over here we see a bond breaking but instead of both electrons going to one of the atoms or another one of the atoms, as right over here. When both electrons went to one of the atoms we use the full arrow, this already you can say had one and now it's gaining another one so use the full arrow, but here the bond is breaking and each electron is going to a different atom. Once again the electron is moving, the electron is moving by itself. Maybe I'll put this right, moving by itself, and here is a movement of the electron as part of a pair. Hopefully that clarifies it a little bit. It's important to keep in mind a lot of the notation I use is a departure from the traditional organic chemistry notation, but I think at least in my mind it's helped me build more of an intuition of what's going on in the mechanisms and account for the electrons. If you're in a course, and especially depending on how it's graded, you might want to stick to whatever the professor uses, which is probably going to be a little bit closer to the using the full arrow as the whole pair, and going from the middle of the bonds, the middle of the pairs, as opposed from one of the electrons moving as part of the pair.