Intro to organic mechanisms

- [Instructor] In the last video, we learned about nucleophiles and electrophiles. And in this video, we're gonna look at some simple organic chemistry mechanisms and learn to identify the electrophiles and nucleophiles and also think about how to show the movement of electrons during a mechanism. Remember from general chemistry that mechanisms show the steps by which a reaction occurs, and so, for this reaction, let's look at this alkyl halide on the left here. We know that chlorine is more electronegative than this carbon, so the chlorine is going to withdraw some electron density away from that carbon, which makes it partially positive. From the last video, we know that since this carbon is partially positive, this is the electrophilic center of this compound. If we look at hydroxide ion, which we could have gotten from something like sodium hydroxide, we know that this negatively charged oxygen would be the nucleophilic portion. So hydroxide is gonna act like a nucleophile, and this carbon on our alkyl halide is gonna act like an electrophile. We know that opposite charges attract, so the negatively charged oxygen is going to be attracted to the partially positively charged carbon on the alkyl halide. And we say that the nucleophile attacks the electrophile, so I could draw a curved arrow showing the movement of two electrons over here to this carbon. At the same time, these two electrons come off onto the chlorine. So the chlorine had three lone pairs of electrons around it, let me go ahead and draw those in. We're gonna add in an extra lone pair of electrons, and let me highlight those electrons in magenta. So these two electrons come off onto the chlorine, so I'll make these this pair. And that gives the chlorine a negative one charge, so this is the chloride anion and we call this a leaving group. So we're gonna form a bond between the oxygen and this partially positive carbon, so let me say that this lone pair of electrons on the oxygen is gonna form a bond between the oxygen and that carbon. So, on our product, on this alcohol, those two electrons must be these two electrons right here in this bond that formed. So that's a simple organic chemistry mechanism. We had only one step, the nucleophile attacked and the leaving group left, all in the same step. The goal is not to understand every single thing about this organic chemistry mechanism in great detail right now. Our goal right now is just to appreciate how nucleophiles and electrophiles are used in organic chemistry mechanisms and start to get a feeling for how these curved arrows show the movement or flow of electrons. Let's look at another organic chemistry mechanism, and we're gonna start by identifying our nucleophile and our electrophile. So let's look at this compound first. We know that oxygen is more electronegative than carbon, so this oxygen is going to withdraw some electron density from this carbon. And this chlorine is gonna do the same thing because chlorine is more electronegative than carbon too. So this carbon is electron deficient, right, it is partially positive, and that can act as an electrophile. On the right, we have the acetate anion, which could come from sodium acetate, and this oxygen has a negative one formal charge, so the oxygen is the nucleophilic center of our acetate anion. And our acetate anion can act as a nucleophile. Opposite charges attract, so this negatively charged oxygen is attracted to this positively charged carbon. And we can say that the nucleophile attacks the electrophile and I draw a curved arrow to show the movement of these two electrons. Now, I can't show a bond directly from this oxygen to this carbon until I take these pi electrons and move them off onto the top oxygen here because remember, carbon can never exceed an octet of electrons around it. So let me draw the movement of all of those electrons here. So let's draw, let's draw everything in. So we now would have an oxygen at the top here with three lone pairs of electrons, which give this oxygen a negative one formal charge. So if I'm showing movement of electrons, I'm saying that two electrons from here moved off onto the oxygen, which gives it a negative one formal charge. I still have this chlorine attached to this carbon, so let me draw in that chlorine down here. So this had three lone pairs of electrons around it. And now we formed a new bond, right, we formed a bond between this oxygen and this carbon. And let me highlight these two electrons in red. So those two electrons in red form a bond between that oxygen and this carbon, and this oxygen still has two lone pairs of electrons around it like that. And now let's draw in the rest of this over here, so we have a carbonyl, so let me draw that in, put in my lone pairs of electrons. And then, we have a methyl group coming off like that. So the first step of this mechanism is a nucleophilic attack, so let me write first step here. The nucleophile attacks the electrophile. And it turns out this is a two step mechanism, and in the second step of this mechanism, we're gonna get loss of a leaving group. So let's say a lone pair of electrons on this oxygen moves back in to reform a carbonyl, but we cannot exceed an octet of electrons to this carbon. So that must mean that these two electrons come off onto chlorine, come off onto our leaving group, which would be the chloride anion. Let me draw in those electrons here, which gives the chlorine a negative charge. So the electrons in, let's make them, let's make them blue here, so these electrons in blue come off onto the chlorine to form the chloride anion, which is our leaving group. So we get loss of a leaving group and we reform our carbonyl, which gives us our final product. Again, don't worry too much about the details of the mechanism. Our goal is to identify our nucleophile, electrophile and appreciate our electron flow and start to think about different steps of a mechanism. Nucleophilic attack is a very common one, so is loss of a leaving group. Let's look at one more organic chemistry mechanism. We're gonna start by identifying our nucleophile and our electrophile. Let's look at this compound first. We know that oxygen is more electronegative than this carbon so the oxygen is going to withdraw some electron density away from that carbon, which makes this carbon partially positive. So that's the electrophilic center of this compound. In the first step, we're adding propyl lithium, and we know that carbon is more electronegative than lithium. So carbon's gonna pull these two electrons closer to it, giving this carbon a partially negative charge. Or since the electronegativity difference is so great, we can take those two electrons in that bond and put them on that carbon. So we have three carbons here, and let me draw them in, and we have two electrons on that carbon. Let me highlight those electrons in magenta. So these two electrons in magenta go onto that carbon here, which gives this carbon a negative one formal charge. And we have a carbanion. Since we have a negatively charged carbon, this carbanion is an excellent nucleophile, and this is going to attack the electrophilic portion that we already identified. So opposite charges attract and the negative charge is attracted to the positive charge. And in the first step of our mechanism, our nucleophile attacks our electrophile. Now, we can't just show a bond between those two carbons because that would give 10 electrons around this carbon, right, that would exceed an octet of electrons. So we have to show some electrons going somewhere else. We can take these two electrons and move them off onto our oxygen. So in our first step, our nucleophile attacks our electrophile, and we form a carbon-carbon bond. So we also have an oxygen here with three lone pairs of electrons around it, which give it a negative one formal charge, so we can follow some of those electrons, let me make them blue here. So these electrons come off onto our oxygen, giving the oxygen a negative one formal charge. I'm going to form a bond between our two carbons, and this is where people get messed up a little bit because counting carbons can sometimes be difficult. We need to account for three carbons on our nucleophile, and we also are going to form a bond. So if you look at our product over here, it gives us a clue as to how to draw that. We have three carbons, one, two, and three, and the electrons in magenta are forming this bond in here. So let's go ahead and draw that on our intermediate. Alright, so we have our three carbons accounted for, let me highlight those, so one, two, and three. And then, the electrons in magenta, right, these electrons form a bond between this carbon and this carbon, so the electrons in magenta form a bond between those two carbons. This is our intermediate. So our first step is nucleophilic attack. In the second step, we have a source of protons here. So I'm using the hydronium ion, so let me draw that in really fast. So we have H three O plus, so positive formal charge on the oxygen. And the second step of this mechanism is acid-base chemistry. It's a proton transfer. This negatively charged oxygen on our intermediate acts as a base and takes a proton. So let's say it's this lone pair of electrons takes a proton from H three O plus, leaving these electrons behind on the oxygen. So let me highlight those electrons in red, so these electrons in red are gonna take this proton. So let's say that the electrons in red form this bond, and here was the proton that we took. So we form an alcohol as our final product here. Again, our goal is just to appreciate nucleophiles and electrophiles, and this is a reaction that comes much later in the course. The last thing that I wanted to talk about in this video is something I'm calling the Schwartz principles. And Dr. Schwartz was my organic chemistry professor in college, and he was by far the best teacher that I've ever had. One day, he said to me, "Organic chemistry is only five things." You need to know valence electrons. You need to understand electronegativity. You need to know your acid-base chemistry. You need to know about oxidation-reduction reactions, or redox. And finally, you need to understand nucleophiles and electrophiles. Let's go back to that previous mechanism and see how it actually has all five of the Schwartz principles. First, let's talk about valence electrons. So when we're showing these curved arrows in our mechanisms, like this curved arrow or that curved arrow, we're showing the movement of valence electrons. We used electronegativity a lot. That's how we figured out our nucleophile and our electrophile. The third Schwartz principle was acid-base chemistry. Well, that was the second step of this mechanism, right? So the second step of the mechanism was acid-base chemistry, and you see acid-base reactions a lot in organic chemistry mechanisms. The fourth principle was redox. This is actually a redox reaction. If you assign some oxidation states, you'll see that our starting compound is reduced, right? This ketone is reduced to an alcohol. And finally, nucleophile and electrophile. Obviously, that was the first step of our mechanism and something that we've been focusing on. So you don't necessarily have to have all five things in one mechanism. This one just happens to have all five things, and so, I wanted to talk about the Schwartz principles. And if you understand those five concepts really well, mechanisms will be a lot easier for you.