9. Alkenes and Alkynes
Zaitsev Rule
Elimination reactions often can yield multiple products. However, not all of these products will be of equal stability. Zaitsev’s Rule (also spelled Saytzeff’s Rule) helps us predict the major product.
Zaitsev vs. Hofmann Product
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concept
Defining Zaitsev’s Rule
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Now I have to talk about one of the most important aspects of elimination reactions. I know that you guys thought we were done learning about them, but we're not. There's something else that you guys need to know, And that's how to predict if you have multiple products possible, how to get the major product and the minor products. And this relates to, Zeit says rule so many times in an elimination reactions. What we're gonna find is that there's multiple al canes that are present at the end. Okay, so how do we determine? Are they all made equally or are they made in different ratios? How do we know that? And we use it saves rule to figure that part out. Okay, so whenever you have more than one unique AL Keen as a product, that's when you use site's rule. What does items rules say? Say Well, basically, at this point, you should already know how to tell when a double bond is more stable or less stable, based on the number of our groups that are around it. Okay. And based on that rule, the most stable product is gonna be called my sights of product, okay, And that is based on how many are groups that has around it. So the more our groups it has, the more than the other one. That's the sites of product. Okay, now the one with the less are groups around it or the least stable product is going to be called the Hoffman product. All right, slash just some basic vocabulary that you need to understand before we can even start using this rule.
Zaitsev’s Rule predicts that in most cases, the most substituted product will be favored. This is also known as the Zaitsev product. Draw the Zaitsev product of the following reaction:
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example
Zaitsev product
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Now I've given you an example here of basically an alcohol. Hey, Light with a nuclear file. Let's use the big Daddy flow chart to figure out if this what mechanism this would be. So let's go ahead and ask our first question notice that my nuclear follows N a O E t. Is that negatively charged? Is that neutral? For those of you that said neutral, you're forgetting that sodium can associate. So what's actually gonna look like is like O E T negative. So it's gonna be a negatively charged nuclear file that's going to go down the left hand side of the flow chart. Okay, so let's go toe. Step two. Step two is now 81 of my bulky bases. No. Okay, we have a list of bulky bases. N a O A. T is not one of them, so I'm just gonna say no. Let's go to my third question. What type of alcohol? Hey, light, do we have or what type of leaving group? Well, this carbon right there is attached to two other carbons. One too. So this would be a secondary. Al Kyohei lied. So do we know the mechanism now? No, we have to ask one more question. The last question is I'm just gonna put it down. Here is my base a better nuclear file or better base? So for this yet to remember, what were the strong bases is N a o a t one of those strong bases. Yes, it is. Remember that one of the strong basis was oxides. Oxides have the general formula of O r. Negative. And that's exactly what we have. We have O E t, which is an ethyl group. Negative. So this is an oxides. This is gonna favor e to Alright, Cool. So now we've got e to Now we have to figure out OK, how do we actually draw the mechanism for this? And how do you predict the products? Just remember what the first step of E two is. Figure out how many different beta protons I have. Okay, So what do we got? We've got two different beta carbons. Let's say this is beta one, and let's say this is Beta two. Okay, those are my two different options. Do both of them have hydrogen on them? Yes, they both do. On the green one. I've got ah, hydrogen towards the front and a hydrogen towards the back on the red one. I just have a hydrogen towards the back. Okay, So are you ready to eliminate? Yet? We have to ask ourselves one more question. Now that we know all of our beta protons, which is three. How many of them could actually react in an e two reaction using the Antico plane or rule? Remember, you always have to think of that rule. The answer is two of these could react. I could eliminate in the green direction with this one right there. And I could also eliminate in the red direction using that one right there. The reason is because my chlorine is facing towards the front so I can Onley eliminate with the hydrogen that's facing towards the back. Okay, so now that I know that, let's go ahead and draw one of the mechanisms, we don't have to draw both. Let's just draw one of them and then predict the products. So the mechanism would be three arrows, just like it always is. Let's say I'm taking off the red age. Then what I'm gonna do is I'm gonna dump my electrons into the bond between the Alpha and the beta, and then I'm gonna kick out my CEO. Okay, So what that means is I'm gonna have to different products possible. I'm gonna have a products possible where I eliminate where I just did. So it would be a dull bond right there with now. No seal. I don't to draw that. And a methyl group facing down like that. Now, why is it important that I drew my metal group on a stick? Because remember, now this is tribunal plainer. If I draw it on a wedge, that would look like I have no clue what I'm doing. Okay, I would kind of look like an organic nube. And you don't wanna look like a nube when you're trying to get points from your professor. So that's the way we would draw that product. Obviously, I would have my cl negative leaving group, but there's also another product I could have gotten. The other product would have been if I eliminated up to the green hydrogen and that would have given me a product that looks like this adult bond there. And let me just straighten that out a little bit. and that method group would still be on the wedge. Why am I drawing it on the wedge here? Because that carbon is still tetra hydro because it still has an H on it. So that is a carbon with four different groups around it, so that one should be drawn as a tetra hydro. All right, so now we've drawn are two different products. Are they both going performed in equal amounts? No, It turns out that site's rule explains. Just look down here that we will always favor the mawr substituted thermal dynamically stable products. Now what the hell does thermal dynamically stable mean? I know that whenever you bring, like, thermal dynamics or kinetics, things get confusing. It just means overall which one's gonna have the least energy? Which one is gonna be the overall most stable at the end. But guess what? We know how to figure this out because we can just use the ALC instability rules to determine that. So out of these two double bonds, which of them is gonna be more stable in which one of them is less stable? What do you think this is like its own question? Well, this one here is Try substituted. This one here is Di Substituted. Okay, So which one is gonna be the overall? More stable product? Red Red is gonna be more stable than green. Which one is the sites of product? Red? Okay, because sites of is the more stable one. This one's the less stable. It's Hoffman. Okay, So instead of saying less stable, more stable from now on, I'm just gonna be using the words Zaitsev and Hoffman, because guess what? Those are just synonyms of the less stable and more stable. Just like something Hoffman. I could just use those words. Okay, So now we have to figure out which one is major and which one is minor. Which one I'm gonna form in Higher Mountain. The other. And what sites of your explains is that I'm gonna favor the sites of product. Okay, So that means that this is gonna be a major product, and this is gonna be my minor product. Okay. Does that mean that I only get one of these? No, I still get both, but I'm gonna get a lot more of the red and a lot less of the green. Okay. Now, sometimes your professor may ask a question that just says, Give the major product for the reaction. If they're asking for major product, then you would just draw red. But if you're professor asks, give all the products, then you would draw both of them. Does that make sense? But many times, especially in multiple choice exams. If you have any multiple choice component many times your professor is just gonna say, draw the major product or select the major product. If you're doing online homework and says, Select the major product, it would be this one. Okay, if you're just checking it out, different resource is online. This would be the more stable, more favorite product. Awesome guys. So Zeit says. Rules pretty easy, right? We just use the number of our groups to figure out which ones more stable, which ones less stable, select the more stable one.
The exception to this rule comes with the use of bulky bases. These promote the formation of the les substituted product.
- The most common bulky bases are lithium diisopropylamide (LDA) and tert-butoxide (t-BuO-)
The less substitued product is called the Hofmann product. Let's draw the Hofmann product of the following reaction:
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example
Hofmann product
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now. One more thing. It turns out that there's an exception to this rule, because guess what? Organic chemistry always has. One exception, right? So the exception is gonna be unless we're using a bulky base. Okay, bulky bases, remember that They're not very nuclear filic. They're very basic. They're very good at pulling off protons, but they're not very good at donating electrons. So what that means is that ah, bulky base is going to promote the formation of the less substituted kinetic product. What does kinetic mean? It means it's the one of the lowest activation energy. The one that's the easiest to grab, even though it's not stable at the end is going to be the one that is favorite at the end. So let me show you guys how this works. Let's say that here I'm reacting with a bulky based turkey talk side. Okay, so that's my turkey talk side molecule. Notice that it's kind of bulky. It's gotten Oh, and then it's got that Terp Udal group on the side. Let's do the same reaction. I have my green hydrogen here and then I have my red hydrogen here. Okay, so we have the same option to pull, pull the green one and get the less stable product or the red one and get the most stable product. All right, so overall on, we know that if we pull the red proton, we're gonna get the more stable product. But this Turk beauty group is also pretty big. So what that means is that it can either come in here and fight through this metal group and through the leaving group to try to get to that age. Or it could just easily pull off this green one where there's a lot of space. And it turns out that even though the red one is more stable, the turkey talks. That is gonna be such a strong base, and it's gonna be so bulky that it will prefer to take the easy way out. It's gonna prefer to take the less substituted age just because it can get to that one better. Isn't that interesting? So I'm gonna get the same exact products here where I'm going to get in elimination product. That looks like this. Plus, I'm gonna get in elimination product that looks like this. Okay. In fact, the site seven Hoffman hasn't changed either. This is still going to be my sights of product. And this is still gonna be my Hoffman product. Okay, so nothing has changed. The only thing that's changed is that since I'm using a bulky base, I'm actually gonna favor the Hoffman. So that means that my major product will actually be the Hoffman, and my minor product is going to be the sites of, even though the minor product is the one that's more stable. But it's the one that's harder for the turkey talks like to get to, so it's just gonna pick the easiest way possible, all right?
Thermodynamic vs. Kinetic Product
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concept
Using a Free Energy Diagram to explain thermodynamic vs. kinetic products.
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Now, the last thing I want to do is I just want to show you guys with an energy diagram that we're going to sketch up really quick what these words mean between thermodynamic and kinetic, because this is gonna come up Maurin Orgel one and or go to. So I just want to show you guys remember that you have an energy diagram. And the way that it works is that you have some kind of, you know, spontaneity here, And you have a reaction coordinate here where? Basically, at the end, I have a double bond, and at the beginning, I have just my alcohol. Hey, lied. Plus the nuclear file. Okay, So what I want to show you guys is that this is a concerted reaction, so it all happens at the same time. Okay, so I'm only gonna have one hump. I'm gonna have one transition state. Remember that you two just has a transition state. The thing is that what it looks like with the kinetic versus a thermal dynamic, um, energies are gonna look like you're gonna be different. So for the thermal dynamic one, I'm gonna start up here at this energy level I'm gonna pass through a pretty big I'm sorry. A pretty big hump in energy. And then I'm gonna gain a lot of energy at the end because my double bond is overall going to save me some energy. Okay, so that's what the elimination products would look like for the first one. Okay. Are you guys following so far? Cool. Now for the second one. What I would find is that my energy levels at the same place at the beginning. Okay, so it's right here. I'm still there's my kinetic pathway, but it turns out that for the kinetic pathway, I'm gonna have a much lower activation energy. But then I'm also gonna have a much lower gain instability. Okay, So what you can tell is that check out the entropy for a second or the spontaneity. Okay, overall, I'm going to gain. I'm sorry. It's supposed to change adult egy. Okay, overall, my product for the red for the sites of product is gonna be overall more stable at the end. I'm going to gain Mawr, delta G or more and free energy by going in that direction than by going in this direction. Is that making sense so far. So basically, the red one is overall more stable in the green. Okay, but what we're also going to notice is that the activation energy off the first one is much higher. And the activation of the second activation energy of the second one is much lower. Okay, so the green is what we would call kinetic control. Kinetic control. Kinetic control basically means that all I'm looking at is the one with lowest activation energy. Okay, I'm saying whichever one is the easiest one to form, that's the one that's gonna be favored. Okay, Whereas thermal dynamic control, I'm just gonna put your thermo is the one that looks at the overall lowest Delta G. The one that changes that get that gets the most free energy at the end. That's the most stable at the end. That's gonna be the one that I favor. Okay? And that's the difference between Zeit seven Hoffman. Basically Zaitsev is thermal dynamic control, where all I care about is the stability of the end product. Okay. Whereas on whereas Hoffmann is gonna be kinetic control, because I'm going to care about is the one that's the easiest to form or the one with lowest activation energy. Is that different? Kind of making sense Now, The reason I'm telling you guys, this is because this is gonna come up later when we talk about other reactions in or go to, there's gonna be Connecticut control and thermal dynamic control. And it's gonna be the same kind of principle where I'm looking at either the stability of the end product or I'm looking at which everyone is just the easiest one to format the beginning. All right, so I hope I didn't confuse you guys here. I just really want you guys to understand the difference between site seven Hoffman and how one is thermal dynamic and one is kinetic. All right, so let's go ahead and do some practice problems based on site. Several
- The Zaitsev product is also known as the thermodynamic product, since it is the one that releases the most free energy overall (most negative ΔG°).
- The Hofmann product is also known as the kinetic product, since it is the one that overcomes the lowest activation energy (Ea).
Practice
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ProblemWhat is the major product of the reaction?
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ProblemWhat is the major product of the reaction?
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Both A and B
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None of these
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ProblemWhat is the major product of the reaction?
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D
Additional resources for Zaitsev Rule