Which of the following carbocations would you expect to rearra...

Question

Which of the following carbocations would you expect to rearrange? If you expect rearrangement, draw the carbocation you expect to form and the mechanism by which it will form.

(e) Diagram of a carbocation with a positive charge and a proposed rearrangement mechanism.

Accepted Answer

Hi, everyone. Welcome back. Our next question says is the following Carbocaine expected to undergo rearrangement if so provide the mechanism for the formation of the expected carbo Cron. So the Carbocaine we're given is a cyclops ring with one carbon attached that has a positive charge. So that would be ach two positive group. Now we have four different answer choices here, but they don't all fit on the screen. So I'm going to scroll up so we can see them all. And then before I do that, I'm just going to draw on the right hand side, our Carbocaine that we're given just so that we can refer back to that even after we scroll up. So though it's fairly easy to remember, but let's scroll up. So we can see all four of our answers choices A through C all say the Carbocaine is expected to undergo rearrangement. The mechanism for the formation of the expected Carbocaine is as followed. And then we have three different proposed mechanisms. We'll look at those in more detail when we think through what we expect to happen. So we'll look back to those and then choice D says the Carbocaine is not expected to undergo rearrangement. So we need to look at our Carbocaine and think about what would make it liable to undergo rearrangement. And what type of mechanism would that be? Well, we recall that the more substituted car orca iron is more stable, that's because our carbon is missing, an electron alkyl groups tend to be electron donating. So the more alkyl groups on our positively charged carbon, they're helping out by donating some electron density and it's more happy that way. So when we look at our original molecule, we see that we have a primary carbo cion, that carbon has no, just the pentane ring attached to it, no other R groups. So then we need to look at our adjacent carbons. Is there an adjacent carbon where that positive charge would be more stable? And yes, there is our adjacent carbon to the one here is a tertiary carbon. So that has three R groups, it'd be way happier with a positive charge. So we do expect that our Carbocaine will undergo rearrangement here. Now, one quick caveat before we say for sure that it will undergo rearrangement, we look at that adjacent carbon. It needs to have either a hydrogen or a methyl group attached to it. So we need to ask, is that adjacent carbon more substituted? If yes, does it have a hydrogen atom or a methyl group? That's because the mechanism of rearrangement will be either a 12 hydride shift or I won to Maple shift. So you have to keep in mind that methyl shift because it's tempting or it's, you know, uh when you look at this, if you saw that you had a quaternary carbon, for instance, next to your positive charge, you might think that there was no rearrangement possible. But if one of the substituents on their quaternary carbon is a methyl group, you're good. So what do we have here? Well, we know the, we have no methyl groups, even if we did a hydride shift is easier to do than a methyl shift. So our adjacent carbon does have our answer is yes, our adjacent carbon is tertiary. Does it have a group capable of a shift? Yes, it has a hydrogen atom because it's a tertiary carbon so that hydrogen atom and its two electrons will move over to the positively charged carbon. So let's look for the mechanism where that is going on. So let's look at answer choice. A we have our Carbocaine with that positive charge on that terminal carbon. We see in terms of our electron pushing, we see a hydrogen atom with its electrons going to that positively charged carbon resulting in a Carbocaine product where we now have a methyl group attached with no charge. And our positive charge is on the carbon that's in the cyclops ring. So choice A is going to be our correct mechanism for a 12 hydride shift which is what we'd expect here generating a tertiary carbo CAV ion. So choice A will be our correct answer. Let's look at our other answer. Choices briefly just to understand why they are not correct. When we look at Choice B, what is happening here? This is showing a hydride shift happening from not the adjacent carbon but two carbons away. So this is not adjacent, we wouldn't expect to see a shift happening here. So choice B incorrect choice C we have a hydride shift but something's wrong here. It shows electrons going from a positively charged carbon to the hydrogen to generate the correct Carbocaine is drawn here. So we have the correct product. Write that here. Correct product. So that might mislead you, you see that neighboring hydrogen, you see an arrow but the arrow is going the wrong way. So wrong electron pushing happening here. So that's why choice C is an incorrect mechanism. And then of course, choice D is incorrect because it says it's not expected to undergo rearrangement. But rearrangement is possible since we've got that tertiary carbon adjacent with a hydrogen available for a hydrate shift. So choice A is our answer here to is will we expect to see rearrangement? How will it happen? We do expect rearrangement and the mechanism involves a 12 hydride shift to create a tertiary Carbocaine. See you in the next video.

Which of the following carbocations would you expect to rearrange? If you expect rearrangement, draw the carbocation you expect to form and the mechanism by which it will form.
(e)

Key Concepts

Carbocation Stability

Carbocation Rearrangement

Mechanism of Rearrangement

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