Acid-catalyzed halogenation is synthetically useful for conver...

Question

Acid-catalyzed halogenation is synthetically useful for converting ketones to α,β-unsaturated ketones, which are useful in Michael reactions (Section 22-18). Propose a method for converting cyclohexanone to cyclohex-2-en-1-one, an important synthetic starting material.

Accepted Answer

Hey everyone and welcome back in today's video, we are going to solve this problem. Alpha beta unsaturated ketones are useful in michael reactions. These compounds can be produced using the acid catalyzed hydrogenation of ketones, propose a method to convert Penton three own to plant one in three own a useful starting material. Now, as the problem suggests, we are starting with a ketone right on the left hand side, we have our key zone because we have that carbonell group with the alcohol substitutes and we are producing an alpha beta unsaturated key zone. Now, the reason why this is an alpha beta unsaturated ketone is because if you look at the carbondale group, the adjacent carbon atom over here is called an alpha carbon and the next one is called the beta one. So we notice that there is in saturation. So it's the presence of that double bond. We essentially want to keep in mind that this would be an acid catalyzed hydrogenation. So let's see how such a reaction works now in the world. First stop, we are going to draw our key tone. So let's redraw it. And now let's understand that if this is an acid catalyzed hydrogenation, the world first step is the protein ation of the carbon steel group. Right? So that we can actually make the carbon here more electra filic. Now recall that oxygen house lone pairs, we are going to protein eight hour oxygen with an asset. But in general for this reaction, it's sufficient to use a weak acid such as acetic acid. So now if we use acetic acid, we can essentially write the expression where the formula for acetic acid, that will be our C three c 00. H. So let's expand that oxygen hydrogen bond. And now the first step is the protein nation step as we previously assad. So we're going to show that the lone parent oxygen gets protein aided by this hygiene. And then we have the loss of the acid ate an ion. So in this step we are producing our protein ated intermediate. So let's redraw our chain and we understand that oxygen is now protein ated so that it has a formal positive charge. Alright, now, what's going to happen from here is that the base or the acetate and I'm produced can easily attack the alpha position. Right? Because as we previously said, the protein nations that allowed us to produce a more electro Felix center here and therefore because it's more electro filic, the adjacent hodgins become more acidic and more easily removable. Now, it doesn't really matter which one you think about because as you can see the intermediate symmetrical. So let's suppose that we're looking at this hydrogen bonded to the alpha position and now the acidity and anna and produced in the first stop will act as a base. So that would be our acetate which now essentially attacks the proton and after we attack it, we are going to transfer these electrons into a newly formed pi bond afterwards. You may also notice that if we are forming a pi bond here, we have to break the adjacent pi bonds, we're transferring these electrons onto oxygen in the form of a lone pair. Now in the step, we're going to get our email. So let's see how that looks like let's redraw our carbon chain. Now we understand that we have a single bond here, This is our alcohol group. Now, here is where we get our pi bon. Okay, perfect. So we got our email and we understand that we also reformed acidic asset. And then the next step because we have an email, we are ready to think about the nuclear attack on an electro file. Keep in mind that the problem suggested that we can use halogen ation right now. The question is which kind of halogen can we use? It doesn't really matter, right? We will be thinking about elimination leader to make sure that we produce that double bond. So we can use either bro mean chlorine or iodine. Let's suppose that for the purpose of this question, we are going to use chlorine. So we can essentially draw a chlorine chlorine Bon here, rights, we have cl two as our material. The lone pair on oxygen essentially will restore the original double bond, right, This is really favorable because those are not perfectly stable. And now the pi bond here performs the nuclear attack at the electra filic chlorine. Now, the reason why it's actually electra filic is that because the pi electrons polarized at chlorine chlorine bonds. So the left one becomes partially positive and the right one becomes partially negative. When chlorine is attacked we have the loss of the leaving group. So that would be our chloride and I. And and we may notice that if we draw the next intermediate here we get the following structure. So that would be our carbonell group. It's still protein ated a former positive charge on oxygen and notice how we have that addition of chlorine at alpha position. So we have chlorine over here. Okay, well done. And now the last step is the deep throaty nation step. Right. We want to get rid of this Haijun notice how we have formed that chloride and I on. So we can simply say that the chloride and iron previously formed will act as a base And that will the protein eight as hygiene warming our carbon compound. So now let's draw the resultant structure at this point. That would be our carbonell group and we have chlorine in this position. Okay, perfect. So now we understand that the only thing we want to perform is basically at this double bond that we have in our final structure. Now this would be basic elimination reaction. Right recall that we want to identify the leaving group that's chlorine and the only position that contains beta hydrants is this position. So if we expand one of the hydrogen atoms, we have three total. Let's show one of them we need to use a strong base right? We call that an E. Two reaction, requires us to use a strong base such as hydroxide and our hydroxide essentially attacks that hydrogen. We are forming a pi bond followed by the loss of the leaving group. So that's our E two elimination. Now we may simply draw the final product. You may already notice that we indeed get our desired product. So that will be our carbon chain, our carbonell group. And then we got our alpha beta unsaturated zone. That's the desire double bond that we needed to get. Okay, so now that we see the logic and rationale behind this problem, let's just summarize our answers. So to summarize the whole synthesis we can essentially illustrate it as follows. We essentially started with our key zone and then the world first stop. We said that we needed to add chlorine in the presence of acidic asset which corresponds to our asset, capitalized, halogen ation. Okay, so this is our first step and now we formed our halogen aged kids only said that here we may add chlorine at the alpha position. Now when the hell originated ketone is formed We are done performing a basic elimination E two using our strong base. Let's suppose that for an un spectator we choose but as cm. Right Because we said we need to use hydroxide. So let's suppose that we're using potassium hydroxide and of course we need to make sure that we are using heat for this reaction because heat promotes elimination. And we previously said that the only position that has beta hydrogen with respect to the leaving group is this position. So this would be the only product, our final product, which is the alfa, beta unsaturated ketone. So we can just draw it out and we are nearly done right we essentially got are found product. This is the synthesis scheme. And looking at this whole sentences, we can conclude that the correct answer choice. This multiple choice question is A. D. However, if you have any questions, don't hesitate to leave a comment down below and thank you for watching.

Acid-catalyzed halogenation is synthetically useful for converting ketones to α,β-unsaturated ketones, which are useful in Michael reactions (Section 22-18). Propose a method for converting cyclohexanone to cyclohex-2-en-1-one, an important synthetic starting material.

Key Concepts

Acid-Catalyzed Halogenation

α,β-Unsaturated Ketones

Michael Reaction

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