Cyclic Hemiacetals - Video Tutorials & Practice Problems

Cyclic Hemiacetals Concept 1

In this video, we're gonna take a look at cyclic hem ays. Now, remember we're gonna say recall that an aldehyde ketone reacts with an alcohol to form a hem acetyl or hem ay depending on how you pronounce it. Now remember it's one mole of an alcohol reacting with an aldehyde ketone to create this structure. Here. We're gonna say that a cyclic meaning they're not in a ring heals are unstable and convert back to the reactants. If we take a look here, we have an aldehyde to begin with and we have our alcohol in essence, what occurs here is that we have our alcohol being used to create our hemal here. This or group is part of the alcohol that we had and just for simplicity, we can say that the H here add it here. Now, if we look what makes this a hem a hem ay is when we have what used to be a carbon carbon, which is, this carbon is now connected to an oh group and an or group are here being a carbon, right? So just remember that a hemio cy is a carbon that used to be a carbon carbon used to be double bonded to an oxygen. But now it is single bonded to an oh group and an or group, this is an a cyclic hemy, it's not in a ring, it's not stable. So if you notice our arrows, you'll see that you have a small arrow going forward, a smaller one going forward, meaning that we're not gonna make very much of this product. You have a much larger arrow moving backwards. That means that the reaction prefers to stay in its two reactive forms. This one and this one, it prefers to stay as an aldehyde in alcohol individually, right? So just remember for a non cyclic hemio, it's better to have it in its original form. This structure here is unstable later on, we'll see what's the difference between a a cyclic hemal and what's a cyclic hemy, right? So just keep this in mind for right now.

Cyclic Hemiacetals Concept 2

Now we're gonna take a look at cyclic heals here. We're going to say that cyclic hemio cys with five or six membered rings are stable and we're gonna say cyclo chemic CS are produced in intra molecular reactions. But what does that mean? Well, an intra molecular reaction is a reaction that takes place within a single molecule. So basically, it reacts with itself. If we take a look here, remember the whole premise of a hemy is that we're gonna have an aldehyde ketone reacting with one mole of an alcohol or to do this intra molecularly, we're gonna have the alde hyder ketone being part of the structure and the alcohol being somewhere else on the structure here, we would redraw it so that we can bring in the oh which will call position six and the carbon carbon closer together. So we started from the carbon carbon. So that's one and we counted to the oxygen, which is six. All I've done is I've kind of like uncoiled it and kind of like wrapped it around so that they come closer to each other. Now that they're in position with each other, they're gonna form a bond with each other. So you would form a connection between the two to make our new bond. But remember, oxygen ideally wants to make two bonds by making this new connection with the carbon, carbon, they'd be making three. So you, you would eliminate this each year. That's why it's no longer there. And then carbon itself only wants to make four bonds max here. It would no longer need to make this double bond to the oxygen. So it would disappear. So you could just have a single bond here with the oxygen. This age here is still present. We're just not showing it. And so it's still there. That oxygen that used to be make, used to make two bonds is not only making one bond. Remember, oxygen ideally wants to make two bonds. So what it does is it picks up an H. So that's how we get this oh group right here. Now, remember we talked about this in earlier chapters when we talked about heal. So if you want, you can go back and take a look at that under alt and ketones. If we look at the arrows, now, we can see that the bigger arrow is definitely pointing towards our product and the smaller arrow is pointing towards our initial material or initial reactants. This is telling me that the cyclic form is more stable. That's why it's preferred. So again, a cyclic hem acetyl is stable if it helps to make a five or six membered ring. Here, it's making a six membered ring, oxygen becomes part of that ring. And if it's not cyclic, then it's not very, not as stable. And we prefer to stay as our initial reactant. So just remember the difference, a cyclic acetal are not as stable. So they prefer to stay as reactants. Cyclic aces are way more stable. And if we can make a five or six membered ring, we'll stay in that form.

Cyclic Hemiacetals Example 1

In this example, it says highlight the hem acetyl functional group in the following molecules. So remember the hem acetyl group is basically shown as a carbon connected to an oh group and connected to an or group where R here represents a carbon. If we take a look here, let's just spot where the carbon is connected to an. Oh, here's a carbon connected to an oh here's a carbon connected to an oh let's look a little bit further. We know that that carbon is connected to an oh. So part of it is satisfied. Now we need to check to see is it also connected to an or group? Well, this carbon here is connected to an oxygen cool and then oxygen is connected to another carbon. So yes, this carbon here with the star would represent our hem acetyl group or hem ayl carbon. Let's look over here, here's our carbon connected to an oh, it is also connected to an oxygen. Is that oxygen connected to a carbon, which again means in our group. Yes, it is. So this would also be a hemi acetyl functional group or hem acetyl carbon. So here we've identified both positions with these stars. Again, remember a hemi acetyl is just a carbon connected to an oh and also an or group where R represents a carbon.

Does the following cyclization reaction show the correct product?

• A. Yes
• B. No