Cyclic Hemiacetals - Online Tutor, Practice Problems & Exam Prep

Cyclic hemiacetals, formed from aldehydes or ketones reacting with alcohols, are stable in 5 or 6 membered rings due to intramolecular reactions. In these reactions, the carbonyl carbon and hydroxyl group come closer, allowing bond formation. This stability contrasts with acyclic hemiacetals, which are unstable and revert to reactants. The equilibrium favors cyclic forms, making them more prevalent in chemical reactions. Understanding these structures is crucial in organic chemistry, particularly in the context of carbonyl compounds and their transformations.

Cyclic Hemiacetals Concept 1

In this video, we're going to take a look at cyclic hemiacetals. Now remember, we're going to say, recall that an aldehyde or ketone reacts with an alcohol to form a hemiacetal or hemiketal, depending on how you pronounce it. Now, remember it's 1 mole of an alcohol reacting with an aldehyde or ketone to create this structure. Here we're gonna say that acyclic, meaning they're not in a ring, hemiacetals 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 hemiacetal. Here, this OR group is part of the alcohol that we had. And just for simplicity, we can say that the h here, added here. Now, if we look, what makes this a hemiacetal? A hemiacetal is when we have what used to be a carbonyl carbon, which is this carbon, is now connected to an OH group and an OR group, r here being a carbon. Right? So just remember that. A hemiacetal is a carbon that used to be a carbonyl 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 acyclic hemiacetal, 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 backward. That means that the reaction prefers to stay in its 2 reactant forms, this one and this one. It prefers to stay as an aldehyde and alcohol individually. Right? So just remember, for a noncyclic hemiacetal, 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 an acyclic hemiacetal and what's a cyclic hemiacetal. Alright, so just keep this in mind for right now.

Cyclic Hemiacetals Concept 2

Now we're going to take a look at Cyclic Hemiacetals. Here we're going to say that Cyclic Hemiacetals with 5 or 6 membered rings are stable. And we're going to say cyclic hemiacetals are produced in intramolecular reactions. But what does that mean? Well, an intramolecular 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 hemiacetal is that we're going to have an aldehyde or ketone reacting with 1 mole of an alcohol. For it to do this intramolecularly, we're going to have the aldehyde or ketone being part of the structure, and the alcohol being somewhere else on the structure. Here we will redraw it so that we can bring in the OH, which we'll call position 6, and the carbonyl carbon closer together. So we start from the carbonyl carbon, so that's 1, and we count it to the oxygen which is 6. All I've done is I've kind of uncoiled it and wrapped it around so that they come closer to each other. Now that they're in position with each other, they're going to form a bond with each other. So you would form a connection between the 2 to make our new bond, but remember oxygen ideally wants to make 2 bonds. By making this new connection with the carbonyl carbon, it would be making 3. So you would eliminate this H here, that's why it's no longer there. And then carbon itself only wants to make 4 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 H here is still present, we're just not showing it. So it's still there. That oxygen that used to make 2 bonds is now only making one bond, remember oxygen ideally wants to make 2 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've talked about this in earlier chapters when we talked about hemiacetals, so if you want, you can go back and take a look at that under aldehydes 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 hemiacetal is stable if it helps to make a 5 or 6 membered ring. Here it's making a 6 membered ring, oxygen becomes part of that ring. And if it's not cyclic, then it's not very stable, and we'd prefer to stay as our initial reactant. So just remember the difference. Acyclic hemiacetals are not as stable, so they prefer to stay as reactants. Cyclic hemiacetals are way more stable, and if we can make a 5 or 6 membered ring we'll stay in that form.

Cyclic Hemiacetals Example 1

In this example, it states, "Highlight the hemiacetal functional group in the following molecules." Remember, the hemiacetal group is essentially depicted as a carbon connected to an OH group and connected to an OR group, where R here represents a carbon. Let's locate where the carbon is connected to an OH. Here's a carbon connected to an OH. Here's another one connected to an OH. Let's investigate further. We know that this carbon is connected to an OH, so part of the requirement is satisfied. Now we need to check to see if it is also connected to an OR group. Well, this carbon here is connected to an oxygen—cool, and this oxygen is linked to another carbon. So yes, this carbon here, marked with the star, represents our hemiacetal group, our hemiacetal carbon.

Looking at another example, here's our carbon connected to an OH; it is also connected to an oxygen. Is that oxygen linked to a carbon? Which means an R group. Yes, it is. So, this would also be a hemiacetal functional group or hemiacetal carbon. Here, we have identified both positions with these stars. Again, remember, a hemiacetal 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

Here’s what students ask on this topic:

What is a cyclic hemiacetal and how is it formed?

A cyclic hemiacetal is a stable compound formed when an aldehyde or ketone reacts with an alcohol within the same molecule, resulting in a ring structure. This intramolecular reaction brings the carbonyl carbon and hydroxyl group closer together, allowing them to form a bond. Typically, cyclic hemiacetals are stable when they form 5 or 6 membered rings. The stability arises because the ring structure minimizes strain and allows for favorable interactions between atoms. This contrasts with acyclic hemiacetals, which are unstable and tend to revert to their reactants.

Why are cyclic hemiacetals more stable than acyclic hemiacetals?

Cyclic hemiacetals are more stable than acyclic hemiacetals primarily due to the formation of 5 or 6 membered rings, which reduce strain and allow for favorable atomic interactions. In these rings, the carbonyl carbon and hydroxyl group are positioned optimally to form a stable bond. This intramolecular reaction is energetically favorable, leading to a stable product. In contrast, acyclic hemiacetals do not benefit from this ring stability and are prone to reverting to their original aldehyde or ketone and alcohol forms.

What is the difference between a cyclic and an acyclic hemiacetal?

The primary difference between cyclic and acyclic hemiacetals lies in their stability and structure. Cyclic hemiacetals form stable 5 or 6 membered rings through intramolecular reactions, where the carbonyl carbon and hydroxyl group within the same molecule react. This ring structure is stable and preferred. Acyclic hemiacetals, on the other hand, do not form rings and are unstable. They tend to revert to their original aldehyde or ketone and alcohol forms, as the open-chain structure does not provide the same stability as the ring structure.

How does the formation of cyclic hemiacetals relate to intramolecular reactions?

The formation of cyclic hemiacetals is a result of intramolecular reactions, where the reacting groups (carbonyl carbon and hydroxyl group) are within the same molecule. This proximity allows them to form a bond, creating a ring structure. Intramolecular reactions are energetically favorable because they reduce the distance between reacting groups, leading to a more stable product. In the case of cyclic hemiacetals, this results in the formation of stable 5 or 6 membered rings, which are preferred over their acyclic counterparts.

What role do 5 or 6 membered rings play in the stability of cyclic hemiacetals?

5 or 6 membered rings play a crucial role in the stability of cyclic hemiacetals. These ring sizes are optimal because they minimize ring strain and allow for favorable interactions between atoms. The angles and distances in these rings are close to ideal, reducing the overall energy of the molecule. This stability makes cyclic hemiacetals more prevalent and favored in chemical reactions compared to acyclic hemiacetals, which do not benefit from the same structural advantages and are therefore less stable.