Acetal: Study with Video Lessons, Practice Problems & Examples

Acetals form when carbonyls react with alcohols, remaining stable in basic conditions but easily hydrolyzed back to carbonyls in acidic environments. The mechanism involves an acid-catalyzed process, starting with protonation, followed by nucleophilic addition, and concluding with deprotonation. To create cyclic acetals, diols are necessary. Understanding this mechanism is crucial, as it is frequently tested in organic chemistry, emphasizing the importance of recognizing the roles of hemiacetals and the choice of protonation to drive the reaction forward.

General Mechanism

In this page, we're going to discuss another product that forms when carbonyls react with alcohols, and that's called acetals. In general, I'm just going to say a few facts, and then we're going to go straight into the mechanism. The first fact I want you to know is that acetals, once formed, are actually stable in base. If you want to keep an acetal for a long time, keep it in a neutral to basic solution. However, they're easily hydrolyzed back to carbonyls using acid. That makes sense, guys, because remember that this is a reversible reaction. It's acid catalyzed. It makes sense that if you use acid, you're going to go back to the carbonyl and it's going to be in equilibrium. If you want to specifically make an acetal that not only has ROR but is actually cyclic, meaning it forms a ring, then you're going to have to use a diol because a diol is going to have carbons in the middle that are going to link together. For example, the diol that I would need here would be 1,2-ethanediol

Provide the chemical steps necessary for the following synthesis.

Hey guys, let's take a look at the following question. Here it says, Provide the chemical steps necessary for the following synthesis. What do we have here? We're starting out with a ketone. And to this ketone, what am I changing it into? Well, what we should notice here is that the carbonyl carbon is no longer a carbonyl carbon. And instead, what does it have? It has 2 OR groups. So we say to ourselves, what do I call a carbon that has 2 OR groups? We call that an acetyl group. Or in this case, because it came from a ketone, we call it a keto group. So then we say to ourselves, "Okay, how do I change a ketone into a keto group?" And remember, going back to the earlier videos, all we have to say is to change this ketone into that group there, we'd have to use 2 moles of that alcohol formed over your acid catalyst. You can use H+ or H3O+ in the same thing. Then what would happen here? We get our 2 OR groups getting added. We go from a ketone to a keto group. Again, the ketosynthesis is figuring out what's your functional group at the end. Usually, there are 2, at most, 3 different ways to get there. We figured out that it was a keto and we know the only way to get to a keto is to use 2 moles of an alcohol with an acid catalyst on the ketone. And in that way, we got our final answer.

Provide the chemical steps necessary for the following synthesis.

Hey, guys. Let's take a look at the following question. So here it says provide the chemical steps necessary for the following synthesis. So, what we're beginning with here is a ketone and what we're ending with is a carbon with this cap that we have on it. Now, remember, if you're observing, we say that this is called a ketal cap. How do we make these caps? Remember, in order to make a ketal cap, I have to use a diol. What does this diol look like? Well, the diol had this portion here connected to it. So, just make that the diol. And don't forget your acid catalyst on the bottom because this requires an acid catalyst in order to form the cap. So what we get here now is our final answer. That is the key to this question. Knowing what our functional group is at the end is key to knowing what reagents we need to get there. Again, there are only a handful of ways to get to certain types of functional groups. And in this case, to get a ketal cap, we have to use a diol with an acid catalyst.

Determine the starting materials based on the acetal group present.

Hey guys. Let's take a look at the following question. So here it says, Determine the starting materials based on the acetyl group present. Now technically this is a keto group. But acetyl group, keto group, very much similar. Now, to answer a question like this, it's a retrosynthesis question. We're going backwards. We have the product. We have to determine the starting material. To answer a question like this, you look for the carbon that is connected to both OR groups. That carbon there is the only one attached to 2 OR groups. Here is an oxygen with a carbon, so that's an OR group and it's connected to this OR group. Now, here all we've got to do is first, locate the carbon connected to 2 OR groups. 2. What you're going to do here is you're going to cut the carbon-oxygen bonds. So we're going to cut here and cut here. 3, you're going to add an H to each oxygen. Then finally, you're going to add a double bond O to the carbon. If you notice, we're adding hydrogens to each o

Here’s what students ask on this topic:

What is the mechanism for acetal formation?

The mechanism for acetal formation involves several steps, starting with the protonation of the carbonyl oxygen, making the carbonyl carbon more electrophilic. This is followed by nucleophilic addition of an alcohol to form a hemiacetal. The hemiacetal then undergoes protonation again, leading to the formation of a good leaving group (water), which departs to form a carbocation. Another molecule of alcohol then attacks the carbocation, followed by deprotonation to yield the final acetal. The overall process is acid-catalyzed and involves protonation, nucleophilic addition, formation of a leaving group, and deprotonation.

Why are acetals stable in basic conditions but not in acidic conditions?

Acetals are stable in basic conditions because bases do not provide the necessary protons to initiate the reverse reaction back to carbonyls. In contrast, acidic conditions provide protons that can protonate the acetal, making it susceptible to hydrolysis. The protonation of the acetal oxygen creates a good leaving group (water), which facilitates the breakdown of the acetal back to the original carbonyl compound. This reversibility in acidic conditions is due to the equilibrium nature of the acetal formation reaction.

What is the difference between a hemiacetal and an acetal?

A hemiacetal contains one -OH group and one -OR group attached to the same carbon, whereas an acetal has two -OR groups attached to the same carbon. Hemiacetals are intermediates in the formation of acetals and can be formed under both acidic and basic conditions. However, to convert a hemiacetal to an acetal, an acid-catalyzed mechanism is required. The formation of an acetal involves the loss of water and the addition of a second alcohol molecule.

How do you form a cyclic acetal?

To form a cyclic acetal, a diol (a molecule with two hydroxyl groups) is used. The diol reacts with a carbonyl compound in the presence of an acid catalyst. The first hydroxyl group of the diol forms a hemiacetal with the carbonyl, and then the second hydroxyl group reacts to form the acetal, resulting in a ring structure. For example, using 1,2-ethanediol with a carbonyl compound will form a five-membered cyclic acetal.

What role does protonation play in the formation of acetals?

Protonation plays a crucial role in the formation of acetals by increasing the electrophilicity of the carbonyl carbon, making it more susceptible to nucleophilic attack by an alcohol. The initial protonation of the carbonyl oxygen facilitates the formation of a hemiacetal. Subsequent protonation of the hemiacetal's hydroxyl group creates a good leaving group (water), which departs to form a carbocation. This carbocation is then attacked by another alcohol molecule, followed by deprotonation to yield the final acetal.