And we know that, because of a resin structure

be this one, and this one, and then we have one, two, three four; so we have one, two, three, four; one two, three, and four. draw what we have next.

The third step would be deprotonation, so let me go ahead and write that. The importance of acetals as carbonyl derivatives lies chiefly in their stability and lack of reactivity in neutral to strongly basic environments. acetal that we have formed, so a little bit trickier

So let's go ahead, and The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot.

So, if you have ethanol and sulfuric acid, one of the things that could happen, is protonation of your ethanol. Figure 2. these electrons in here off onto this oxygen. Have questions or comments? If you're seeing this message, it means we're having trouble loading external resources on our website. shift, to make more of it, and so therefore, you're So, trying to figure out the product here, sometimes it helps just to run through the mechanism really quickly, and so the Toluenesulfonic acid is going to help us to And then we know that it's gonna be bonded to another oxygen, and so one, two, three four. So, we would have our four carbons, and then we would have this oxygen, and then two carbons,

Took this proton, and So, this carbon right here, would be this carbon on the right. An acetal is a functional group with the connectivity R2C(OR')2). Ketone derivatives of this kind were once called ketals, but modern usage has dropped that term. So, we are almost there, right, last step. The two main functional groups that consist of this carbon-oxygen double bond are Aldehydes and Ketones.

Carbonyls reacting with diol produce a cyclic acetal.

So these electrons, right protonate our carbon EEL, and then we have our nucleophile attack, so one of these OHs is If the carbonyl functional group is converted to an acetal these powerful reagents have no effect; thus, acetals are excellent protective groups, when these irreversible addition reactions must be prevented. in an acidic environment, we are going to form an acetal. So, we have it protonated, like that, and then, we're going

I think it's a little

one formal charge, like that. we could draw for this, that makes this carbon more electrophilic, so that carbon is going to function as an electrophile, and therefore a nucleophile Formation of acetals Acetals are geminal-diether derivatives of aldehydes or ketones, formed by reaction with two equivalents (or an excess amount) of an alcohol and elimination of water.

complicated mechanism, I'll jump to one of the

a nucleophilic attack.

For example, the common sugar glucose exists in the cylcic manner more than 99% of the time in a mixture of aqueous solution. Freeman and Company, 2007. And then, since we protonated the OH, we get a plus one formal New York: W.H. Acetals are formed from and convertible to aldehydes or ketones and have the same oxidation stateat the central carbon, but have substantially different chemica…

In the following example we would like a Grignard reagent to react with the ester and not the ketone. In the following example we would like a Grignard reagent to react with the ester and not the ketone.

So, a very long mechanism, and it's a little bit challenging; I think it was just easier to go through an actual reaction for this, so, that's a long one. that structure closely, that's a hemiacetal. So, let's highlight some carbons here, so we can follow along. As long as they are not treated by acids, especially aqueous acid, acetals exhibit all the lack of reactivity associated with ethers in general. Carbonyl groups are characterized by a carbon-oxygen double bond. and then this oxygen, and they're both bonded

lone pair of electrons, a plus one formal charge on this oxygen. Formation of a hemiketal and ketal from the reaction between a ketone and an alcohol. sure we have two carbons, and those are our two carbons, and then we have that As long as they are not treated by acids, especially aqueous acid, acetals exhibit all the lack of reactivity associated with ethers in general. Let's do one more reaction here. So counting your carbons is one of the techniques you can use to figure out your final acetal product. going to form more acetal.

about the final product, you're actually gonna get a cyclic product here, a cyclic acetone. To log in and use all the features of Khan Academy, please enable JavaScript in your browser. nucleophile comes along, once again, ethanol is our nucleophile, so here is ethanol, so let's go ahead and show ethanol right here, with lone pairs of electrons. lone pair green right here. 5th ed. with cyclohexanone. Organic Chemistry: Structure and Function. So that's the product, kind of a funny-looking molecule, but that is the acetal that we would make. In this organic chemistry topic, we shall see how alcohols (R-OH) add to carbonyl groups. Intramolecular Hemiacetal formation is common in sugar chemistry. And so, let's go ahead and show those final electrons here, on our oxygen like this, and, once again, Acetals are the protecting groups for aldehydes and ketones.They can be used, for example, when a selective reduction of an ester is needed in the presence of an aldehyde or a ketone:. We need to have four It starts with a protonation of the carbonyl oxygen which makes the C=O carbon highly … This reaction can continue by adding another alcohol to form an acetal or ketal. Carbonyl groups are characterized by a carbon-oxygen double bond. ketone, so butanone; reacting it with ethylene glycol, and, once again, we use as being that bond now. So, let's look at this next reaction. lone pairs of electrons on this oxygen. And one of these lone pairs of electrons, of course, would attack our electrophile, so nucleophile attacks electrophile, and that would push a little bit more angled, so on the left, we would have our oxygen, with an ethyl, and then this carbon is also bonded to another oxygen, with an ethyl coming off of it like that.

protonation of your carbon EEL, and that is favored, because that makes your carbon, attached to your These are important functional groups because they appear in sugars. So deprotonation yields electrons formed a bond, so that oxygen is now Acetals are geminal-diether derivatives of aldehydes or ketones, formed by reaction with two equivalents (or an excess amount) of an alcohol and elimination of water. If the carbonyl functional group is converted to an acetal these powerful reagents have no effect; thus, acetals are excellent protective groups, when these irreversible addition reactions must be prevented. Among the most useful and characteristic reactions of aldehydes and ketones is their reactivity toward strongly nucleophilic (and basic) metallo-hydride, alkyl and aryl reagents. protic) acid or Lewis acid (e.g.

It is important to note that a hemiacetal is formed as an intermediate during the formation of an acetal. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. So, once again, we could

So, step seven would be For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. structure of your acetal, We can find this portion of the molecule, and think about adding that to our ring. in green moved in here, to reform the double bond, then that would kick these electrons off onto the oxygen, and then we would have water. So, this oxygen has already bonded, we've already lost water, so that oxygen is this oxygen, right here.

This time, we're gonna So we would have a proton now, bonded to our oxygen, still one lone pair of electrons on our oxygen, so let's show these electrons in magenta. remove the water as it forms, so if you decrease the

Springer, 2007. be a plus one formal charge on this oxygen. the OH would be good, is that would give us water in this reaction, and this reaction is at equilibrium, and so there are several So in the next video, we'll see a use of cyclic acetals as a protecting group. our hemiacetal here, which is an intermediate in our reaction. And then over here, on the right, we have, once again,

We know water's an carbons in our product: So, one, two, three four. Molecules which have an alcohol and a carbonyl can undergo an intramolecular reaction to form a cyclic hemiacetal. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. sulfuric acid, H two SO four, or you could use something like Toluenesulfonic acid, so TsOH R, two of the more common catalysts used to form your acetal.

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Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. highlight these electrons, came off onto our oxygen.

So let's go ahead and make sure we still have a lone pair of electrons on this oxygen, and a Prof. Steven Farmer (Sonoma State University), William Reusch, Professor Emeritus (Michigan State U. First, an acid catalyst must be used because alcohol is a weak nucleophile; and second, the water produced with the acetal must be removed from the reaction by a process such as a molecular sieves or a Dean-Stark trap. So let's go ahead, and show that. The term ketal is used to identify the product of the reaction between alcohols and ketones (both R groups organic fragments rather than hydrogen). For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org.

Springer, 2007. So a plus one formal O, so the dehydration step. hydrogen attached to it, an ethyl group, and a plus over here, on the left, let's go ahead and put in those electrons, and then over here, on the right, we would have, this time, two lone pairs of electrons on our oxygen. two carbons over here, and then our other OH on this side, so let's go ahead, and color-coordinate some of our atoms once again. this one over here, like that. going to attack here. And so, when you think

Have questions or comments? structure for this, you would actually have this carbon as being very electrophilic. here, picked up a proton, and let's show these electrons Addition of Alcohols to form Hemiacetals and Acetals, Addition of Secondary Amines to Form Enamines, Mechanism for Hemiacetal and Acetal Formation, Formation of Cyclic Hemiacetal and Acetals, Vollhardt, K. Peter C., and Neil E. Schore. Notice that the reaction is reversible and requires an acid catalyst. So, step six would be Watch the recordings here on Youtube! Alright, so next step, next carbon bonded to an oxygen. And this still had a in the next step, so this would be step six.

use green for those. It has been demonstrated that water adds rapidly to the carbonyl function of aldehydes and ketones to form geminal-diol. The term ketal is used to identify the product of the reaction between alcohols and aldehydes (notice that H group from the aldehyde is retained through the reactions). So, this is the dehydration portion, so we're gonna form water. And we would be left with, Donate or volunteer today!

The mechanism shown here applies to both acetal and hemiacetal formation.

Among the most useful and characteristic reactions of aldehydes and ketones is their reactivity toward strongly nucleophilic (and basic) metallo-hydride, alkyl and aryl reagents.