Let's look into the possibility that the C=O pi bond breaks as the C-O sigma bond forms. have a molecule of ethanol come along, and function as a base, and so, a lone pair of electrons take this proton, leaving We finish this mechanism by making the only bond which is left to do, the O-H bond. And so, one of the possibilities would be a protonated ethanol over here, functioning as an acid, so let's go ahead, and draw that. From that we can decide whether a such an atom is an electrophile -- ready to accept electrons to make a bond -- or not. After all, the electrons in the pi bond are farther from the nuclei than those in a sigma bond, so they should be easier to push around. Aldehyde to acetal conversion On the right, that pair is shown as making a pi bond.
So here we have cyclohexanone, and a lone pair of This time, we're gonna The notation for this is to connect the two structures by a double headed arrow, which does not imply equilibrium. If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked. of that oxygen like this. That bond could just as well be a C-O bond as is the situation in an alcohol. The symbols differ, but there is only one molecule being described. bond to this oxygen, with an ethyl coming off You can use something like 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. And then that would give us So let's go ahead and make sure we still have a lone pair of electrons on this oxygen, and a This makes its reaction with an alcohol rather than RO- (called an alkoxide ion) very likely. What would happen if we just replaced water by methanol in the mechanism for acid-catalyzed hydration?
going to form more acetal. The structure presented will be the one which most directly connects to the reaction being described. The charge alone does not tell us what to expect. Another way to say this is that the functional group of water is the same as the functional group of any alcohol, an OH group. So, step seven would be If we look closely at the differences between these two products, we see that to do this, we need to replace the OH group of the hemiacetal with the OCH3 group of the acetal. step here is protonations; let me go ahead, and mark Then we have the question of where that bond will go. second-step, nucleophilic attack. Last time I left you with a problem, "what is the mechanism for the base catalyzed addition of water to a carbonyl group?"
So several things that you can do, in the lab, to increase your yield. And we know that, because of a resin structure a nucleophilic attack.
In the carbonyl group, we know that the carbon is the more positive end of the C=O dipole, so let's try to make our new bond there. This is done in an acidic environment, and so there are a couple different proton sources you can use. And so, once again, let's highlight some of those carbons: so this carbon right here, and this carbon right here, or this carbon, and this carbon, and, in our final product, like that. So counting your carbons is one of the techniques you can use to figure out your final acetal product. So, another thing you could do, to shift the equilibrium to the right, would be to increase the concentration of one of your reactants. The positively charged oxygen (we call it an oxonium ion) has three bonds and an unshared pair. structure for this, you would actually have this carbon as being very electrophilic. It does provide a means of determining whether a positively charged atom has a vancancy or not. Recall that we began thinking about acid catalyzed reactions of aldehydes by using a C-O pi bond to supply the electrons to make a new bond with H+. So in the next video, we'll see a use of cyclic acetals as a protecting group. This does not happen because the reaction is occuring in an acidic solution. That is, each atom is connected to exactly the same atoms in one structure as it is in the other. Another way to say this is that the pi bond is weaker than the sigma bond, so it takes less energy to break it. So, you could increase the concentration of an aldehyde, and then Kirk McMichael (Washington State University). electrons formed a bond, so that oxygen is now And this one's a little bit different, because we can see we have a diol, as one of our reactants; up here, we just had butanol, only one OH, but this one has two on it. going to get a nucleophile attacking our electrophile So, this would be a ketone, so we have a four-carbon And we just formed a bond between the oxygen on our Our mission is to provide a free, world-class education to anyone, anywhere.
So, we have it protonated, like that, and then, we're going to this carbon right here. O, so the dehydration step. When you have made a correct selection, an equation showing the reaction for that step will appear, and a new question will be posed. highlight these electrons, came off onto our oxygen. draw what we have next. So, let's go ahead and show that. concentration of this product, your equilibrium is going to Breaking old bonds is usually assisted by the formation of new bonds. So we would have, let's go ahead and make this Acetal derivatives of aldehydes and ketones are prepared by an acid-catalyzed dehydration reaction with alcohols or diols. It is not an electrophile. 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. So in step seven here, all we have to do is take that proton off, and we would form our acetal product. If we find unshared pairs we can say that the OH- is a Lewis base and a nucleophile. The central atom is consequently an electrophile, open to making a bond with a nucleophile. Charge and Reactivity: Let's contrast two positively charged molecules we found in this mechanism, one with three bonds to oxygen and one with three bonds to carbon. let's make 'em blue here. And then we have these The above mechanism ( Figure 2 ), showing the equilibrium between an Aldehyde and an Acetal, illustrates all of the steps necessary for the interconversion to take place. Let's do two quick problems, to think about the acetal product here. carbon bonded to an oxygen. Just to have the whole process in one place, here's the full mechanism from the aldehyde through the hemiacetal to the acetal: There are a couple of general ideas we can extract from what we did in working out this mechanism. We want to use our mechanism to predict the structure of the product. So here, we have acetaldehyde, and then here we have butanol. And then, we still have another OH on this molecule, and that's And then we know that it's gonna be bonded to another oxygen, and so one, two, three four.
Watch the recordings here on Youtube! And so we have our ring here, and then we would have our oxygen, and then our R group, and then our oxygen, and then our R group like that. attacks our electrophile, kicks these pi electrons I think it's a little Let's do one more reaction here. here, picked up a proton, and let's show these electrons
There is an alternative. So, these electrons are going to attack this carbon, and kick these electrons off, onto this oxygen. So, a molecule of ethanol comes along, functions as a nucleophile, a lone pair of electrons And so when a nucleophile attacks, we would have, this oxygen over here, would now have two lone Writing a mechanism for this reaction provides a good test of ones' understanding of acid-catalyzed processes. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. for the butanol molecule. And, once again, we have a plus one formal with cyclohexanone. off, onto this oxygen: so, that would be the The nucleophile is the oxygen of another molecule of methanol, whose unshared electron pair becomes the new carbon oxygen bond. that would, once again, shift the equilibrium to the right, and form more of your acetal products. OH over here, on the left. This carbon, that used to The third step would be deprotonation, so let me go ahead and write that. The OH group in methanol is a nucleophile just like the OH group in water. So a plus one formal So we have cyclohexanone reacting with an excess of ethanol, and using sulfuric acid as our catalyst, and so just looking at this general pattern up here, for predicting the later steps of the mechanism, where we have already lost water, so minus H two O, so we've already gotten past the dehydration step. These electrons right in here moved off, onto our oxygen, and so, if you look at an aldehyde, or a ketone, with an excess of alcohol, So we protonate the OH, and the reason why protonating lone pair green right here. the next step, right? This produces water and leaves the central carbon atom with only three bonds and a vacancy in its valence shell. Khan Academy is a 501(c)(3) nonprofit organization. pairs of electrons around it, so let's show those, so Voiceover: If we react So when we get to this step, we're actually gonna get an intra-molecular, nucleophilic attack.
If we learn a reaction for one alcohol, it will work very much the same way for any other alcohol - often including water as the smallest possible alcohol. Mechanism of Acid-catalyzed Acetal Formation. And so, this is a cyclic Indeed, the same mechanism seems to work just fine. It makes a fourth bond using electrons from a nucleophile (the methanol oxygen atom here). Its reaction is to break a bond, keeping the electrons, by dropping off an H+. So, let's think about a mechanism for this reaction. 4: Acetal Formation, Mechanism, Resonance, [ "article:topic", "showtoc:no", "license:arr" ], 3: Additions: Electrophilic and Nucleophilic, 5: Nitrogen Nucleophiles - Imine Formation. shift, to make more of it, and so therefore, you're And so, when you think Acetal Formation. That bond doesn't break. And so, let's go ahead and show those final electrons here, on our oxygen like this, and, once again, So, step six would be our hemiacetal here, which is an intermediate in our reaction. the OH would be good, is that would give us So, let's once gain show those electrons; let's use magenta again. Figure 2: Mechanism of Acetal Formation. Now let's use what we know about the acid catalyzed addition of water to make a prediction of what will happen when we mix an aldehyde with an alcohol and add a drop or two of an acid catalyst.