hydrogen attached to it, an ethyl group, and a plus The positively charged cation (we call it a carbocation) with three bonds has only six electrons in its valence shell. So, step six would be If we find unshared pairs we can say that the OH- is a Lewis base and a nucleophile. So, another thing you could do, to shift the equilibrium to the right, would be to increase the concentration of one of your reactants. So, over here on the right, is our acetal, and you can see the OR double prime, from our alcohol, and OR double prime, from our alcohol. The Mechanism of Acetal Formation. here, picked up a proton, and let's show these electrons use Toluenesulfonic acid, as our acid catalyst, and one of the things you could do is increase the concentration a) The first step in this process must be an acid-base proton transfer. in the next step, so this would be step six. So therefore, we need to make oxygen, more electrophilic. that would, once again, shift the equilibrium to the right, and form more of your acetal products. structure of your acetal, We can find this portion of the molecule, and think about adding that to our ring. Donate or volunteer today! So, another molecule of ethanol could come along and function as a base, and a lone pair of electrons on ethanol could take this proton, which leaves these electrons 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. Notice first that the "connectivity" of these two structures is identical. be our carbon EEL carbon, is going to be right here, and then, let's switch colors Recall the mechanism of acid-catalyzed addition of water. for the butanol molecule. That would tell us that an electron pair is ready to be used to make a new bond. The notation for this is to connect the two structures by a double headed arrow, which does not imply equilibrium. 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. We finish this mechanism by making the only bond which is left to do, the O-H bond. So, let's go ahead and write these out: so we had step one, An example is shown below. (The new product is called an acetal. It has a complete octet in its valence shell. It remains to break the bond between the hydrogen and the positively charged oxygen which produces the acetal and replaces the H+ to regenerate the catalyst. So, this is the dehydration portion, so we're gonna form water. Took this proton, and So, a molecule of ethanol comes along, functions as a nucleophile, a lone pair of electrons And so, this is a cyclic 4: Acetal Formation, Mechanism, Resonance, [ "article:topic", "showtoc:no", "license:arr" ], 3: Additions: Electrophilic and Nucleophilic, 5: Nitrogen Nucleophiles - Imine Formation. So, let's highlight some carbons here, so we can follow along. excellent leaving group, so, if these electrons 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.
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