Another disclaimer before we begin. The mechanism that I'm about to show you contains a resonance structure as you can see here. We're going to fill this one out. I like to add resonance structures into my mechanisms because I think it makes it more clear where the arrows are being moved to. The only thing is that your professor may not use a resonance structure. Your professor just might decide to push the arrows without it. If that happens, it's okay. You can draw it his way, her way. You can draw it my way. It doesn't matter because in the end of the day, as long as the arrows are going to the same places, it doesn't matter if you add a resonance structure or not. This is going to be true with a lot of the mechanisms in clutch prep. I go out of my way to try to make the mechanisms extra clear and your professor might not really be explaining every arrow. Feel free to draw it my way even if it's slightly different than the way your professor drew it. Just know that the arrows are being pushed the same way in the end, so it's okay that you draw my mechanism which should be equivalent to the one your professor is drawing.
That being said, why don't you help me out with what the first step of this mechanism is? Since it's acid catalyzed, what's the very first thing we should be doing? Protonation. The very first thing we're going to do in an acid catalyzed mechanism is protonate. Notice that my acid in this case is a protonated version of alcohol. What I'm essentially using is ROH2+. You could have used any acid source. It doesn't have to be that. You could have used H+. You could have used H3O+. I'm just doing it like this because then the conjugate is going to make more sense for you guys, the conjugate base. But you could use any other acid source. What that's going to make is a resonance structure because I'm going to get a positively charged oxygen. But we know that this double bond could join the oxygen to make a lone pair and then I would get a formal charge on the carbon. I like drawing this resonance structure because it makes it clear to me that that carbon is now very electrophilic, even more electrophilic than it was when it was unreacted. Notice that the whole point of the acid catalyst is to make this carbon even more reactive than it was before, so reactive in fact that alcohol is going to want to attack it. That's the next step guys. The next step is what we call nucleophilic attack. I'm just going to put NA, nucleophilic attack. Nucleophilic attack is going to attach the O, make a new single bond and we're going to get a protonated version of an ether attached to that central carbon.
What do you think the next step is guys? Deprotonation. We have to regenerate that catalyst. I'm going to take my alcohol, my neutral alcohol. Since I started with an alcohol acid catalyst, I need to regenerate it. Then I would just grab the H and lo and behold, what do I have at the end? Now I have my O, my OR, my H, and my H. On top of that, I have my catalyst still there. Awesome guys. That is a hemiacetal. I always like to draw it in this cross structure because I like to always keep it consistent in terms of what I'm looking at, so that when I try to recognize the function group later, I always just put it into that cross and I'm like do I have all the groups that I need? Just a little peculiarity of mine. Anyway, that was the acid catalyzed mechanism to get to a hemiacetal. Not bad at all. Now are we going to stay there? No. Because this hemiacetal is not cyclic the way I drew it. This hemiacetal is either going to go back to the original carbonyl or it's going to keep reacting with alcohol to get to an acetal. More on that later. Now in this next video, I want to show you guys the base catalyzed version of the same reaction.