Thioacetal - Online Tutor, Practice Problems & Exam Prep
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Thioacetals are similar to acetals but utilize thiols (RSH) instead of alcohols (ROH). They can be formed using a dithiol for cyclic structures or two equivalents of thiol for acyclic ones, typically in the presence of a Lewis acid like BF3. Thioacetals serve as protecting groups and can undergo reduction with Raney nickel, a strong reducing agent, to convert the thioacetal into an alkane by replacing sulfur with hydrogen. This reaction is crucial for removing carbonyls in organic synthesis.
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concept
Thioacetals and Raney Nickel Reduction
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This is going to be a quick video on thioacetals and Raney nickel reduction. We're not going to spend a whole lot of time on this topic because it turns out that a thioacetyl is really the same exact thing as an acetyl except that instead of using an alcohol, you're using a thiol. A thiol would just be a group that's RSH which is really the same thing as ROH. Remember that S is right below O on the periodic table, so it even reacts very, very similarly. It has those two lone pairs and everything. In this case, you can see that I have my carbonyl and I'm reacting it with what's actually called a dithiol here. You don't always have to react with a dithiol. But if you want to get a cyclic thiazotel, then you need to use a dithiol. If you want to get an acyclic one, then you would use two equivalents of just a regular thiol. The biggest difference being that between this direction and acetyls is that we do use a specific acid instead of how for acetals I told you guys it doesn't really matter. It could be any proton donating acid. For thioacetals, typically we use BF3 which if you guys remember is actually a strong Lewis acid.
The biggest difference between this mechanism and the Bronsted Lowry mechanism for acetyls is going to be that in your first step instead of protonating the O, the O actually just starts to donate its electrons to the empty orbital which again makes a resonance structure with a positive charge down here. Other than that, the mechanism is really pretty much identical. It's just that you're using a Lewis acid instead of a Bronsted Lowry acid in this case.
Now we get to the thioacetal which is this. Thioacetals are also important protecting groups in organic chemistry. You could use a thioacetal as a protecting group and then that would be it. You would just stop there and then you'd go back when you want to regenerate the carbonyl. But there's a very important secondary
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example
Raney Nickel Reduction
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Product A is just going to be the reverse reaction of my acetal leading to the original carbonyl because dilute acid, as we know, is going to be able to carry out the reverse reaction. How do we know what that carbonyl will look like? It's actually not that hard. What I would do is I would split this down the middle and I would say anything past the Os. Actually, I'll split it on the Os. Anything past the Os must have come from the alcohol, and anything on this side must have come from the carbonyl. What that means is that my alcohol originally looked like this. As you can see, the 2 carbons that were in between the Os are still there, and that must have reacted with an R group that looked like this. The only part we're missing is the carbonyl. The carbonyl goes where the acetal is. It would be cyclobutanone. Now I'm going to react that molecule with a thioacetal reaction. What we're working with now is the ketone because the alcohol, I don't care about that can leave. What I care about is the actual carbonyl. Now the carbonyl is going to react with 2 thiols and some kind of acid. It doesn't tell me which one but I don't really need to know. It's catalytic anyway. What this is going to do is it's going to give me structure B which is going to be a thioacetal. What I could do to get this structure is I could just draw the general structure and plug in the R groups. That's what I always do. I would do something like this: Sr, Sr, carbon-carbon. Now I'm just going to swap out the Rs for what the R was in my thiol. My thiol is going to be a propanethiol, 3 carbons. I would do 1, 2, 3 and 1, 2, 3. Now I've got my thioacetal. That's structure B. We have A. We have B. Now what's Raney nickel going to do to this? Oops. Woah. Woah. Woah. Woah. Just a second. I made a mistake an