Thioacetals and Raney Nickel Reduction

Concept: Concept: Thioacetals and Raney Nickel Reduction

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Video Transcript

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 thioacetal is really the same exact thing as an acetal 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. It even reacts very, very similar. 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 thioacetal, 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 these acetals 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 acetals 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 structured with a positive charge down here. The mechanisms are 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 wanna regenerate the carbonyl. But there's a very important secondary reaction that only thioacetals can undergo that we want to make sure that we learn here. That secondary reaction is when you combine a thioacetal with Raney nickel. Raney nickel is a strong reducing agent. I’m going to put here strong reducing agent that work specifically with thioacetals. What it does is it replaces the entire sulfur ring or the dithioesther ring with Hs. The Raney nickel catalyst is a great way to remove carbonyls altogether. If you’re intention from the begging was just let me get rid of that carbonyl and make it into an alkane, then thioacetal with Raney nickel is the way to go.
I’m going to let you guys do this example here. Notice that there's multiple steps. First I'm actually starting off with an acetal then I’m reacting with dilute acid so you can try to think what's going to go on there. Draw all the structures for A, B, and C. Then we'll be done with this topic. Go ahead and do that and then I’ll do it for you.