Dihydroxylation: Videos & Practice Problems

Synvicinal dihydroxylation is an addition reaction that introduces two alcohols (diols) to a double bond in a single step using reagents like potassium permanganate (KMnO₄) or osmium tetroxide (OsO₄). This reaction results in alcohols that are syn to each other, meaning they are adjacent and have a cis relationship. The mechanism involves a cyclization where the double bond interacts with the reagent, leading to the formation of diols. Understanding this reaction is crucial for grasping concepts in organic synthesis and stereochemistry.

General properties of syn vicinal dihydroxylation.

So now we're moving on to yet another addition reaction, and this addition reaction is going to be one that adds two alcohols to the same double bond, and it's going to do it all in one step. And this reaction is called synvicinol dihydroxylation. Okay? So how does this work? Basically, we have these two very special reagents. We have potassium permanganate and we have osmium tetroxide, This is KMnO₄ and OSO₄ that are both highly capable of adding oxygens to double bonds. Now it turns out that this mechanism, you're not going to need to know the whole thing. Okay? But what you do need to know is you need to know what the end product is going to look like. And as you can see, it's really easy. All we have is a double bond. We add one of these two reagents, it doesn't matter which one. Sometimes you're going to see catalytic NMO, that's just a catalyst that sometimes comes up, sometimes it doesn't. Don't worry too much about it. And what you're going to get at the end is what we call diols. Why? Because there are two alcohols in the same molecule. They have a vicinal relationship, which means that they're next to each other, they have a one, two relationship, and they are syn to each other or they're cis, and that means the reaction was syn. Okay? So how does this actually work? Okay? The reason this works is because potassium permanganate and osmium tetroxide both have a very, very similar structure, where it's basically one central atom surrounded with as many oxygens as possible. Now, the way that I like to think of this, maybe just to make it really kind of fun, is I kind of visualize that these look like spaceships. So this is like a flying saucer and there's a flying saucer and there's like a little like cockpit here with an alien inside. But of course he's got like four arms because aliens don't have the same amount of arms as we do. And whatever. So we've got these UFOs, and what do they do? Well, they fly down to earth and they go attacking double bonds. Alright? So this UFO is like coming, it's like swooping down on the double bond, and it decides, hey, I'm gonna leave humans a gift. I'm gonna leave this double bond a gift. And what it's gonna do is it's just gonna add an oxygen to both sides of the double bond. So they're going to take one of the oxygens here, one of the oxygens there, and add them to both sides, leaving a diol at the end. Okay? So they're going to leave two alcohols behind and they're going to take off back into space. Alright? Now if you do want to know the mechanism really quick, I am gonna show you the first step. You don't need to know the whole thing, but it's going to look like this, where if you have a double bond and you have, let's say we're using osmium tetroxide, so double bond. K. What's gonna happen is you're gonna get a cyclization reaction where my, and there's some methyl there, I'm sorry, where my double bond grabs one of the O's, these electrons go down onto the osmium. Okay? And then these electrons go and make a bond to the double bond. Okay? So what winds up happening is that we get this cyclization reaction, you wind up getting two alcohols at the end, and then your OSO₄, I mean, obviously it's missing two oxygen, that thing just leaves and it goes back into space. Alright? So you might need to know the first step, but regardless you're never going to need to know the whole mechanism. Alright? At least I've never seen that and I've been teaching Ordo for a very long time at a lot of universities. Alright? So let's go ahead and do a multi-step reaction here, and what I want to do is just literally, you know, just have you guys go ahead and try to figure this out by yourself, and then I'll go ahead and jump in.

Here’s what students ask on this topic:

What is synvicinal dihydroxylation in organic chemistry?

Synvicinal dihydroxylation is an addition reaction that introduces two alcohol groups (diols) to a double bond in a single step. This reaction uses reagents like potassium permanganate (KMnO₄) or osmium tetroxide (OsO₄). The resulting alcohols are syn to each other, meaning they are adjacent and have a cis relationship. This reaction is important for understanding organic synthesis and stereochemistry.

What reagents are used in synvicinal dihydroxylation?

The primary reagents used in synvicinal dihydroxylation are potassium permanganate (KMnO₄) and osmium tetroxide (OsO₄). These reagents are highly effective at adding oxygen atoms to double bonds, resulting in the formation of diols. Sometimes, a catalyst like N-methylmorpholine N-oxide (NMO) is also used to enhance the reaction.

How does the mechanism of synvicinal dihydroxylation work?

The mechanism of synvicinal dihydroxylation involves a cyclization reaction. When a double bond interacts with a reagent like osmium tetroxide (OsO₄), the double bond grabs an oxygen atom, and the electrons form a bond with the osmium. This results in a cyclic intermediate, which eventually breaks down to form two alcohol groups (diols) that are syn to each other. The reagent, now missing two oxygen atoms, is released.

What is the significance of the syn relationship in dihydroxylation?

The syn relationship in dihydroxylation means that the two alcohol groups (diols) added to the double bond are on the same side, resulting in a cis configuration. This is significant because it affects the stereochemistry of the resulting molecule, which is crucial for understanding its chemical properties and reactivity. The syn addition ensures that the diols are adjacent and have a specific spatial arrangement.

Why is osmium tetroxide (OsO₄) used in synvicinal dihydroxylation?

Osmium tetroxide (OsO₄) is used in synvicinal dihydroxylation because it is highly effective at adding oxygen atoms to double bonds, resulting in the formation of diols. Its structure, with a central osmium atom surrounded by oxygen atoms, allows it to interact efficiently with double bonds, facilitating the cyclization reaction that leads to the syn addition of alcohol groups. Additionally, OsO₄ is often used because it provides high yields and selectivity in the reaction.