Up to this point in the course, we’ve learned 4 methods to make alcohols. This will be a brief review of those methods.
Concept: Forming alcohols through Oxymercuration-Reduction.3m
In this section, we're going to talk a lot about alcohols. It's important for us to take a minute to remember all the different ways that we can make alcohols from prior chapters. Let's go ahead and get started.
It turns out that there are four different ways that we've already learned to make alcohols. I'm going to go ahead and remind you what those are now. Let's say that I have a double bond and I'm trying to add a hydroxyl group to it. I want to add it specifically in this position. What addition reaction could I use to put a hydroxyl right on that secondary position? The answer is that there's only one reaction that could yield this kind of alcohol and that would be oxymercuration or what we just called oxymerc.
Why is that? Because if you remember, oxymercuration is an addition reaction that adds Markovnikov alcohol, which is good because you can tell that that is the Markovnikov or more substituted location. But there's another thing that's special about oxymerc which is that there's no carbocation in the intermediate. And remember that carbocations have a tendency to do what? They shift. In this case, this is an example of where I can add Markovnikov alcohol without worrying about a rearrangement.
Now all I have to do is fill in the reagents for oxymerc. Does anyone remember them? Basically what we would have is some kind of mercury molecule, so it would be Hg(OAc)2. Then what we would put with that is water. Why is that? Because you want to show specifically that we're using water to add the alcohol.
On the bottom of the arrow then we did the reduction step. Remember that the full name of this reaction is oxymercuration-reduction. The reduction is completed by NaBH4, which later on we will learn is a reducing agent, and a base, so NaOH. Those are the reagents. I know it looks like a lot, but we have been over this before and this is one of the ways that you can add alcohol to a double bond.
So we've been over one of the four ways, but let's keep going because it turns out that there's a lot more that we can do to a double bond.
Concept: Forming alcohols through Hydroboration-Oxidation.2m
So what if we take that same double bond, but we want to add the alcohol here instead. Notice that in this molecule, I have the alcohol on the least substituted position. Does anyone remember what kind of reaction that would be? Well, it's going to be another addition reaction but it has a completely different name. This is hydroboration.
Remember hydroboration is really unique because it's going to be the only anti-Markovnikov way we have to add alcohol or going to the least substituted position. Let's just go ahead and remember what the reagents were. Does anyone remember?
It was really – hydroboration means that you need boron in there. It was really going to be any boron source, most commonly BH3, but your professor, your textbook may use a different source of boron. There's all kinds of different sources that we can use. BH3, and many times that will be complexed to THF, but sometimes you won't see that step. But sometimes – THF is a solvent, so it's not absolutely required that you write that. But sometimes you'll see it.
That's it for the top. The top is just adding boron. The bottom is the oxidation step because remember that it's hydroboration-oxidation. Let's fill those reagents in. That was hydrogen peroxide and, once again, a base. Those are the reagents that could generate an anti-Markovnikov alcohol from a double bond.
Concept: Forming alcohols through Acid-Catalyzed Hydration.3m
Concept: Forming alcohols through SN2 reactions.2m
Mixing 2-propanol (CH3CH(OH)CH3) with sodium cyanide (Na+CN-) produces
A. (CH3)2CHO- Na+.
E. no reaction.
Show how you would carry out each of the following reactions. You do NOT need to draw the mechanisms