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
Cool so we've gone over two ways now let's go on to the next one, the next one notice that let's say that I start off with that double bond but now what I wind up getting is an alcohol not in the secondary position but in the tertiary position, Ok? That's kind of odd and notice that the double bond used to be here but now my alcohol isn't even attached where the double bond was before, OK? Any clue what this might be? This is also going to be a Markovnikov addition but it's a Markovnikov addition that also has a Carbocation in the intermediate...In the reaction so what this means is that this is going to have to be in acid catalyzed hydration.....Sorry acid dash catalyzed hydration there we go and the reagent for this one were much easier, it was really just any strong acid so it's going to put here HA O for water but most commonly the acid that we would use is sulfuric acid so most commonly what you're going to see is like H2O over H2SO or the other way around to both are fine, OK? So that's usually what you would see and remember what that's going to do is it's going to add a water but it's going to add it to the most stable location which is the one that has created the tertiary positions it's created after the shifts, OK? So those are the three different ways that we have to make alcohol from an addition reaction and this is just meant to be an overview of the reactions that we've already learned, OK? If you need to brush up on your addition reactions I would recommend to go to the additional reactions chapter and I always teach all these three in a row as well so you'll be able to really learn the full mechanisms and catch up on some of the stuff that you might be rusty on.
Concept: Forming alcohols through SN2 reactions.2m
So now let's go to this last way to generate alcohol that we've learned in the past and this one's actually really easy, OK? Notice that what I have here is an Alkyl halide and at the end I have an alcohol, there is a dead giveaway as to what this reaction might be because I have stereo chemistry given and what I'm showing is that the halogen used to be on a wedge or towards me and after reaction the alcohol is going to be on a dash away from me so does anyone know what kind of reaction might invert the stereo chemistry? That would be SN2, OK? So it turns out that we can generate alcohols through SN2 reactions but it's only in some specific situations, basically it's when you have an Alkyl halide that is open to backside attack so that would be basically a Methyl, a primary or secondary Alkyl Halide because remember that tertiaries are just too backed up they can't do a backside attack and I need my nucleophile to be NaOH, OK? And if can use NaOH what I'm going to wind up getting is the backside attack where I kick out the X and I wind up getting my alcohol, alright? So these are the four commonly used methods to make alcohol this is all intended to be somewhat of a review for you guys of reactions that hopefully you've already talked about but if not, I would just encourage you to reference those chapters so you can get a little bit more of a feel of what's going on, alright?
Which reagent would you use to accomplish the transformation shown here?
Provide the reagents or starting materials to perform the indicated transformation. More than one reaction may be required. You should number your steps if it is important for the synthetic transformation to take place.
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
Based on the principles of acidic and basic cleavage as well epoxidation determine the necessary reagents for the each of the following transformations.