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Oxidation of alcohols (examples)

Video transcript
In the last video, we took a look at the mechanism for the oxidation of alcohols. In this video, we'll do specific examples for different types of alcohol. So we'll start with a primary alcohol. And we identified the carbon attached to the OH as my alpha carbon. And in order for this mechanism to work, we need at least one hydrogen attached to our alpha carbon. So if we react to our primary alcohol with sodium dichromate, sulfuric acid, and water, which we call the Jones reagent, in that mechanism we're going to oxidize our alpha carbon and lose one of those protons attached to the alpha carbon, which would give us an aldehyde functional group. So we increased the number of bonds of carbon to oxygen, we lost a bond of carbon to hydrogen. Now, the difficulty is trying to isolate this aldehyde. Usually, it's very difficult to isolate, and oxidation will continue. And you get a second oxidation to produce a carboxylic acid as your final product. So if you react a primary alcohol with the Jones reagent, you're going to end up with a carboxylic acid. Let's look at an example, and we'll use ethanol as our primary alcohol here. So if we react ethanol with the Jones reagent, the chromium in the sodium dichromate is chromium-6 plus, which has kind of an orangish color to it. So when you're starting off with your reaction, it's going to look a little bit orangish due to that chromium presence. And when we oxidize our primary alcohol, when we oxidize our ethanol, we're going to turn it into a carboxylic acid. We're not changing the number of carbons, so there's still going to be two carbons like this, but we're now going to change it into a carboxylic acid. So acetic acid will be the product. So we went from this carbon having one bond to oxygen, and we oxidized it so this carbon now has three bonds to oxygen atoms. And in that process, if we oxidized that alpha carbon, we're going to reduce the chromium. So the chromium is going to go from an oxidation state of 6 plus and eventually it's going to reach an oxidation state of 3 plus like we talked about in the last video, which has kind of a greenish color. So it's very easy to monitor this reaction by just looking for the color change. And this is a very, very fast reaction, so this was originally used for the breathalyzer test to determine if ethanol is present. So let's see what would happen if you wanted to actually stop it at the aldehyde? You don't want the oxidation to continue to the carboxylic acid. Let's say you wanted to actually stop it at the aldehyde. Well, to do that, you would have to use a different reagent. So let's go ahead and look and see how we could stop the reaction after the first oxidation. So if we started with a primary alcohol-- so I'll just re-draw a primary alcohol really fast here like this. And if we wanted to oxidize it only once, so that we end up with an aldehyde, the best reagent to use for this is something called pyridinium chlorochromate, or PCC. So let's take a look at the structure of the PCC reagent really fast. So pyridinium, let's go ahead and show what that looks like. So it's derived from pyridine, so let's go ahead and sketch that in like that. So pyridine as a base is going to pick up a proton to form a positive charge here. And then we have a CrO3 and then Cl and then with a negative charge. So this would be the pyridinium part, so let's go ahead and write it. Pyridinium. And then we have chlorochromate over here on the right, so I'll go ahead and write chlorochromate. And then that makes it easier to see where the P-C-C comes from. So this is the PCC reagent, which is a much more mild agent than the Jones reagent. It'll oxidize your primary alcohol and stop at your aldehyde. So let's go ahead and react to ethanol again. This time, we'll use PCC instead of Jones. So if we started with ethanol and we added PCC-- so here we go-- we're going to end up with an aldehyde. And it's a two-carbon aldehyde, so we can say those two carbons are still there. And we are going to form a double bond, and this time it's going to be an aldehyde. So this is ethanal, or acetaldehyde, which will be the result of this oxidation reaction. So that takes care of primary alcohols. Let's look at the oxidation of secondary alcohols now. So we'll start with a general reaction over here. So we'll have a secondary alcohol, so two different alkele groups, or they could be the same, attached to our alpha carbon. Our alpha carbon is attached to an OH, and remember for the mechanism to work, you must have a hydrogen attached to that alpha carbon. So this is my secondary alcohol like that. Now, for secondary alcohols, we can only get one product. We saw in the last video that when you oxidize a secondary alcohol, you are going to end up with a ketone. So for a secondary alcohol, you could use either Jones or you could use PCC. So either one of those two reagents will oxidize a secondary alcohol to a ketone. So let's take a look at an example. So let's start with a secondary alcohol. So I'm just going to draw a benzene ring on here and then attach that benzene ring. There will be a secondary alcohol presence. So there's my secondary alcohol. And if I were to add either Jones or PCC, I look at my secondary alcohol. I identify my alpha carbon. It's the one attached to the OH. And I can see there is one hydrogen attached to that alpha carbon. This is a secondary alcohol. So when I draw the product, I'm going to convert that secondary alcohol into a ketone. So if I were to do that, I would just real quickly redraw my benzene ring here, and I would convert that alpha carbon into a ketone, so that would be my product. All right. So let's look at a tertiary alcohol. So if I had a tertiary alcohol, something like tert-butanol here like that, and if I attempted to oxidize that tertiary alcohol with either Jones or PCC, we saw in the last video no reaction. Because if I find the alpha carbon, this carbon right here, there are no hydrogens attached to that alpha carbon. And again, we saw in the mechanism that that was necessary. So something like tert-butanol would not be able to be oxidized in this fashion. So that sums up the oxidation of alcohols.