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Oxidation of alcohols II: Examples

Examples of oxidation reactions for primary, secondary, and tertiary alcohols.  Created by Jay.

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  • blobby green style avatar for user nandu.nans97
    Instead of PCC, can we use PDC to stop the reaction at ?
    (2 votes)
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  • leaf green style avatar for user khjhzw
    at , for the primary alcohol, oxidation reaction happens twice with Jones reagent to yield carboxylic acid but it happens only once with PCC to yield aldehyde. Why is this the case?
    (2 votes)
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    • spunky sam blue style avatar for user Ernest Zinck
      The Jones reaction uses aqueous, acidified dichromate.
      In aqueous solution, the aldehyde forms a 1,1-diol, which is easily oxidized to a carboxylic acid. RCH₂OH → RCHO ⇌ RCH(OH)₂ → RCOOH
      PCC is used in organic solvents, so there is no water present to form the diol, and the reaction stops at the aldehyde stage.
      (4 votes)
  • blobby green style avatar for user Yash Verma
    in my school textbook it is written that secondary alcohol can be converted into ketone by CrO3 ,, can that be done rather than what is shown at
    (2 votes)
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  • purple pi purple style avatar for user udayaditya1998
    Why is PCC milder ? Is it because of resonance by any chance?
    (2 votes)
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  • piceratops seed style avatar for user Varun
    PCC is used to prevent further oxidation . So how can sec. alcohol be converted to COOH ?
    (1 vote)
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  • marcimus pink style avatar for user Grace Gemar
    talks about differences in PCC and Jones reagent. In my class my professor said Jones reagent was the same thing as PCC....?
    (1 vote)
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    • leaf green style avatar for user patrickbrown404
      PCC = Pyridinium chlorochromate, Jones Reagent = Chromium Trioxide in Sulfuric acid.

      They're similar in that they both rely on the oxidizing abilities of Chromium, however their countering is different and thus they will exhibit different properties.

      TL;DR They are different, but effectively the same oxidising reagents.
      (2 votes)
  • blobby green style avatar for user Tina Ismail
    Is it right to say that tertiary alcohols can be oxidised by highly oxidising agents like KMnO4 even though this will break the carbon-carbon bond.
    (1 vote)
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    • blobby green style avatar for user isabelpark12
      I think tertiary alcohols are as oxidized as they can be and so do not react with any of the oxidizing agents....I think if you want to oxidize it by breaking the C-C bond, you would need to make the one of them a really good leaving group somehow...
      (2 votes)
  • male robot hal style avatar for user Mirko Nikolovski
    You said there must be at least one hydrogen atom attached to the alpha carbon if we want a reaction. But, where are the hydrogen atoms at , and ?
    (1 vote)
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    • spunky sam blue style avatar for user Ernest Zinck
      At , there are 2 H atoms on the C atom with a blue dot.
      At , there are 2 H atoms on the same C atom, but it doesn't have a blue dot.
      At 600, there is 1 H atom on the C atom with a blue dot.
      Remember that C must have 4 bonds, and the H atoms are not shown in bond-line structures.
      (2 votes)
  • leafers seed style avatar for user Kennady Crayne
    Can someone show an example of when oxidation of a primary alcohol to a carboxylic acid using potassium dichromate in aqueous sulfuric acid.
    (1 vote)
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  • blobby green style avatar for user Athira Panthariyil
    how will convert n-propyle alcohol to n-pentyle alcohol
    (1 vote)
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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 alcohols. 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 needed at least one hydrogen attached to our alpha carbon. So if we react 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. All right. 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'll 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. We'll use ethanol as our primary alcohol here. So if we react ethanol with the Jones Reagent right here, the chromium in the sodium dichromate chromium 6 plus, which has kind of an orange-ish color to it. So when you're starting off with your reaction, it's going to look a little bit orange-ish because you've got chromium present. 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. We went from this carbon having one bond to oxygen. And we oxidized it, so this carbon now has three bonds, two oxygen atoms in. In that process, if we oxidized that alpha carbon, we're going to reduce the chromium. So the chromium's 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 tests to determine if ethanol is present. Let's see. Well, what would happen if you wanted to actually stop it at the aldehyde? Right? 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-- I'll just redraw 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 they called "pyridinium chlorochromate" or "PCC." 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. Let's go ahead and sketch that in like that. 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 PCC comes from. This is the PCC reagent, which is a much more mild agent than the Jones Reagent. It will 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. All right. 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 2-carbon an aldehyde. Right? 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. This is a ethanyl or assets aldehyde, 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 alkyl 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, we 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. Right? 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 present. So there's my secondary alcohol. And if I were to do 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 converge that alpha carbon into a ketone. So that would be my product. All right. Let's look at a tertiary alcohol. If I had a tertiary alcohol-- so 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. Something like tert-Butanol would not be able to be oxidized in this fashion. So that sums up the oxidation of alcohols.