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Current time:0:00Total duration:7:02

Video transcript

in last video we took a look at the mechanism for the oxidation of alcohols and this video will do specific examples for different types of alcohols so we'll start with a primary alcohol and we identified that the carbon attached to the O H 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 in water which we call the Jones reagent alright in that mechanism we're going to oxidize our alpha carbon and lose one of those protons attached to the alpha carbon which will give us an aldehyde functional group alright so we increase 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 an 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 right here the chromium in the sodium dichromate is chromium 6 plus which has kind of an orangish color to it so when you're when you're starting off with your reaction that's going to look a little bit orange due to that chromium present and when we oxidize our primary alcohol right 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 are now going to change it into a carboxylic acid so acetic acid will be the product right so we went from 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 oxidize 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 tests right - to determine if ethanol is present so let's see 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 all right 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 a primary alcohol so 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 a an aldehyde alright the the best reagent to use for this is something called pyridinium chlorochromate or p c c so let's take a look at the structure of the PCC reagents really fast so pyridinium let's go ahead and show what that looks like so so it's derived from pyridine so let's go ahead and sketch that in like that so cárdenas a base is gonna pick up a proton to form a positive charge here and then we have cro3 and then CL and then with a negative charge so this would be the the pyridinium part so let's go ahead and write its pyridinium and then we have chloro chromate over here on the right so I'll go ahead and write chloro chromate and then that makes it easier to see where the p c c comes from right so this is the pcc reagent which is a mild it's it's 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 alright so if we 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 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 so this is ethanol 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 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 or an alpha carbon is attached to an O H and remember for the mechanism to work we must have a hydrogen attached to that alpha carbon so this is my 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 for for a secondary alcohol you could use you could use either 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 right so there's my secondary alcohol and if I were to add either Jones or PCC alright so Jones or PCC I look at my secondary alcohol I identify my alpha carbon right it's the one attached to the O H and I can see there there is one hydrogen attached to that alpha carbon right this is a secondary alcohol so when I draw the product right 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 alright so let's look at uh let's look at a tertiary alcohol alright so if I if I had a tertiary alcohols and like tert-butanol here like that and if I attempted to oxidize that tertiary alcohol with either Jones or PCC right we saw in the last video no reaction right because if I find the alpha carbon right this carbon right here there are no hydrogen's attached to that alpha carbon and again we saw in the mechanism that that was necessary so sunlight tert-butanol would would not be able to be oxidized in this fashion so that sums up the oxidation of alcohols