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Oxidation of alcohols I: Mechanism and oxidation states

Video transcript

let's see what happens when you oxidize alcohol so in the top left here we're starting with a primary alcohol and the carbon that's attached to the O H group is your alpha carbon to oxidize an alcohol you must have alpha hydrogen's you must have hydrogen's attached that alpha carbon in order for the mechanism to work so in that mechanism you're actually going to lose one of those alpha hydrogen's and we'll take a look at the mechanism in a few minutes so if I were to oxidize this primary alcohol I'll add something I'll add something to oxidize oxidize my primary alcohol like that one way to think about on the oxidation of an alcohol is to think about the number of bonds of carbon to oxygen on the left side here we have a one bond of our alpha carbon to this oxygen and the mechanism we're going to lose a bond of carbon to hydrogen and we're going to gain another bond of carbon to oxygen so you're increasing the number of bonds of carbon to oxygen so that would of course give me two bonds of carbon to oxygen if I if I oxidize my alcohol one time and I'm going to lose one of those hydrogen's so one of those hydrogen's is still left and my alkyl group is still attached so obviously this would give me an aldehyde functional group so if you oxidize a primary alcohol one time you will get an aldehyde let's take a look at the oxidation states of my alpha carbon and see what happened to it all right so if I I want to assign an oxidation state to my alpha carbon on the left once again I have to put in my electrons right each bond consists of two electrons like that and I need to think about electronegativity differences oxygen is more electronegative than carbon so it's going to take those those two electrons carbon versus carbon is a tie so each carbon will get one of those electrons carbon actually is slightly more electronegative than hydrogen so carbon will win and take those electrons right there carbon normally has four valence electrons and in this instance it is being surrounded by five electrons so four minus five will give me an oxidation state of negative one for my alpha carbon so let's look and see what happened to that alpha carbon after we oxidized it right so over here on the right if I wanted to assign an oxidation state to two what is now my carbonyl Carbon alright once again I think about my electronegativity differences and I know that oxygen is going to beat carbon carbon versus carbon is a tie and carbon versus hydrogen carbon will win so the oxidation state of that carbon right normally four valence electrons surrounded by three this time so four minus three will give me plus one so I can see that my my oxidation state went from negative one to plus one so an increase in the oxidation state is of course oxidation so if you oxidize a primary alcohol one time you will get an aldehyde what about if you keep going right so if you if you form an aldehyde and sometimes it's hard to stop the reaction mixture from from continuing to oxidize so if you oxidize an aldehyde alright you think about what functional group you would get well again a simple way of doing it would be to think on the left side I have two bonds of carbon to oxygen is there any kind of functional group where carbon is bonded to three times to an oxygen so that of course would be a carboxylic acid so if I think about the structure of a carboxylic acid right I can see that carbon is actually bonded three times to an oxygen if you will I had three bonds of carbon to oxygen and over here I have my alkyl group like that so if you oxidize an aldehyde you're going to get a carboxylic acid let's look again at the at the at the oxidation state of my carbonyl carbon right so once again I put in my electrons here and I think about electronegativity so oxygen of course beats carbon right tie between these two carbons and oxygen beats carbon again so in this case right normally four valence electrons now there's one so four minus one gives us an oxidation state of plus three so once again an increase in the oxidation state means oxidation if you oxidize an aldehyde you will get a carboxylic acid let's look at a secondary alcohol now alright so we'll go down here to our secondary alcohol and once again identify the Alpha urban the one attached to your Oh H group we need to have at least one hydrogen on that alpha carbon and so we have one right here so if we were to oxidize our secondary alcohol all right so we're going to oxidize our secondary alcohol once again a simple way of doing it is thinking my alpha carbon has one bond to oxygen so I could increase that to two bonds and that should be an oxidation reaction in the process I'm going to lose a bond to my alpha hydrogen so I'm now going to have two bonds of carbon to oxygen and I'm going to lose the bond that that alpha carbon had with with the with the hydrogen there so that leaves my two alkyl groups like that so now I have two alkyl groups and of course this would be a ketone functional group so if you oxidize a secondary alcohol you're going to end up with a ketone I can I can assign oxidation states so once again let's let's show that this really is an oxidation reaction here and I go ahead and put in my electrons on my alpha carbon and think about electronegativity differences so once again oxygen beats carbon carbon versus carbon is a tie carbon versus hydrogen carbon wins and then carbon versus carbon of course is the tie again so normally four valence electrons in this example it's around by 4 so 4 minus 4 gives us an oxidation state of 0 for our secondary alcohol and when I oxidize it I'm going to get this ketone over here on the right so let's take a look at the oxidation state of the carbon that used to be our alpha carbon on the Left which is now our carbonyl carbon so once again we put in our electrons and we think about electronegativity difference right so oxygen is going to beat carbon so we go like that Carbon versus carbon is a tie carbon versus carbon is a tie once again so normally 4 minus 2 this time around that carbon giving us an oxidation state of plus 2 so to go from a secondary alcohol to a ketone right we see there's an increase in the oxidation state so this is definitely an oxidation reaction let's look now at a tertiary alcohol so here is my tertiary alcohol all right and when I find my alpha carbon I see that this time there are no hydrogen's bonded to my alpha carbon so according to the mechanism which we'll see in a minute there there is no there's no way we can oxidize this tertiary alcohol under normal conditions anyway so if we attempted to oxidize this we would say there's no reaction here since we are missing that alpha hydrogen let's take a look at the mechanism and let's see why we need to have that alpha hydrogen on our on our alpha carbon so if I if I were to start my mechanism here with with an alcohol remember this this must be either a primary or a secondary alcohol in order for this oxidation to work so I'm going to go ahead and show my alcohol there alright so again Oh either primary primary or secondary like that and when we have our primary or secondary alcohol and it's going to be reacting with chromic acid so here is the dot structure for chromic acids like that so I'll just I'll just simplify it right here I won't worry too much about my lone pairs of electrons and chromic acid can come from several different reagents probably the most common reagent would be would be sodium dichromate so na 2 CR 2 O 7 sulfuric acid h2so4 and water and water and and all of this together is usually referred to as the Jones reagent so a mixture of sodium dichromate sulfuric acid and water it's called the Jones reagent and that will mix together to give you chromic acid in solution okay so another way to do it what be you could you could start from chromium trioxide so you could also use a different reagent which consists of CRO 3 chromium trioxide and h3o plus and acetone and that will also generate chromic acid in solution so whichever one you would like to use the first step of the mechanism is similar to the formation of nitrate esters that we saw in the previous video okay so this is going to be a reaction equilibrium or it's it's reversible and if you remember in the formation of nitrate esters it's a similar mechanism for the formation of all inorganic esters here and we're going to we're going to lose all right we're going to lose this hydrogen and this o H and those are going to produce water and we can we can stick those two molecules together all right and so we would get we would get this as the initial as the initial product here we're going to have the end result of putting that oxygen right bonded to that chromium atom like this so this is a chromate ester intermediate alright so this is what we would make alright so in the next step of the mechanism we need something to function as a base and water is going to do that for us so water comes along like this two lone pairs of electrons one of those lone pairs can function as a base and it's going to take that alpha proton remember this is this is our alpha hydrogen on that carbon right and over here right we're going to take it just the proton right just the nucleus of that hydrogen atom and so this lone pair of electrons in here could take that proton that's going to leave the electron that hydrogen brought to the dot structure behind and these two electrons are going to move into here to increase the number of bonds of carbon to oxygen at the same time that is going to kick these electrons in this bond off on to the chromium so let's go ahead and draw the result the product of that reaction here so let's see if we can get some space all right so right here well we're going to lose that alpha hydrogen right so now our carbon still is bonded to two other things we lost that alpha hydrogen and now it's double bonded to that oxygen so that would be the mechanism we went from one bond of carbon to oxygen on our primary or secondary alcohol we've now increased it to two bonds of carbon to oxygen so the the other products the other products here we would make h3o plus of course so we'll go ahead and put h3o plus when water picks up the proton we would form would form H CRO 3 - as our other product now if the alpha carbon is the one being oxidized right so if this carbon right is oxidized to this carbon it's the same carbon but this carbon is being oxidized something must be being reduced right so this is a redox reaction if you oxidize something something else is reduced and that something else is chromium so if you were to assign an oxidation state to chromium in the sodium dichromate over here alright so so in this guy over here chromium has an oxidation state of 6 plus alright so when we look at our products and we find chromium in our products here if you were to assign an oxidation state to this chromium you'd get 4 + so CR 4 + and there's there's some other chemistry that goes on which ends up converting the chromium from 4 plus into 3 + and so overall you can see that you're starting out with with 6 + over here and you're ending up with with 3 + over here that's a decrease in the oxidation state so chromium is being reduced so that that alpha that alpha carbon is being oxidized and chromium is being reduced in this redox reaction in the next video we'll take a look at several examples involving primary and secondary alcohols