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Current time:0:00Total duration:6:47

Video transcript

here's a dot structure for propane dioic acid or malonic acid and if you heat it up it's going to undergo a decarboxylation reaction so if we show free rotation about this bond turn it's a sigma bond so we can show a different conformation let me go ahead and draw in this carboxylic acid on the left and then we were going to have a carboxylic acid on the right - this time the carbonyl is going to be going to the right so let me go ahead and put in those electrons and the O H would be going to the left so there we have it alright so in this mechanism we're actually going to form a bond between this oxygen and this proton and it's a cyclic mechanism so if these electrons in here move into here that's going to push these electrons into here and then these electrons are going to form the bond between the oxygen and the hydrogen so let's go ahead and show the result of our cyclic mechanism all right would have a carbon bonded to an oxygen right bonded to hydrogen and then we have an OHA over here and then we'd have a double bond between this carbon and another carbon on the right we would actually form co2 so let me go ahead and put in lone pairs of electrons on that oxygen alright so we can see that we would form our carbon dioxide molecule here so let me go ahead and draw those in and let's follow some of those electrons right so the electrons in magenta right in here are going to move in right to form this bond right to form our double bond for co2 and then these electrons in here in blue so between this carbon and this carbon alright are going to move over here to form this double bond right between this carbon and then there's a carbon right here and then there's also two hydrogen's bonded to this carbon let me go ahead and draw those in so we can see it a little bit better and then finally let's make these electrons in here red so these electrons are the ones are going to form this bond between the oxygen and the hydrogen so we formed our co2 and we've also formed an acid enol alright so this right here is called an acid enol and we saw in earlier videos how the enol is in equilibrium with the keto form right with keto enol tautomerization and so this is actually the enol form of acetic acid and so that's actually going to be our product so let me go ahead and draw acidic acid up here and and here we have the O H on the left side and then we would have a carbon over here with three hydrogen's bonded to it right so this would be if we're thinking about the keto type form right so the difference between the enol and the keto form are the movement of one proton right so there's a proton here in the oxygen and here it's one of these in the carbon and then the double bond here we have the double bond between the two carbons and here we've moved the double bond between the carbon and the oxygen so once again we've seen how to do that in earlier videos and then we also produce co2 so carbon dioxide as the other product for this reaction so the key to a decarboxylation reaction is having a carbonyl beta to a carboxylic acid so for example here's our carboxylic acid and we know the carbon next to a carboxylic acid is the alpha carbon and the carbon next to that is the beta carbon and we saw how this carbonyl was necessary in the mechanism and so the fact that there's an O H here isn't really necessary and what we really need is a carbonyl that's beta to our carboxylic acid an order for a decarboxylation reaction to occur so let's look at another example alright where we don't have a a dioic acid anymore right we have a carboxylic acid on the right and then um and then over here on the Left we have a ketone but again the key point is here's the Alpha carbon and here's the beta carbon and we have a carbonyl that's beta 2 our carboxylic acid and so therefore a decarboxylation reaction can take place so if we heat up this molecule once again thinking about the mechanism rotating about that Sigma bond alright let's go ahead and redraw this alright so we have our benzene ring and then we have our carbonyl right here and then we would have once again our carbonyl going off to the right this time and then our over here on the left so thinking about our mechanism once again we know it's a cyclic mechanism we know this oxygen is going to bond to this proton and so these electrons are going to move into here and these electrons move into here and these electrons move into here so that's our cyclic mechanism when we draw we draw what happens right moving all those electrons around we have our benzene ring we would have it bonded to an oxygen this oxygen was bonded to this proton now and then we have a double bond right in here and then we also form co2 so once again let's let's run through let's run through those electrons and try to follow some of those electrons here so let me draw on these lone pairs of electrons on our oxygen right so we can see where the co2 comes from so once again these electrons in here and magenta are going to move in here to form the double bond on co2 at the same time these electrons in here are going to move into here to form this double bond and then finally these electrons in here are going to move out to form the bond between the oxygen and the hydrogen so we've made our co2 so the oxygen carbon oxygen right comes from this oxygen carbon oxygen right here on the molecule on the left so hopefully that's a little bit easier to see now and once again we have an enol alright so we form our co2 we also create our enol and so we can think about keto enol tautomerization for our product right so the enol is going to be an equilibrium with the keto form so let's go ahead and draw the keto form which we know is moving one proton and moving the double bond so we move the double bond between the carbon and the oxygen and we move that proton to this carbon right here so this carbon right here picks up a proton so down here that carbon had two hydrogen's alright and it doesn't have to be this one right here but we are going to add a hydrogen to that carbon and that gives us our final product our ketone so this decarboxylation reaction produces a ketone and once again it doesn't really matter what this r group is here right here we have a benzene ring instead of the O H in the previous example and then of course we're also going to make co2 so decarboxylation reactions are going to be important in in future videos where we're synthesizing some comp molecules and so here is the this is the cyclic mechanism for it