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Video transcript

here's another carboxylic acid derivative so this is an acid anhydride over here on the right and we can form those from carboxylic acid so if we start with the carboxylic acid and our first step at a base like sodium hydroxide and our second step at an acyl chloride then we'd form our acid anhydride as our product if you think about a mechanism sodium hydroxide to base the hydroxide anion is going to take this proton right leaving these electrons behind on the oxygen and there are already two two lone pairs of electrons on an oxygen to start with so if we go ahead and draw the product alright would form a carboxylate anion so this oxygen right here would have three lone pairs of electrons on it like that and so if we follow some of those electrons alright just have a negative 1 formal charge and if we put these electrons in magenta right those electrons come off onto our oxygen to form our carboxylate anion and that's going to function as our nucleophile so in the second step alright we add our acyl chloride it's going to go ahead and draw in our acyl chloride it's going to be our electrophile and let's think about why here we have the oxygen right withdrawing electron density from this carbon because oxygen is more electronegative than carbon so we have that and then we also have this chlorine doing it as well chlorine is more electronegative than carbon as well so we have these two things withdrawing electron density and so this carbon is definitely electrophilic right here and so we have a nucleophile that's going to attack our electrophile so our nucleophile attacks our electrophile these electrons kick off onto our oxygen and we can go ahead and show the result of our nucleophilic attack alright so we would now have this carbon double bond to this oxygen and then this oxygen is now bonded to this carbon and then we have an oxygen up here alright with three lone pairs of electrons negative 1 formal charge we still have this carbon bonded to a chlorine and we still have an R prime group here like that so following some electrons alright let's go ahead and put in these these lone pairs here the electrons in magenta right formed the bond between the oxygen and the carbon and then we could say that these electrons in here right moved off onto our oxygen to give that oxygen a negative-one formal charge all right when we we think about the next step right we know that the chloride anion is an excellent leaving group so if these electrons in blue move in here to reform our double bond then these electrons will kick off onto the chlorine to form the chloride anion which we know is a stable on its own as a leaving group and so that forms our acid and hydride so let's go ahead and draw in the final structure here all right so we would have our oxygen with lone pairs of electrons on our oxygen we would have this bond right we we reformed our carbonyl like that and then the chloride anion was our leaving group so now we have an R prime group and so we formed our acid anhydride and so we could make these R groups the same or we could make them different and so this is a good way of forming a mixed anhydride as well as one that is symmetrical so let's let's look at an example all right so let's take acetic acid and we're going to do two different reactions with acetic acid so the first thing we're going to do is add thionyl chloride to acetic acid and we've seen that addition of thionyl chloride converts a carboxylic acid into an acyl halide so let's go ahead and show the conversion of that carboxylic acid into the corresponding Asil halide and then we could take that acetic acid in a separate reaction right we could add sodium hydroxide and the hydroxide takes this proton right which leave these electrons behind on the oxygen so lets us scoot up a little bit so we can see the mechanism that we talked about before all right and so this would form a sodium acetate so this would form sodium acetate drawing our lone pairs of electrons on this oxygen negative one formal charge and a plus like that all right so this oxygen already had two lone pairs of electrons and so now you have the situation that we talked about up here your carboxylate anion functions as a nucleophile attacks our electrophile like carbon on your ACL chloride all right so we could show we could show these electrons attacking this carbon right these electrons kick off onto the oxygen and then when those electrons move back in I have to reform your double bond these electrons will kick off on to your chlorine and then so we can go ahead and draw our product right so just thinking about what happens in this mechanism we can go ahead and draw our products which would be asymmetrical and hydride so we would have our oxygen right here and then we would have we would have our group over here so thinking about the our groups this time and so this our group right is a methyl group alright and then we could think about this oxygen being this oxygen and then this portion of the acyl chloride right is this portion for our final our final product for our acid anhydride and so this is acetic anhydride this right here is a Sita can hydride which is the one that's used most commonly in an undergraduate lab and so this is a nice way of preparing acid anhydride alright let's look at another way to form a seat again hydride you could start with two carboxylic acids right and this would be acetic acid and acetic acid so the same carboxylic acid and apply high heat and this time you think about a dehydration reaction so you can think about losing and Oh H from one and a hydrogen from the other alright to form water so you can think about losing the water here and your dehydration reaction and then you can you can you can stick those portions of the molecule together right so you could you could take this portion right and then this portion and put them together and you can see that that is once again acetic anhydride so let's go ahead and draw that so we would form acetic anhydride here by dehydration so this way of doing it is not always the best way it's it's it works for acetic acid but it doesn't work for most carboxylic acids alright here's one more case where it can work if you have a situation like this this is phthalic acid all right so it's a dicarboxylic acid and if you apply heat to it you don't need as high of a heat as you need for the previous reaction this heat is higher than this one but you can once again form an and hydride so if we think about a dehydration right losing Oh H from one and H from the other and then we can go ahead and draw the product here so we would form we would form our benzene ring and then we would form our and hydride like this alright so once again loss of water so the name of this acid anhydride would be phthalic anhydride because it's derived from phthalic acid so this is a good way to form five or six membered rings in this case we have a five membered ring right we have a carbon and oxygen a carbon a carbon and a carbon so we have a five membered ring this time it also works for six membered rings