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Current time:0:00Total duration:7:56
The molecule should be labeled "triacylglyceride."

Saponification - Base promoted ester hydrolysis

Video transcript

okay now I said in a previous video that try glycerols and I've drawn an example here triglycerol czar hydrolyzable lipids which means that their lipids that can be broken down into smaller smaller pieces in a hydrolysis reaction so hydrolysis hydrolysis reaction and maybe a more general way of looking at this reaction is as an ester hydrolysis because really what's being broken down in here are the individual ester bonds you see this is an ester this carbon is double bonded to an oxygen and then bonded to an additional oo r group as well here this would be an ester in this when we break these triglycerides down this bond right here and right here and right here these are the bonds that we're breaking so this is actually more generally an ester hydrolysis reaction now this reaction can occur in the presence of either an acid or a base but I want to concentrate on the base promoted ester hydrolysis because it has some specific implications for these triglycerides that we've been talking about that I want to build towards so the base the base promoted ester hydrolysis so to give us a head start on this on this base promoted ester hydrolysis I went ahead and I kind of drew us in a backbone here for this reaction mechanism and I'll I'll just try to go ahead and walk us all the way through it so starting off starting off what we have is an OHA up right here an O H group this is going to be our base it comes in and it nucleophilically attacks this carbon so this is a nucleophilic addition reaction and when these electrons came here that forces the electrons some of the electrons from the carbon oxygen double bond to kick up to the oxygen which the oxygen pretty readily accepts these electrons it's pretty electronegative as a as an atom so in this tetrahedral intermediate we end up with an an and I excuse me at this oxygen this is our our carbon anion tetrahedral intermediate but these electrons end up coming back down to reform this this carbon oxygen double bond because the double bond is a little bit more stable and it forces these electrons from this oo our group to push out and this o our group becomes a leaving group so this would be step one we kind of have the addition of this Oh our group and in step two here we have the oo our group which is now got an extra pair of electrons leaves so you can see in this step RRR o our group which would be negatively charged has left and what happens is is this o our group now is a really strong base it's got this extra pair of electrons it was a good leaving group but it's a strong base and we formed a carboxylic acid you see here we've got a carbon double bonded to an oxygen and then with an OHA up and this is a strong acid so in this in this third and this that would be the second step in this third step we have an acid base proton transfer so this so our group this strong base is going to pick up this proton and these electrons are going to move back onto this pretty electronegative oxygen which was willing to give up this hydrogen because it's such a strong acid and you end up with a product of a carboxylic excuse me a carboxylate anion so we've got that negative charge and then our alcohol so this would be the base promoted ester hydrolysis reaction and you can see that this is the bond that we're breaking we've broken that bond right here now in our body this base promoted ester hydrolysis that allows us to break up these these triglycerides is the starting point for all of our fat metabolism reactions so if we want to take a fad and metabolize and break it down so that we can get the energy out of it remember that we had nine Killick Al's of energy if we want to metabolize fats I'm going to write that up here as one of the uses of this base promoted ester hydrolysis reaction we're going to start with this mechanism right here so in the cells of our body were able to use specific enzymes called lipases so lipases lipases to facilitate this reaction so that we can break down fats and eventually get energy out of them and that reaction is hugely significant in our bodies but another application for for this reaction is actually in the creation of soaps so so so then what happens is when you use a strong base like NaOH so maybe like in a oh is sodium hydroxide as this base promotion you end up with a sodium a sodium cation at the beginning of this relationship that's where this hydroxyl base group comes from from the sodium hydroxide and you end up with this sodium cation that just kind of floats through this reaction unused until the very end because what you have is a cation here and an anion this is a polyatomic anion this carboxylate anion and it links up with the sodium cation to form an ionic salt this is an ionic bond so we've got a salt here now if the if the carboxylate anion in the salt that you just formed happens to be from a fatty acid which is the case in this triglycerol would remember these are individual these are three fatty acids then you have a really neat property in the product because this R group right here is actually a really long tail remember with the fatty acid tails this is I'll just go ahead and draw a whole fatty acid here so this this might be an example of the of the carboxylate anion with a fatty acid you have this really long carbon tail and then you have this sodium cation that comes in if this is the salt product which again is the case with the triglycerol then you have a long nonpolar section here for the tail so nonpolar with a really polar head in this ionic bond and this allows this salt which is a soap to dissolve both nonpolar molecules and polar molecules so they the nonpolar side is going to wash away it's going to dissolve things like grease and oil and then the polar side is going to allow this mixture to wash away with water because the polar side can dissolve in with water and this is exactly what allows you to take the grease which gets all over your hand when you're working say on a car or on a bike and wash it away with soap with heavy soaps so this soap actually kind of leans towards the the reaction named for a base promoted ester hydrolysis which is saponification kind of a long word but saponification this is exactly what saponification is referring to the base promoted ester hydrolysis reaction