Reaction of Esters

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Reaction of Esters
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This is the third (and final) quarter of the organic chemistry series. Topics covered include: Fundamental concepts relating to carbon compounds with emphasis on structural theory and the nature of chemical bonding, stereochemistry, reaction mechanisms, and spectroscopic, physical, and chemical properties of the principal classes of carbon compounds. Index of Topics: 00:22 - Reactions of Esters 01:22 - Reaction of Esters with Water: Hydrolysis of Esters 01:56 - Acid Catalyzed Hydrolysis of Esters 09:22 - Base Promoted Hydrolysis of Esters 32:43 - Application: Lipid Hydrolysis 37:20 - Percent Fatty Acid in Triacylglycerides 37:56 - Reaction of Esters with Alcohols: Transesterification
Separation process Ionenbindung Ester Reaction mechanism Carboxylate Chloride Hydroxyl Atomic number Methoxygruppe Hydrolysat Hydrocarboxylierung Setzen <Verfahrenstechnik> Walking Veresterung Adamantane Chemical reaction Kupplungsreaktion Alcohol Hydroxide Wine tasting descriptors Lone pair Hydrogen Gesundheitsstörung Acid Computer animation Acid anhydride Functional group Etomidate Base (chemistry) Cobaltoxide Methanol
Sense District Activation energy Ester Reaction mechanism Carboxylate Resonance (chemistry) Acetic acid Hydroxyl Pathology Methoxygruppe Molecule Acid dissociation constant Hydrolysat Mixture Conjugated system Oxide Hydrocarboxylierung Setzen <Verfahrenstechnik> Grading (tumors) Saponification Walking Alkoxide Carbon (fiber) Hybridisierung <Chemie> Chemical reaction Alcohol Kupplungsreaktion Hydroxide Wine tasting descriptors Food additive Acid Epoxidharz Gesundheitsstörung Computer animation Metabolic pathway Functional group Materials science Base (chemistry) Carboxylierung Cobaltoxide Soap Thermoforming Isotopenmarkierung
Sense District Acyl Ester Reaction mechanism Carboxylate Carbon (fiber) Alkoxide Chemical reaction Alcohol Grade retention Hydroxide Nucleophilic substitution Molecule Lone pair Gesundheitsstörung Computer animation Electron Metabolic pathway Addition reaction Cobaltoxide Isotopenmarkierung
Prolyloligopeptidase Chain (unit) Chemical property Resonance (chemistry) Lye Methoxygruppe Umesterung Ethanol Enzyme Electron Lipide Öl Omega-6-Fettsäuren Walking Veresterung Solution Chemical reaction Wine tasting descriptors Fat Hydroxide Toxicity Acid Glycerin Chemical compound Cobaltoxide Stuffing Methanol Ethylgruppe Biosynthesis Ester Stereochemistry Reaction mechanism Carboxylate Hydroxyl Wursthülle Methylgruppe Lipasen Sodium hydroxide Hydrolysat Schweflige Säure Conjugated system Setzen <Verfahrenstechnik> Hydrocarboxylierung Oxidans Carbon (fiber) Kupplungsreaktion Amrinone Computer animation Functional group Teilentrahmte Milch Salt Base (chemistry) Soap Primer (film)
Computer animation
so what we talked about last time it's easy to go downhill in these reactions it's usually easy to go from a more reactive type to carbonyl to a less reactive one if you want to go to a less reactive carbonyl to a more reactive one
you want to go uphill you're going to have to use special reagents or forcing conditions okay so forcing conditions we're going to see both the examples of both of those okay so for esters so so based on that yeah easy to go downhill we can go back and forth between the ester and the carboxylic acids and when we come up with talk about carboxylic acids you'll see that reaction so we could take an ester we can hydrolyze it to make a carboxylic acid well that's the one we will have to drive the equilibrium so that's this one right here we can easily go back and forth here we get also good downhill we can go from an ester to an amide but going uphill from an ester to an anhydride or to an ester to a to an acid chloride can't do okay so let's talk about some let's talk about the reactions at esters can do certainly hydrolysis if we hydrolyze an ester we get a carboxylic acid so we're going to have an acid catalyzed and a base catalyzed and you need to know both of them okay the base catalyzed is not actually really catalyzed we'll talk about that in just a minute but the acid catalyzed is catalytic acid so h3o plus catalytic here and our product is a carboxylic acid and notice we get when we hydrolyze that we also will get methanol so that's from this piece right here you'll also get that so this is a this is has a special name it's called a no no that's not that's a hydrolysis I'm jumping ahead here to fast okay so this is a popular mechanism for exams by the way this reaction and the reverse reaction taking a carboxylic acid plus an alcohol to make an ester those are very those are popular mechanisms that I put on exams either one or the other it's very likely for which midterm not this one no not one oh man don't you're going to fill up my inbox here yeah no for midterm 2 okay so you just definitely want to keep on top of that all right so acid catalyzed mechanism what's the very first step that we do protonate the carbonyl make it more electrophilic so I'm going to Prout that fruit neat that with hydronium ions we're going to break the hydrogen oxygen bond alright so what we've done is we've made that a souped up electrophile the carbonyl is much more electrophilic now so that even a weak nucleophile can attack our weak nucleophile is water and we know that's a weak nucleophile don't make the mistake of using hydroxide ion here because we wouldn't expect to see hydroxide ion in an acid catalyzed mechanism so we're going to have water attack all of these steps are reversible all right so here's our tetrahedral intermediate here so as you can see we're part of the way there we have a tetrahedral intermediate with two hydroxyls and a methoxy group if we're going to be going from the ester to the carboxylic acid we need to lose the methoxy group I didn't show this protonated so that would look like that okay so the water is going to stay so we deprotonate the methoxy is going to go so we protonate the methoxy and that's two different steps so first we'll deprotonate here all right so there we have two hydroxyls and a methoxy in an acid catalyzed mechanism the leaving group is always going to be protonated before it leaves so we're going to protonate the methoxy group that comes from the ester we're trying to go from the ester to the carboxylic acid so notice we need lone pairs just on the every acting adams and sapling they want lone pairs on everything but for me I just want lone pairs on the reacting atoms all right so our that's going to be our leaving group methanol is going to be our leaving group not methoxy we don't want to make methoxy in an acid catalyzed mechanism all right so methoxy group is ready to go pronating it makes it a much better leaving group so electrons at either one of these oxygens it does not matter can come down and kick off that group and you can see that after we do that we are we are only one step away from our carboxylic acid in the last step we deprotonate so that that actually regenerates our catalyst you only need a catalytic amount here so we get our ester I mean the carboxylic acid plus methanol plus methanol which we kicked off in the previous step plus our regenerated catalyst h3o plus questions on that mechanism anybody yes they are and that's why you have to drive the equilibrium because each one of these oxygens right here is equally likely to be protonated so if we protonate this one instead we're going to go start going back this direction if we protonate this one instead and then you know a third at the time you'll get this one a third of the time you'll get this in a third of time you get this and so that's why you have to drive to equilibrium any one of these steps you could go all the way here voom voom voom voom voom and then go backwards at this last step or you could go right here and backwards in here and and that sometimes feels like that when you know if you've heard the same take two steps forward and one step back and sometimes it feels like that way when you have a goal any what any one of these steps can go back again so we wanted that's why we need to drive to equilibrium what would be a way to drive this equilibrium here what do you think based on Liz Shockley its principle I mean I wrote the balanced equation here what do you think easiest way to use a large excess of water right okay so you want a lot more water around than nothing all around okay that's a good way to drive it okay more questions anybody all right so a couple points I want to make about this carbonyl oxygen is in the first step is pro need to make it more electrophilic so that a weak nucleophile water can attack and so we've made this point several times that a protonated carbonyl so here's a protonated carbonyl it is more electrophilic than a carbonyl that's not protonated we talked about why that is in discussion last week all right so we and we also just talked about how we're going to drive the equilibrium excess water is one way or the other thing is you can do and
there's more than one more than two ways but the main thing is we can remove the alcohol as it's formed sometime that's a good way to do that that would be another way to drive the equilibrium for this one but you definitely need to drive it or you're going to end up getting about a 50/50 mixture of carboxylic acid and ester all right that's the acid catalyzed so now notice I say base promoted here so it's not really catalyzed because you need a full equivalent of base one equivalent of bass and you'll see why when we do the mechanism here this reaction has a special name it's called saponification it's how you make soap this is the reaction that's used been used for probably what centuries right to make soap and so we'll talk about that in just a minute the different types of soaps alright so the overall the product is carboxylate not a carboxylic acid is a carboxylate mold and and actually formation of this drives the equilibrium and then we have to add acid in a second step formation drives the equilibrium we'll talk about why that is and then in a second step is that's when you have to add acid so it's two steps here and when you add an edge to h3o plus in the second step you get the carboxylic acid alright so let's go through that mechanism this is another possible mechanism very popular type mechanism for midterm 2 alright so base catalyzed mechanism do we protonate the carbonyl first no we've got it we got a strong nucleophile hydroxides a good nucleophile we don't need to protonate the carbonyl we're not going to protonate the carbonyl so we're going to attack directly just like we did with the POC sides base catalyzed app oxide ring opening we attacked directly and then we protonate it later all right so we have an alkoxide this is looking more like chapter 20 right we have the strong nucleophile attacking we've got to two possible leaving groups we have a methoxy and a hydroxy are those about the same leaving group ability pretty much they're pretty close so half the time we'll have the methoxy leave half the time we'll have the hydroxy leave ok the hydroxy leaves we go right back to starting material but it's reversible so the hydroxide can attack again half of the time we're going to have half the I mean half the time we're going to have the hydroxy leave half the time we're going to have the methoxy leave if we have the methoxy leave this is what happens this is going to actually bring us to the product now most students would like to stop there can we stop there we have this plus we just kicked off from a foxy group are those two regions going to react with each other absolutely very highly favored so what's going to happen is the methoxy group is going to come and deprotonate the carboxylate you cannot stop that from happening so if you only add catalytic hydroxide to this you will get a catalytic amount of carboxylate and the rest will be unreacted starting materials you have to use a full equivalent alright so why does this drive the equilibrium let's I'm going to draw a little bit of an energy diagram here why does formation of this our carboxylate drive the equilibrium well we know what the energy diagrams look like for a relative energy we know that if we haven't let's draw a little one a little miniature energy diagram here we start with an ester here they're at the same energy level approximately right ester and acid and we know we have to drive the equilibrium so when we do acid catalyzed we use we have two two different ways to drive the equilibrium in the base catalyzed reaction as soon as the acid forms it's going to be deprotonated and so what happens is what's carboxylates way down here right it's the most stable of all of the carbonyl compounds so when we do this this second step here where we come down and we form the carboxylate it's going to stay because what what energy would it require to go back again we'd have to come up with this much energy all the way up to here to go back again right that's enormous lehigh so this is a good example of the acid gets deprotonated and falls into the thermodynamic well it's down at the bottom of the well and it can't come back out again because it has to come up with this much energy to go back okay so let's write the energy for the reverse reaction EA reverse reaction right here enormous energy of activation of the reverse reaction is huge not too huge okay huge energy of activation for reverse reaction doesn't happen doesn't happen alright so if we go back to our our tetrahedral intermediate here again half the time the pathology group is going to come off half the time the hydroxy group comes off when when the half of the time that the methoxy comes off comes off before the carboxylic acid how fast is this acid-base reaction extremely fast and so it forms the carboxylate falls down into the well and there it stays and so eventually even if we're reversing this reaction as soon as we get back to that last step it doesn't go back again so I really should change this arrow hide it we shouldn't I should we change that arrow what do you think should be just going in the forward direction now don't worry about that on the test because I don't grade that ok once that happens it doesn't tend to go back again so I'm just going to do a forward error in that direction questions on the base mate of what did I say promoted mediated promoted base base
promoted hydrolysis of an ester anybody know okay okay so we're I think we're no longer alarmed we should be alarmed because we have what we have what are those called can't even think of the name I can't think today exclamation points we should be alarmed but we're not alarmed because we've already seen that before and where have we seen that alkoxide as a leaving group we certainly didn't see it in an sn2 reaction but it's it when we see is when we have a really big difference in stability between reactants and products so we don't see it here so we said no in Chapter seven this doesn't work so that's a no product and starting material have similar energies or will just say product and react and have similar energies on the next page we're comparing that with this reaction here we have a really big difference in energy really big difference in energy we're going from a tetrahedral intermediate to carbon and we know that carbonyls are very stable and then of course as soon as we deprotonate this carbonyl and we go to the carboxylate that's even more stable so product much more stable than reactant and then of course the other reaction where we seen in alkoxide as a leaving group was back in 51 B so the sn2 that we can't have happen that was 51 a and this is in 51 B when we open up an epoxy and using a base catalyzed reaction we are having an alkoxide as a leaving group probably didn't register as an alkoxide leaving group but it really is even though it's in the same molecule it still is so here's our o - and as you can see that is an alkoxide leaving group and again product much more stable than reactant whoops gosh so touchy questions anybody so you may have been thinking I have another possible pathway for this reaction how do we know the mechanism goes by way of an addition-elimination because what we could do instead what we certainly could do instead is do this how about this idea possible pathway number two tell me if you see any problems with this okay let's draw the product we get the same exact product so how do how are we supposed to know which pathway is operating this would be here is our alcohol so so here here's our this is more of our why not why not sn2 this would be our leaving group is it a decent leaving group well we if we look at the pKa of the conjugate acid and our conjugate acid is acetic acid what's the pKa of acetic acid rounded to the nearest 5 it's about 5 right so that's a decent leaving group remember our criteria for leaving groups back in Chapter 7 was here you know we want the pKa of the conjugate acid to be less than about 7 or 8 and then it can lead okay so this is definitely certainly well below 7 or 8 so that's a decent leaving group who likes that mechanism do you think well the person that you like it I do too I like it it's decent right so we really need to if we're going to go and write a textbook and tell you what the mechanism is we need to distinguish between those two mechanisms so we want to we want to know that we want to prove that the mechanism that I already showed you which is the correct one is the correct mechanism so here's a couple ways to do this the first clue is labeling studies so we take and we make you know so I've had like labeling reactions where you have label product and this is one of the reasons why you make labelled reactants so that you can probe mechanisms both in the lab and in your body okay so you want to be able to figure out how things are happening in your body so if something goes wrong we can kind of figure out how to how to fix it so on this labeled ester was made and then it was subjected to the reaction conditions and this is what was isolated and what was found was that the label only appears in the alcohol all right so can you see that if the labels only appearing in that alcohol that this mechanism doesn't work let's imagine that this is a labeled oxygen right that's a labeled oxygen we attack with hydroxide we kick that off where's the label label is going to be right here it's going to be on it's going to be on that oxygen it's going to be this sense residence stabilization so it's going to be on either one of these oxygen it's not going to be at all in the alcohol so that rules out this mechanism entirely okay can't have that mechanism so let's go through that let's write the arrows for that okay so boom-boom-boom sn2 we kick that off see where that labels appearing it's going to be in one of these two oxygens these remember this is resonance stabilized we have 50/50 let me put that up higher here we have 50/50 one resonance structure and the other in fact I'll just going to draw the resonance structure here and then we'll go back and put the label on so you can see it a little better label would be in the carboxylate so remember Delhi it's the hybrid of the two so if we go back and we put our label and it's going to be either on the carbonyl oxygen or then with the hydroxide oxygen so here's our label right here 18 right here Oh 18 right here there's none in the alcohol
questions on that so that that means if we only have the label in the alcohol we have a way to detect where the label is can't possibly be that mechanism but maybe we're on maybe we're not satisfied with that maybe we want to do another reaction to disprove that mechanism so the second strategy is to use a chiral ester so you make a chiral ester you subject it to the same reaction conditions is that retention or inversion of configuration what do you think it's the same configuration right the carbon booth at this are here are here that's there it's not the mirror image its exact same so it's it's 100 percent retention of configuration does sn2 give you retention of configuration no one gives you 100% inversion so that means we can't be sn2 so that's another experiment that rules out the other mechanism and this is this is how we pretty much do things when we're trying to figure out what a mechanism for reaction is so this is consistent with the mechanism involving a nucleophilic substitution at the acyl carbon we will need water here also so let's just go through the right the correct mechanism here for this particular chiral alcohol to show you that this the correct mechanism will not invert the configuration of the alcohol so look at we attack the carbon we still haven't touched the carbon of the alcohol so that's addition some of the time the hydroxides going to leave some of the time the alcohol is going to leave we will just show the alcohol leaving because that's our productive pathway going to put lone pairs on this oxygen electrons on oxygen come down and kick off with oxy notice we have not touched the carbon in order to invert that carbon we have to tack the carbon directly in an sn2 reaction we're not ok so then in the line' so so then that gives you carboxylic acid which is very very quickly deprotonated notice that the alkoxide from the alcohol portion is right nearby so it's going to attack very fast I notice that this last reaction that this is going to do does in no way affects the stereo genic carbon so 100% retention to configuration if the MEK is involved nucleophilic substitution you'd get inversion here's the alternate mechanism that's incorrect so we're going to draw this very nicely and we're going to put a big X through it because there is no data that is consistent with this mechanism so chemically it makes sense there's nothing wrong with it is just not operating remember the molecules being behaved by their own rules we try to figure out what those rules are but they're going to do what they want to do whether we like it or not oh look what I did I do that I don't do that okay so forget that try that again an inversion of configuration so I'm going to have this I'm going to have this command opposite right we want to do the sn2 right you don't want to do it like that because and then you cuz then you get it wrong on the test and then
you'd say well that's the way you showed it in lecture and then I'd have to give everybody points back again okay so if you look like that right so a good example of how your your mind doesn't work very well when you haven't had a lot of sleep which is why it's always a good idea to sleep before an exam because you make a lot of mistakes that you don't normally make if you're well-rested okay so that's some hundred percent inversion of configuration alright questions yes excuse me it isn't the same so if you assigned configuration what you can try to do on your own assign of one of these on like maybe make our primer methyl make our double Prime at ethyl and you'll see if you assign configuration here and here that you have inverted okay that brings us back to those stereochemistry days okay application of base promoted hydrolysis is lipid hydrolysis lipids are the most prevalent naturally occurring esters and this is what a choice of glycerol we've talked about these a couple of times here's the R groups our groups have 11 to 19 carbons and so this is a try Oscar so you can see the esters here we have won our group of one side o our on the other our group on one side o our on the other our group on one side o our on the other so three esters so a triester so fats and oils hydrolyze in the body with lipases in the lab we use acid or base hydrolysis so this would follow the same mechanism except you do it three times so so this would be an acid or enzymes are going to give you fatty acids here and of course these fatty acids are oxidized them to produce co2 and water and energy if you do this with bass so here we have enzymes are acids so that's acid catalyzed here hydrolysis of an ester here's base when you do an invasion stead of getting fatty acid you get carboxylate salts with our soaps and for soaps our needs to be greater than or equal to twelve twelve carbons and so as you can see our is a long-chain so carboxylate salts so then you have here's your counter ion here from the sodium hydroxide that you use this would be a sodium salt here alright so that's how we do it type of fatty acid in the length determines unique properties of the various soaps and so there's people that it's really rather an art and rather than a science in making soap I made a very big mistake one year and I offered an extra credit I'm not doing that now so don't email me I offered an extra credit of making soap for extra credit I what I didn't realize is that they don't sell lye so do I use sodium hydroxide they don't sell it in the supermarket anymore so people had to go online and buy it large amounts of it like five grams 10 grams you know 20 grams and not only that and when you make soap you have to have the right amount of hydroxide okay so you need to have one equivalent what if you have more than one equivalent what's the soap going to do to you when you put it on your body the lye is going to kind of eat away your skin right so you have to have the right amount if you don't have enough it won't be completely converted you'll have some of the and you'll have some that's not converted so it's really an art rather than a science so but again I made the mistake it was a larger class than this and we had soaked day when everyone brought their soap and I just can't even tell you I I pictured little bars of soap you know people would make a clear solution we had like classic people would cut off the top of a milk carton and they'd bring in this liquid soupy thing and I'm like what were supposed to do about stuff so um anyway is there anybody here who's made soap before successfully what do you think is it's not that easy to do right yeah so I had people looking up preps online and they were phone online procedures it's not it's not very easy so anyway he agrees with me so I will not be doing that that extra credit any longer but if you want to try it there are some great websites that tell you how to do it
higher chains make a more insoluble soap so really hard soaps longer chains and then shorter chains are better here's the different fats that you would use to make soap and what would actually fatty acids they have in them kind of interesting we have unsaturated have saturated and if you use olive oil it makes a really nice soap called castile soap so you could try to make that on your own but again not as easy to get the reagents to do that okay so that's hydrolysis of esters you can also do something called a transesterification doesn't that sound that sounds pretty cool right that's going directly from one answer to another and you you also have to drive the equilibrium alright so here we have a methyl ester we want to convert it into an ethyl ester so let's draw the product and then we're going to go we'll go through the mechanism here so going from one ester to another transesterification let's go through the mechanism here I don't have a huge amount of room so let's let's let's convert this let's put it on our group here instead to make that a little easier so acid catalyzed mechanism we protonate the carbonyl first unless we're making an Indian or an enemy right toxic acid what does that look like remember that it's in the same family of sulfuric acid and you know the only thing that we've done differently is we have put on an R group instead of one of the hydroxyls what does that do for you why is that an advantage what's the advantage of using at oscillate rather than say sulfuric acid what do you think any ideas so you want to raise their hand yeah well you know okay somebody else want to raise their hand a couple of things we take one of those hydroxyls and we put an R group it's going to be more soluble in organic compounds right more soluble number one number two if we use sulfuric acid that has a little bit of water in it right it's about 96 percent it also has water in it what's water going to do with our ester in acid it's going to convert it into a carboxylic acid right so you're going to have if you have water around and and if you use sulfuric acid you're going to lose what you'll have more water around so that will make some some of your esters going to be converted into a carboxylic acid so lots of reasons why we use this instead so that's toxic acid don't be alarmed if you see that on the test all right so we pronated our carbonyl we've made it more electrophilic sore weak nucleophile can attack our weak nucleophile is ethanol don't make the mistake of attacking with a oxide ion that would not be an acid catalyzed mechanism that would be a base catalyzed mechanism all right we want the ethanol to stay on we wanted the the methoxy group to come off because we're converting this into an ethyl ester alright so let's do that so I'm going to have the conjugate base of toxic acid come in deprotonate so do you think we should does it matter if we protonate deprotonate first and then protonate or so we could we protonate the methoxy group first before we deprotonate what do you think yeah I'm thinking if we if we if we protonate the methoxy and leave this here and protonate the methoxy first and we have double positive charges that's not going to be good so we want to deprotonate this first and then we're going to protonate the methoxy group so that it can leave so in an acid catalyzed mechanism we're always protonating the Leavey group before it leaves and I hope that you're seeing that these guys are all looking very much the same and especially in these reversible ones you have to cut it sometimes you have to step back and say okay where am I going because it's easy to get lost in the middle of the mechanism and go back in the wrong direction alright so here I know that we're going so I'm reminding myself we're going from a methyl ester to an ethyl ester so the methoxy group needs to leave okay so now methoxy group is protonated ready to go electrons on oxygen come down and we kick off that leaving group now in your book they just have the leaving group leave and that's also ok with me that's just it's just a resonance structure of what we have here now let's see what it looks like after that step protonated ester we've gone from one ester to the other and in the last step we deprotonate almost there last step is draw the product so we've just converted a methyl ester to an ethyl ester how would we drive that equilibrium what do you think well there's different ways and probably it helps if I write the methanol over here so you can see one of the ways that we can we can play around with this is - we could use a large excess of ethanol use it's a nice cheap reagent we could use that as solvent and that would make an enormous Lee large excess would drive it to the right we could also remove methanol as its formed not too much we can do with the the esters in this case because they're so similar and physical properties but maybe we can remove the methanol and certainly the easiest way is to use a large excess of ethanol so when you're doing these reversible reactions on the exam if you're using it in the synthesis you should indicate how you're driving the equilibrium okay so you could say large excess or something like that questions did I make a mistake yeah where's the water coming from huh but there's no methanol we're using where there's no water formed here so there is no water here if we use sulfuric acid there would be some water there but with this we're using straight ethanol and this I don't see how water can form here see why I'm not hearing you very well it's just lovely once you come up and I'll explain it to you yeah I'm missing an arrow where that's an all attacking where show me I went stamping to give him a laser pointer oh yeah I missed that arrow see you're gonna do that on the test cuz you haven't had enough sleep and then you're going to just say yeah see look at that okay that's time we'll stop right there
and we'll continue this on Monday you