If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:10:18

Video transcript

in order to see how to form enolate anions in this video we're just going to look in more detail how to form any light ant ions from ketones and so the ketone we have here is acetone to find our alpha carbon right we just look at the carbon next to our carbonyl carbon so this could be an alpha carbon and this could be an alpha carbon each one of those alpha carbons has three alpha protons and so there's a total of six I'm just going to draw one in here and this is the one that we're going to show being deprotonated here so the base that's going to deprotonate acetone we're going to use Lda which is lithium diisopropylamide and i could go ahead and draw on the lithium here so li plus and then we see the the two isopropyl groups like that a negative one charge on our nitrogen so this is a very strong base it's also very bulky and sterically hindered so you can think about a lone pair of electrons on the nitrogen taking that proton alright leave these electrons behind on this carbon so we can go ahead and draw the conjugate base here alright we would have an electrons on this carbon now that's a carb anion so let me go ahead and show those electrons these electrons in here in magenta are going to come off on to this carbon and this carbon is a carbon ion because remember there's also two other hydrogen's attached to it so that's what gives it a negative 1 formal charge here we could draw a resonance structure alright we could show these electrons in magenta moving in here and these electrons coming off on to our oxygen so for our resonance structure right we would show the negative charge is now on our oxygens this would be a negative 1 formal charge like that now so the electrons in magenta moved into here to form our double bond and then we could show the electrons in here and blue moving out on to the oxygen so this is our this is our enolate anion alright this represents our enolate anion and for our two resonance structures we have one with a negative charge on the carbons that's our carbon ion and one with our negative charge on our oxygen that's our oxy anion remember the the oxy anion is contributes more to the overall hybrid because oxygen is more electronegative than carbon so that's our enolate anion that is formed so if we if we form our enolate anion we're also going another product right so if I if I think about adding a proton on to our base right we're going to form an amine so let me go ahead and draw the amine that we would make alright so we would have it would now be this amine so let's show those electrons alright so the electrons over here in red these electrons right here pick up this proton forming this bond and we'd form an amine all right so this reaction is that equilibrium so to figure out which which direction is favors one way to do it would be to calculate the keq and you can see over here on the left these these ways for calculating the keq which talked about in the last video we first need to calculate the pKa q which would be the pKa of the acid on the left so the acid on the left would be acetone all right with the pKa of approximately 19 or sometimes you'll see 20 and so from that number we're going to subtract the pKa of the acid on the right which would be our amine the pKa of this is approximately 36 so 19 minus 36 gives us negative 17 to find the keq all we do is take 10 to the negative of that number so 10 to the negative of negative 17 is the same thing as 10 to the 17th which is obviously a huge number much much greater than 1 and so we know that the equilibrium lies to the right so the equilibrium favors formation of the enolate anions and for all for all practical purposes right this is a huge number you're pretty much going to get complete formation of your enolate anion so if you add Lda to acetone you're going to get again pretty much all enolate anion and none and none of your acetone will remain here all right another way of figuring that out is to know that the equilibrium favors formation of the weaker acid the weaker acid is the one with the higher value for the pKa right over the lower the pKa the more acidic something is so acetone is more acidic than this amine and so since the amine has the higher value for the pKa the equilibrium favors formation of this weaker acid also to the right like that all right so let's say let's talk about the pKa of acetone or a lower ketones in general compared to aldehydes right so this PKA is higher than that for our aldehyde like we saw in the last video and we could think about why by looking at the enolate anion and thinking about the fact that now we have a methyl group here and alkyl groups are electron donating and so if it's donating a little bit of electron density right it already has a negative charge on it so donate a little bit of electron density would destabilize this negative charge a little bit so if the conjugate base is destabilized right that means that acetone is not quite as likely to donate a proton all right so that's the situation when you have when you have a ketone here when we had an aldehyde right we didn't we didn't have this electron donating factor and so that's that's just one way to think about why an acetone why a ketone is not as acidic as an aldehyde so in general all right here's a here's an example where we have a very acidic ketone alright so this is a special kind of ketone called a beta dye ketone so this is a beta dye ketone here and if we look for alpha carbons alright we look for the carbon next to our carbonyl also this is an alpha carbon alright this is an alpha carbon and this is an alpha carbon and so the question is which one of those which one of those alpha carbons is the one that has the most acidic protons it turns out to be the alpha carbon in the center the one between our two carbonyls and so there are two alpha protons on that carbon and so the pKa for one of those for one of those acidic protons is about nine so that's very acidic right much more acidic than than acetone or asset aldehyde like we talked about in the last video and so since since updated a ketone has such acidic protons we don't need a super strong base like Lda we could use something like sodium ethoxide so negative one formal charge here on this oxygen alright so sodium ethoxide could be used to deprotonate this beta dye ketone so we think about lone pair of electrons taking this proton leaving these electrons behind on this carbon so let's go ahead and draw the conjugate base right so we would have our carbonyls here and we took a proton away so now we have our electrons on this carbon so negative one formal charge so these electrons right in here magenta all right moved on to this carbon which gives this carbon a negative one formal charge because there's still a hydrogen bonded to that carbon I'm just not drawing it in so we can see a little bit better all right for resonance structures we could show these electrons in magenta moving into here pushing these electrons off onto the oxygen so we could draw a resonance structure for that alright so for our resonance structure right we would form a double bond here and then this oxygen would get a negative 1 formal charge like that and then our carbonyl on the right is still here like that all right so that's one of our possible resonance structures electrons in magenta moved in here to form our double bond and then we could show these electrons in here moving off on to our oxygen like that all right we could have we could have shown our electrons moving over on this side as well so we can push those electrons off so let's go ahead and draw another resonance structure here so let's get some room and let's let's show the formation of another resonance structure this time our our carbonyl on the left is still there all right we move some electrons into here all right and then we would form a negative one charge on this oxygen so negative one charge right here like that and so let's show those electrons electrons in red move in to form our double bond and then let's show these electrons in here coming off on to this oxygen like that so a total of three resonance structures for this enolate anion and so you can see this negative one formal charge is delocalized writes delocalized over this carbon its delocalized on this oxygen its delocalized on this oxygen so the more you delocalized or spread out a negative charge the more you stabilize the anion so this is a very stable anion because of resonance and also because of conjugation if you think about everything about the conjugation present here all right so here's a double bond and then here's a single bond and then here's a double bond so you have some conjugation stabilizing it as well and so we have a very stable conjugate base for all those reasons that we just talked about and since we have a stable conjugate base this beta dye ketone is likely to donate one of these protons and so that's why it's PKA value is lo alright so if a sock side takes one of these acidic protons we're going to form ethanol as our other product so we can go ahead and draw ethanol in here like that and then we could we could calculate the keq for this reaction all right so ethanol is PKA we have already seen is approximately 16 so if I want to calculate the keq first we find the pKa eq so the pKa of the acid on the left yes on the left is this beta die ketones that'd be that'd be nine from that number we subtract the pKa of the acid on the right which is ethanol so nine minus 16 gives us negative seven so that's the pkeq you to find the keq we take 10 to the negative of that number so 10 to the negative of negative seven is of course the same thing as 10 to the seventh and obviously that's a number much much greater than one and so we know the equilibrium lies to the right favoring formation of the enolate anion here so favoring formation of our enolate anion so for all practical purposes we're going to get pretty much nearly complete formation of our enolate anion and so we're going to get our enolate anion and and that's and that's because even though we we used like sodium ethoxide it's not as strong of a basis Lda but it's enough it's enough to deprotonate our very acidic beta die ketone