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Current time:0:00Total duration:11:41

Video transcript

in this video I want to introduce you to a mechanism called the aldol reaction aldol reaction it's easily one of the most important mechanisms and reactions in all of organic chemistry because it's a powerful way to actually create carbon-carbon bonds and it'll actually be a little bit of a review of what we saw with enol and the enolate ions and the the keto enol tautomer I have trouble saying that anyway let's start with let's start with a couple of aldehydes and just for convenience I'll make them identical so let's say that this is one of the aldehydes it just has a carbon chain there that's what the R is it could be of any length who knows what's there and then I have another carbon here and then this is bound to the carbonyl group and we're going to make it an aldehyde although you could do this reaction with a ketone as well and just to make things clear this carbon right here which is going to be involved in a lot of the business here let me draw its hydrogen's normally we don't have to draw its hydrogen's and just as a bit of review the carbon that is next to the carbonyl carbon is called an alpha carbon if this was a ketone this would have also been an alpha carbon if this was a carbon we're going to see in this reaction besides just exploring the reaction is that these hydrogen's are actually much more acidic than traditional hydrogen's attached to carbons on the rest of the chain and it comes from the fact that this this proton can be given to something else the electron can go to that carbon and then it'll be resonance stabilized and we're going to see that in a second now I said I would draw two molecules of that because we need two molecules actually going to be to some degree joining the two molecules so let me draw another aldehyde right over here and I'm going to draw it symmetric to this because it'll make it very it'll make it I think a little bit easier to visualize the two molecules actually let me just draw it let me just draw it the same way so I'll draw in a different color so you have the R group and then you have the oxygen right there and I won't I won't draw all of the hydrogen's on this guy but this and this are the exact same molecule it's just the hydrogen's are implicit here now the aldol reaction I'll show you will be in a basic environment so you could imagine that we have it'll be catalyzed by I am bass and so imagine we have some hydroxide laying around some of the hydroxide anion hydroxide we do that in a different color so let's say we have some hydroxide anion floating around negative charge just like that I just told you that these hydrogen's are much more acidic than hydrogen's anywhere else on a carbon chain these alpha hydrogen's so you could imagine a situation where an electron from the hydroxide is given to one of these hydrogen protons and then then the electron that was associated with that hydrogen is now given back to this alpha carbon and so if that were to happen the next step in our reaction would look like this and I'll draw it in equilibrium actually let me draw it this way so the next step in our a or the the products of that step would be in equilibrium with you have your carbon chain or cart the rest of your molecule right there's and that's just the show that could be anything it's attached here to the alpha carbon which is now going to be negative I'll show that in a second which is attached to the carbonyl group which is attached to a hydrogen and actually I'll I'll stop drawing that hydrogen for now to just we know it's there but I'll keep drawing this hydrogen right over here the other hydrogen was taken away and this alpha carbon now has a negative charge because it it got the electron from that proton and of course we have the hydroxide grabbed it grabbed this hydrogen and it is now it is now water now the reason why this was the reason why this was acidic to begin with is because this is resonance stabilized and I'll show you that it's resonance stabilized right now this alpha carbon right here can give its electron to the carbonyl carbon and if the carbonyl carbon gets an electron it can give an electron to this oxygen up here it'll break the double bond so this configuration is resonance stabilized is resonance stabilized with this so I could draw it like this you have your our and then you have a single bond to this oxygen it now gained an electron it is now negative and you now have N and you now have a double bond just like that and I could draw this hydrogen if I like or I don't have to it's implicitly over there now and you might be familiar with this this is the enol eight anion this right here is the enolate enolate this is the enolate ion if we had a hydrogen right here it would be an enol and we would say hey this is the keto form this is the enol form we've seen this before now what's interesting about the enolate ion is it can act as a nucleophile it can do a nucleophilic attack on the other on the other aldehydes carbonyl group but it does it in kind of a non-conventional way and I'll show you how it does it right now so it does the attack like this so let me draw this guy over here so you have the carbonyl group carbonyl group and then then you have its alpha carbon and then you have an R group right over there there's actually a hydrogen right over here as well I just flipped it over this and this are the same molecule but and let me make it clear these two guys right here are resonance forms and this is once again this is the reason why it was it's easier to take this hydrogen than other hydrogens on a traditional carbon chain easier to take an alpha hydrogen to a carbonyl group because you have this resonance structure but this enolate ion especially this configuration of it you can imagine it doing something like this you can imagine this oxygen giving back the electron to the carbonyl carbon to this carbon right here and when that happens then this then this guy this guy is going to be giving up he's going to be giving up an electron and that electron that he gives up let me do it in a new color this electron that he gives up could go and do a nucleophilic attack on this carbonyl group and so of that if that carbonyl carbon gets let me do this in a new color if this carbonyl carbon gets an electron then it could give away an electron to that oxygen right up there so the next step after this we would have something like this and once again I'll show it as happening in equilibrium so from here we go right over there and what we have is a situation where let me draw this guy on the Left first so we have a double bond to this oxygen now actually let me draw let me draw the second so this is this oxygen we now have a double bond let me do it in this same purple color right over here and then we have the rest of what was an aldehyde what was an aldehyde where you have let me do it in that same color in that same color and then you have your R group right over there but now this electron get an attack on this other aldehyde so this guy right here this alpha carbon is that same alpha carbon we've been dealing with the same alpha carbon is now bonded to this carbonyl carbon so it is now bound to this carbonyl carbon right over here and so it will look like this it will look like let me draw it with the right colors get the orange out so that carbonyl carbon it now has a single bond to this oxygen this electron was taken back by it so this oxygen now has a negative charge and it is bound to its alpha carbon its alpha carbon and then that is that is bonded to another another group another probably a chain of a carbon chain or something that contains a carbon chain or another functional group whatever you want to call it and then the final step the final step this anion can get rid of its negative charge by essentially grabbing a hydrogen from maybe this maybe from this water that was formed before were obviously not going to be the same molecule but it could grab it from this in a previous step this Mount water molecule that was formed in a previous step and of course this is all in a basic environment so it can give an electron to this hydrogen and then the hydrogen would lose the hydrogen proton would lose an electron to the hydroxide and the hydroxide will become negative again and so what will be the final product the final product will be the final product and I'm just going to try my best to redraw this thing right over here you have this part of the molecule so you have this carbonyl group right over here it is attached to this radical group right over there so that is this part and I can even do the same colors this bond right over here is this bond right over here and then this this carbon is attached to a carbon that's attached to a hydroxyl group now so it'll look like this and let me draw it so this oxygen is now this oxygen and it just captured this hydrogen so it is now a hydroxyl group it's now and O H it is now an OHA up and then finally this guy is bound to what was an alpha carbon it's not anymore what was an alpha carbon which is then bound to a radical group and if we want we can remember that there was always from the get-go there was always a hydrogen over here so why is this called the aldol reaction why does it matter well it's called the aldol reaction because what we formed is both an aldehyde notice this is an aldehyde and it's an alcohol so that's where the word aldol comes from but the more important thing about this but you know and and I don't want to mislead you you could have also have done this with a ketone you could have had a methyl group or a ethyl group you could have a big carbon chain here it still would have worked so the aldol reaction doesn't only form things that are aldehydes and alcohols it could have formed something that's both a ketone and an alcohol but that's why it's called the aldol reaction but the more important thing about the aldol reaction is one it shows you how the enol a dyon can be a nucleophile it shows you why the alpha hydrogen's are more acidic than hydrogen's on other parts of carbon chains but the most useful aspect of it is it's a useful way to actually join two carbon chains together notice we were able to join this alpha carbon right here to this carbonyl carbon over here to form this aldol or sometimes this will be called because is still an alpha carbon right here this is an alpha carbon this is a beta carbon and so you'll sometimes this will be referred to as a beta hydroxy and we've probably used things from the from convenient from the from the pharmacy that that has this word in it this is also called a beta hydroxy this alpha this is beta has a hydroxyl group on the beta carbon beta hydroxy aldehyde beta hydroxy aldehyde anyway hopefully you found that entertaining