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Current time:0:00Total duration:6:12

Video transcript

the last video we saw formation of an enolate anion and one of our mechanisms in this video in the next one we're going to go in much more detail about how to form eeling anions and what base to use for example this space right here is the ethoxide anion which you could get from sodium ethoxide so na + OE T - and this could act as a base and take a proton from an aldehyde or a ketone in this case we have an aldehyde so this is assets aldehyde here and we need to find the alpha carbon so the Alpha carbon is the one next to the carbonyl carbon so this is the alpha carbon right here on acetaldehyde there are three hydrogen's attached to that alpha carbon so we have three alpha protons so our base could take any one of those three alpha protons I'm just going to draw one in here to simplify things and so we could show our base taking this proton and leaving these electrons behind on our carbon so we can draw the enolate anion that would form so we have our carbonyl here and then we could show these electrons on this carbon now so let me go ahead and follow those electrons so in magenta these electrons move off onto this carbon to form a carb anion remember there's also - two other hydrogen's attached to that carbon there and we could draw a resonance structure for this alright so we could show these electrons in magenta moving in here to form a double bond push these electrons off onto the oxygen and if we do that right we'll have a we'll have a resonance structure showing the negative charge this time on the oxygen so the oxygen gets a negative 1 formal charge now and then we have a double bond over here on the right and so this would be our other resonance structure so the electrons in magenta moved in here to form our double bond and then let's make these electrons in here blue right move off on to the oxygen to make an oxyanion right so this is our enolate anion here so we have the the carb anion form and then the oxy anion form of our enolate anion so your enolate anion here which is once again extremely important for reactions we'll talk about later so our base has formed our enolate anion if you think about what happens to the base right if you protonate ethoxide you're going to form ethanol so we'll go ahead and show ethanol also formed here in this reaction alright so we have an equilibrium here between our aldehydes and our enolate anion and to figure out which direction is favored we can we need to know some PKA values so the PKA value for this aldehyde is approximately 17 and the pKa value for ethanol is approximately 16 and so one way to figure out which which directions favorite is to use these these equations down here we could first find the PK eq by taking the pKa of the acid on the left so the acid on the left is our aldehyde so the pKa is 17 and from that number we subtract the pKa of the acid on the right the pKa of the acid on the right is 16 which is ethanol so 17 minus 16 gives us 1 and then to find the keq we can take 10 to the negative pkeq you so 10 to the negative 1 is equal to 0.1 which is obviously less than 1 and so we know that the equilibrium favors the reactants right the equilibrium is back in this direction so the equilibrium favors formation of the aldehyde so at equilibrium we're going to have some aldehyde presence we're also going to have some some NH anion present here so if you choose sodium ethoxide as as your base you're going to have both the aldehyde and your eagle 8 anti and present another way to figure out which direction the the equilibrium goes is to think about which one is the weaker acid so we have these two acids here let me go ahead and change colors we have a pKa of 17 and a pKa of 16 the lower the pKa the more acidic something is so ethanol is more acidic than our aldehyde and the equilibrium favors formation of the weaker acid and so since the aldehyde is the weaker acid the equilibrium favors formation of this weaker acid so that's another way to think about which direction for the equilibrium alright what if you wanted to completely make your enolate anion so one thing you could do is add a base like a hydride here we have once again acetaldehyde here so we have acetaldehyde this time our base is the hydride anion so we could get that from something like sodium hydride na plus h minus or potassium hydride k plus h minus and so for once again we identify our alpha carbon which is this one with three alpha protons so we can just go ahead and draw one of them in there and we could show our our hydride anion functioning as a base taking this proton leaving these electrons behind on that carbon so let's go ahead and show the enolate anion that results alright so we have our carbonyl here and then we have our electrons on this carbon giving it a negative 1 formal charge so electrons in magenta move out on to this carbon alright forming the carb anion and just to save time I won't draw in the oxy anion but that's the one that's actually a greater contributor to the resonance hybrid here so we have our carb anion and then we would also form hydrogen gas right so if this hydride anion picks up a proton right we would form a ch2 and so let's let's show those electrons so the electrons in red here on our hydride anion pick up this proton and forming this bond and so we get hydrogen gas which would bubble out of solution and since this is going to bubble out of solution we're going to drive the reaction to completion so we're going to we're going to push the reaction to completion and so we're going to get our enolate anion so we're pretty much going to get complete formation of our enolate anion here so in the next video we're going to talk about another base called Lda which can also give you complete formation of your enolate anion