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Current time:0:00Total duration:9:12

Video transcript

sometimes one dot structure is not enough to completely describe a molecule or anion sometimes you need two or more and here's an example this is the acetate anion and this dot structure does not completely describe the acetate anion we need to draw a resonance structure another resonance structure and so what we're going to do is take a lone pair of electrons from this oxygen and move that lone pair of electrons in here to form a double bonds between this carbon and that oxygen at the same time we're going to take these two pi electrons here and move those PI electrons out onto the top oxygen so let's go ahead and draw a resonance double-headed arrow here and when you're drawing resonance structures you usually put in brackets let's go ahead and draw the other resonance structure so now there would be a double bond between this carbon and this oxygen here this oxygen on the bottom right used to have three lone pairs of electrons around it now it only has two because one of those lone pairs moved in to form that pi bond the oxygen on the top used to have a double bond right now it has only a single bond to it and it used to have two lone pairs of electrons and now it has three lone pairs of electrons that gives the top oxygen a negative one formal charge and make sure you understand formal charges before you get into drawing resonance structures so it's extremely important to understand that all right so next let's let's follow those electrons is to make sure we know what what happened here so these electrons in magenta all right moved in here to form our PI bond like that and the electrons over here in blue moved out on to the top oxygens let's say those electrons in blue are these electrons like that so we have two resonance structures for the acetate anion and neither of these structures completely describes the acetate anion we need to draw a hybrid of these two and so if we take a look at let's say the oxygen on the bottom right here we can see there's a single bond between this carbon and this oxygen if we look at this one over here we see there's now a double bond between that carbon and the oxygen so think about a hybrid of these two resonance structures let's go ahead and draw it in here we can't just draw a single bond between the carbon and that oxygen there's some partial double bond character there so it's a it's a hybrid of the two structures above so let's go ahead and draw on a partial bond here like that the exact same thing for the top oxygen here we have a double bond and then over here we have a single bond so somewhere in between right so in between is going to be our hybrid so let's go ahead and draw that in so we can't just draw a single bond in our hybrid we have to show some partial double bond character drawing the dotted dotted line in there like that and also charge right so we think about charge the negative charge is on the oxygen on the bottom right and then over here the negative charge is on the top oxygen and so that negative charge is actually delocalized right so it's not localized to one oxygen it's delocalized it's distributed evenly over both of those oxygens here and so this is just one way to represent the hybrid here and studies have shown that the hybrid is closer to what the actual anion looks like so studies have been done on these bond lengths here and the bond between the bonding between this carbon and this oxygen it turns out to be the exact same bond length as the bond between the carbon and this oxygen so it's exact same bond length and so the hybrid again is a better picture of what the anion actually looks like let's think about what would happen if we just moved the electrons in magenta in so if I go back to the very first thing I talked about and you're like well why didn't we just stop after moving these electrons in magenta let's go ahead and draw what we would have if we stopped after moving the electrons in magenta so we would have this so the electrons in magenta moved in here to form our double bond right if we don't push off those those electrons in blue this might be our resonance structure the problem with this one is of course the fact that this carbon here five bonds to it so one two three four five so five bonds so ten electrons around it we know that carbon can't exceed the octet of electrons because of its position on the periodic table so this is not this is not a valid structure and so this is one of the patterns that we're going to be talking about in the next video so the pattern is a lone pair of electrons all right so next to a PI bond which is the example we see here for the acetate anion and so these are the two resonance structures the problem with the with the word resonance is when you're a student you might think that the anion will resonate back and forth between this one and this one that's just kind of what the name seems to imply and that's not actually what's happening it's just that we can't draw if we're just drawing one dot structure this is not an accurate description and so the electrons are actually delocalized so it's not resonating back and forth when you draw resonance structures in your head think about think about what that means for the hybrid and how the resonance structures would contribute to the overall hybrid so don't forget about your brackets and your double-headed arrows and also your formal charges so you have to put those in when you're drawing your resonance structures all right let's look at an application of the of the acetate anion here and the resonance structures that we can draw so if we look at the acetate anion so we just talked about the fact that one of these lone pairs here so this is not localized to the oxygen it's delocalized so we can move those electrons in here we push those electrons off onto the oxygen we can draw a resonance structure and so this negative one formal charge is not localized to this oxygen it's delocalized and so because we can we can spread out some of that negative charge that increases the stability of the anion here so this is this is relatively stable so increased stability do 2d localization alright so the fact that we can draw an extra resonance structure means that the anion has been stabilized and so this is called this is called pushing electrons right so removing electrons around and it's extremely important to feel comfortable with moving electrons around and being able to follow them so the only way to do this is just is to get good at this is to do a lot of practice problems so please do that do lots of practice problems in your textbook if we compare that to the ethoxide anion so over here all right if we try to do the same thing if we try to take a lone pair of electrons on this oxygen and move it into here we can't do that because this carbon right here already has four bonds right so it's already bonded to two hydrogen's and then we have this bond and this bond and so moving those electrons in trying to delocalize those electrons would give us five bonds to carbon so we can't do that we can't draw a resonance structure for the ethoxide anion so those electrons are localized to this oxygen and so this oxygen has a full negative 1 formal charge and since we can't spread out that negative charge it's going to destabilize this anion all right so this is this is not a stable so decreased stability compared to the anion on the left because we can't draw a resonance structure if we think about the conjugate acids to these to these bases right so the conjugate acid to the acetate anion would be of course acetic acid so we go ahead and draw in acetic acid like that the conjugate acid to the ethoxide anion would of course be ethanol so we go ahead and draw in ethanol and we think about which one of those is more acidic right we know that acetic acid is more acidic it's more likely to donate a proton because the conjugate base is more stable because of you can think about resonance right or delocalization of electrons if you're looking at ethanol ethanol is not as likely to donate its proton because the conjugate base the ethoxide anion is not as stable because you can't draw any resonance structures for it the negative charge is not able to be delocalized it's localized to that oxygen so this is just one application of thinking about resonance structures and again do lots of practice and the next video will talk about different patterns that you can look for and we talked about one in this video we took a lone pair of electrons so right here in green right and we noticed this lone pair of electrons was next to a PI bond and so we were able to draw another resonance structure for it we don't have that situation with ethoxide we have a lone pair of electrons all right but we don't have we don't have a pi bond next to it and so again more in the next video on that