CH240 Chapter 1 Video 4 Resonance Structures

[00:00:01]chapter one I think this is video four it doesn't matter resonance structures and resin structures are extremely important because the more structures you can draw for a given struck the compound or ion the more stable it is some compounds and structures are not adequately represented by a single Lewis dot and that's what we're gonna find out and most of these are going to be ions format in other words there may be two or more possible structures or rearrangement or possible arrangement of the electrons we're not gonna move any atoms and nuclei we're only gonna move electrons a hint when moving electrons I will always circle them usually and then draw the arrow to where they are going there's a general rule if I draw a double arrow they're both going if I draw a single arrow only one is going that is the common language of organic chemistry we use the double arrow between the structures to represent resonance don't confuse it with equilibrium equilibrium was arrow going one direction arrow going the opposite when they're double when they're that's the equilibrium so let's get started so if I have this ion and I'm gonna use colors to represent this carbon is electron deficient the nitrogen is electron rich so these pair of electrons can form a bond so now our resonance structure is C and a double bond nitrogen and now the nitrogen has a plus charge now with most of these they're always has what is known as a minor and a major contributor we're mainly interested in the major in this case this is our major contributor the reason it is is because carbon here has eight electrons around it and so does nitrogen well if we go over here to this molecule this carbon only had six electrons around it nitrogen still had eight so this is our major contributor this is the most likely structure but both of them work together so if I have two for Bob if I have this at ion here it's the acetate ion to be exact I have this oxygen here marked as like radioactive it's a marker for tracing purposes so what can happen here electron pair here can form a double bond then I realized I didn't put my electrons around this one here it has to pair and when that's happening now the carbon has too many bonds to it so one of the PI bonds breaks and now our resonance structure and I need to put my tracer on here because that we want to trace that all the way through as an example sometimes instances oscillates back and forth you may see it drawn this way in this manner with just a negative charge here as it oscillates back and forth so these are resonance structures so if I have methyl nitro methane the same thing is going to apply this nitrogen though right now has a positive charge on it because it doesn't have a pair of electrons so the electrons can form a double bond here when that happens when the PI electrons from the C double bond o break and this is going to be negative charge it's nitrogen we said it's positive here again it is able to oscillate back and forth Conoco remember we're not moving atoms we're moving only the electrons that's all we're interested in so here I have one this is already the major contributor so what can happen now and I got too many electrons on the oxygen not smart Bob there we go I can delete them very easily so one of the miners remember oxygen is highly electronegative it can attract the oxygen the electrons from the carbon so it's resonance structure it's going to be negative and since that is negative the carbon is going to be positive but hopefully we know that this one here is the major because everything has the octet rule so let's look at this molecule this is a type of ether we're missing a proton over here so this carbon has a positive charge so what could happen is carbon is short electrons it wants to have eight pair of electrons here from the oxygen can form a PI bond a double bond since this oxygen now has three bonds it's going to be positive so let's evaluate and see which one of these two is gonna be the major contributor what would you just have to start counting electrons we didn't mess with this carbon this oxygen has eight electrons around it double bond single bond is to another single bond is four six eight this carbon has six electrons around it it wants to have eight so if we go over here this oxygen has eight electrons around it this carbon here also has now eight electrons around it so it is the major contributor to resonance so now I have another one and I notice I got too many oxygen double bond or electrons on my oxygen that's okay so there is the correct structure right now everything has eight around it have electron pair here so this pair of electrons can't form a double bond but now this carbon here has too many electrons so the PI bond will take a pair of electrons and transfer them to the oxygen the question is which one of these is going to be the major contributor because everything here already has eight around it and in fact the major contributor is the structure that we just put up because oxygen is highly electronegative it can take on those electrons kind of cool still remember we're only moving electrons so here I have another one let's see right now this is eight eight and eight so we're good pair of electrons here could form a triple bond but now this nitrogen has two main electrons so these electrons can just go to the carbon and by the way this nitrogen is gonna be positive and it is positive here also so which is our major well listen has two charges where this one only has one this one is our major this one is our minor oh yeah this is a good one this is a good one give you a little bit of time to draw this structure that's five carbon two double bonds and Oh H and an NH 2 we're going to take a little bit of time on this one I can already tell so electron pair here good form a double bond let me just try to straighten this out that carbon is going to be positive now keep my electrons so far so good I'm gonna scroll down here a little bit so now too long a line we can have another resonance structure factors two possibilities off of this one here we can move a pair of electrons from the oxygen to form the double bond now that auction is going to be positive charged so that's another resonance structure there is also another one off of this same structure because of the double bond so what could happen let me erase a little bit here if this doesn't occur oops the other possibility is this PI bond here can break and make a PI bond between with the carbo-cation the positive charge species so now we have CH - I lost my hydrogen here that's okay I have a tendency to lose hydrogen yeah I lost a lot of hydrogen she's gonna hydrogen there also if you notice hydrogen really don't matter its but here we now we have a another positive charge it is moved to the other side and then there's one more this pair of electrons from the nitrogen can form a double bond so now I have my CH to bond it to a CH bond it to C double bond C H oh wait you're still here bond it to CH double bond it to a nitrogen and it has a positive charge and all these are resonance structures and the more resonance structures that you can draw it increases the stability of the molecule which is a good thing this one wasn't the one that you had to realize there is pot two possible structures coming off of it pair of electrons from the oxygen or the double bond PI bond moving over if you notice the electrons never jump well they always happen to the atom right next to them or form a bond right next to them they never jump across the molecule they cannot do that so if we go back now and count this carbon here only had six electrons around it not good this carbon here had six electrons here everything here yes I supposed to have my hydrogen everything here has eight that's fine that is somewhat of a met major contributor when we go here a little bit further this one here has six but when we get to this one here everything has eight it is also a major contributor and those in or increase the stability or likelihood of the molecule so resonance structures when we start moving them around the better it is the more likely the structure it increases the stability we're always looking for major products or major contributors to that resonance structures Khanna deep we're going to do lots of resonance structures in organic chemistry organic chemistry basically is you're an accountant like an accounting class but your accountant for electron