Hey everyone, professor Dave here. I wanna talk to you a little bit about resonance structures and how to assign formal charge. so a very important concept in organic chemistry is resonance, and in order to understand resonance we're going to have to understand pi electrons because pi electrons are the ones that participate in resonance so a pi bond basically, if we remember a double or a triple carbon-carbon bond consists of one sigma bond and either one or two pi bonds, so here is a carbon-carbon double bond one of those bonds is a sigma bond and the other is a pi bond so let's take a look at some orbitals and see what's going on. so let's take this, a flat alkene remembering that carbons that are participating in a double bond are sp2 hybridized, they have hybridized their atomic orbitals into sp2 molecular orbitals because they have 3 electron domains each, the other carbon and then whatever else they're bound to, so let's take this flat molecule and we're gonna flip it like this and view it edge on so these hydrogens are away from us, further past the board, these hydrogens are towards us precisely in a plane perpendicular to the board and then what we're seeing is that the carbon is bound to these other three atoms, the carbon and the two hydrogen atoms by its sp2 molecular orbitals but that leaves an unhybridized p orbital extending in perpendicular fashion up and down in the opposite directions and it is the overlap of these p orbitals that is generating the pi bond this carbon-carbon bond is the sigma bond and then this is the pi bond now the reason this is important is because it is the electrons in pi bonds as well as electrons that are present as lone pairs that are the ones that are going to participate in resonance. so most resonance structures are going to contain atoms that bear a formal charge, so before we investigate these structures I want to review really quickly how we can tell whether an atom will bear a formal charge so take a look at some of these polyatomic ions for example in this structure why is it that that oxygen atom has a formal negative charge localized on that oxygen atom? well in order to tell if an atom's gonna have a formal charge we can look in the periodic table to see how many valence electrons that atom ought to have. now oxygen, if we look at the table is in the sixth column so it should have six valence electrons but here if we were to go ahead and draw in all the lone pairs we could see that this oxygen atom is actually showing 7 electrons in this lewis dot structure because we have 3 lone pairs so that is six electrons and then one of the electrons in this covalent bond belongs to that oxygen the other belongs to this carbon so this oxygen is showing 7 electrons in this lewis dot structure that's one more than its typical valence of 6 one additional negatively charged particle means that this oxygen has a formal negative charge now for example let's look at the nitrogen atom in the nitrate ion now nitrogen ought to have 5 valence electrons but here it is contributing only four to this lewis dot structure because it is participating in four covalent bonds, one here, one here and then two, both the sigma and a pi bond in that double bond so 1 per bond, 4 electrons that this nitrogen is contributing to this lewis dot structure 4 is one less than 5 one fewer negatively charged particles means that this nitrogen bears a formal positive charge so that is the simplest way to tell if an atom has a formal charge a lot of books and teachers will give you this very strange way of using arithmetic and adding and subtracting all these things but in my opinion this is the simplest way, just compare the number of electrons that an atom is contributing to a lewis dot structure to its typical valence and if they're not the same that atom must bear a formal charge so now that we're able to understand that lets take a look at some of these resonance structures now it is the case that pi electrons are delocalizable within a structure that means that if pi electrons can be rearranged to form some other stable resonance structure they will do so. so once again sigma electrons cannot do this because they are electrons that are involved in direct orbital overlap so those are not delocalizable, but the pi electrons, the ones that are the lateral overlap of p orbitals or lone pairs, those are free to move around so for example we've got a negative charge here which is essentially just a lone pair so let's say a lone pair goes ahead and forms a double bond there the only way that this carbon can accept another electron from some other atom is if it then loses an electron, in other words it must lose a bond. it can accept a bond from this electron sorry, from this oxygen if it then loses a bond and those electrons go somewhere else. well this oxygen is just as capable of accommodating a negative charge as this one is so that means that this is an equally valid resonance structure as this one is, so we can have the negative charge on this oxygen or go ahead and form the carbonyl there and bring the negative charge up to that oxygen, so these are two equally valid resonance structures of the same molecule now the convention is to list them in brackets with this double-headed arrow between them, that's just how we do it and then also with regards to these electron pushing arrows those are something we're gonna see a lot more later with some of the mechanism stuff and these are very important to be drawn in a very particular way this must go from electron-rich to electron-poor, in other words from the lone pair to the bond itself. so we will get much deeper into those arrows later. the important thing that we want to understand about resonance structures is that individual resonance structures do not exist. we draw them that way because they help us understand chemistry and how molecules behave but in reality the only thing that exists is a composite resonance structure that represents the distribution of pi electron density around the molecule so that would be this. so the reality of the fact is that there is partial pi electron density distributed all along that portion of the molecule. and that's what truly exists in nature and there's a lot of ways that we can prove this from molecular shape to just electron distribution, a lot of different things but this is absolutely true that it is not the case that these electrons are flipping back and forth really fast between the two oxygen atoms in fact the reality is this composite structure that bears an overall delocalized negative charge. so taking a look at the nitrate ion just as above we can push these pi electrons to form a pi bond here as long as we push the pi bond up to this other oxygen atom that would give us this structure and then likewise the double bond can also exist connecting to that third oxygen so just as above these are three equivalent resonance structures but once again it is important to understand that none of these discrete resonance structures exist, it is actually only this composite resonance structure that shows delocalized pi electron density distributed about all the portions of the molecule that have the delocalized electron density in this case the whole thing so we have an overall formal negative charge that is delocalized about the whole molecule now in the prior examples we were looking at resonance structures that were precisely equally contributing but there could be other examples in which we have valid resonance structures that are not equally contributing so let's take a look at why that might be for here let's see that we have a formal positive charge on this carbon these pi electrons are delocalizable so we could lose this bond to oxygen and neutralize that carbon if this pi bond moves down here oxygen will lose an electron carbon will gain an electron therefore carbon will neutralize, but now oxygen will have a formal positive charge now this is a case where we have the formal charge on two different kinds of atoms depending on which resonance structure we're looking at. because this is a positive charge the atom of the two that is more electropositive will have an easier time accommodating that formal positive charge so we know about electronegativity and we know that oxygen, because of its position in the periodic table is more electronegative than carbon, therefore carbon is more electropositive. because carbon is more electropositive it can better accommodate the positive charge and therefore this resonance structure is more strongly contributing so they are both valid resonance structures however the composite resonance structure would show a skewing of the partial pi electron density closer to the carbon atom or actually in this case more toward the oxygen because the carbon can better accommodate the positive charge. now let's look at precisely the opposite scenario, over here we have a formal negative charge on that carbon so this is an enolate anion that we will see quite a bit later and we can see that that can go ahead and form a carbon-carbon double bond, kicking this pi bond up onto that oxygen atom giving the other resonance structure with the oxyanion and this is precisely the opposite case we have two resonance structures each with a formal negative charge but they are on two different kinds of atoms so here because it's a formal negative charge it's the more electronegative atom that is going to be able to accommodate the negative charge best in this case it's the oxygen so this is the more strongly contributing resonance structure, this is still a valid resonance structure but the distribution of the electron density on the composite resonance structure is going to more closely resemble this resonance structure. and then lastly if we look at an example like this where let's say this pi bond goes and leaves that nitrogen and just plops a negative charge right there on that carbon this is a valid resonance structure however resonance structures with all neutral atoms are necessarily more contributing than other valid resonance structures that have formal positive or negative charges so that's why this is going to be a much more strongly contributing resonance structure overall. and lastly any resonance structure must be a valid lewis dot structure so we can't have a resonance structure where carbon has five bonds or some other thing that violates the rules of Lewis dot structures. so hopefully this clarifies a little bit about resonance and how to draw proper resonance structures thanks for watching, guys. subscribe to my channel for more tutorials and as always, feel free to email me with questions