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Video transcript

electronegativity is probably the most important concept to understand in organic chemistry we're going to use the definition that Linus Pauling gives in his book the nature of the chemical bond so Linus Pauling says that electronegativity refers to the power of an atom in a molecule to attract electrons to itself so if I look at a molecule I'm going to compare two atoms in that molecule I'm going to compare carbon to oxygen in terms of the electronegativity and to do that I need to look over here on the right at the organic periodic table which shows the elements most commonly used in organic chemistry and then in blue it gives us the Pauling scale for electronegativity so Linus Pauling actually calculated electronegativity values for the elements and put them into the table and that allows us to compare different elements in terms of their electronegativities for example we are concerned with carbon which has an electronegativity value of 2.5 and we're going to compare that to oxygen which has an electronegativity value of 3.5 so oxygen is more electronegative than carbon and the definition tells us that if oxygen is more electronegative oxygen has a greater power to attract electrons to itself than carbon does and so if you think about the electrons in the covalent bonds between carbon and oxygen that are shared they're shared unequally because oxygen is more electronegative oxygen is going to pull those electrons in red closer to itself and since electrons are negatively charged the oxygen is going to get a little bit more negative charge and so it's going to have what we call a partial negative charge on it so partial negative it's partial sign is a lowercase Greek letter Delta and so the oxygen is partially negative it's pulling the electrons in red closer to itself another way to show the movement of those electrons in red closer to the oxygen would be this funny arrow here so the arrow points in the direction of the movement of the electrons in red so carbon is losing some of those electrons in red carbon is losing a little bit of electron density Carbon is losing a little bit of negative charge so carbon used to be neutral but since it's losing a little bit of negative charge this carbon will end up being partially positive like that so the carbon is partially positive and the oxygen is partially negative that's a polarized situation right you have a little bit of negative charge on one side a little bit of positive charge on the other side so it's a it's a still a covalent bond but it's a polarized covalent bond due to the differences in electronegativity between those two atoms let's do a few more examples here where we show the differences in electronegativity so if I we're thinking about a molecule that has two carbons in it and I'm thinking about what happens to the electrons in red well for this example each carbon has the same value for electronegativity alright so the carbon the left has a value of two point five the carbon the right has a value of two point five that's a difference in electronegativity of zero which means that the electrons in red aren't going to move too towards one carbon or towards the other carbon they're going to stay in the in the middle they're going to be shared between those two atoms so this is a covalent bond and there's no polarity situation created here so there's no difference in electronegativity so we call this a nonpolar covalent bond alright this is a nonpolar covalent bond like that let's do another example let's compare carbon to hydrogen so if I had a molecule and I have a bond between carbon and hydrogen and I want to know what happens to the electrons in red between the carbon and the hydrogen we've seen that carbon has an electronegativity value of two point five and we go up here to hydrogen which has a value of two point one and so that's difference of 0.4 so there is a difference in electronegativity between those two atoms but it's a very small difference and so most textbooks would consider the bond between carbon and hydrogen to still be a nonpolar covalent bond all right let's go ahead and put in the example we did above all right where we compare the electronegativities of carbon and oxygen like that when we looked up the values we saw that carbon had an electronegativity value of 2.5 and oxygen had a value of 3.5 for a difference of 1 and that's enough to have a polar covalent bond right this is a polar covalent bond between the carbon and the oxygen so when we think about the electrons in red the electrons in red are pulled closer to the oxygen giving the oxygen a partial negative charge and since electron density is moving away from the carbon the carbon gets a partial positive charge and so we can see that if your difference in electronegativity is one it's considered to be a polar covalent bond and if your difference in electronegativity is 0.4 that's considered to be nonpolar covalent bonds so somewhere in between there must be the difference between nonpolar covalent bond and a polar covalent bond and most textbooks will tell you approximately somewhere in the 0.5 range so the difference in electronegativity is greater than 0.5 you can go ahead and consider to be mostly a polar covalent bond if the difference in electronegativity is less than 0.5 we would consider that to be a nonpolar covalent bond now I should point out that we're using the Pauling scale for electronegativity here and there are several different scales for electronegativity so these numbers are not absolute these are more is more relative differences and it's the relative difference in electronegativity that we care the most about let's do another example let's let's compare let's compare oxygen to hydrogen so let's let's think about what happens to the electrons between oxygen and hydrogen so the electrons in red here alright so we've already seen the electronegativity values for both of these atoms oxygen had a value of 3.5 and hydrogen had a value of 2.1 so that's an electronegativity difference of 1.4 so this is a polar covalent bond since oxygen is more electronegative than hydrogen the electrons in red are going to move closer to the oxygen so the oxygen is going to get a partial negative charge and the hydrogen is going to get a partial positive charge like that all right let's do let's do carbon and lithium now so if I go ahead and draw a bond between carbon and lithium and once again we are concerned with the two electrons between carbon and lithium the electronegativity value for carbon we've seen is 2 point five we need to go back up to our periodic table to find the electronegativity value for lithium so I go up here and I find lithium in Group one of my periodic table it has an electronegativity value of one so I go back down here and I go ahead and put in a 1 and so that's a difference in electronegativity of 1.5 so we could consider this to be a polar covalent bond this time carbon is more electronegative than lithium so the electrons in red are going to move closer to the carbon atom and so the carbon is going to have a little bit more electron density than usual so it's going to be partially negative and the lithium is losing electron density so we're going to say that lithium is partially positive now here I'm treating this bond as a polar covalent bond but you'll see in a few minutes that we could also consider this to be an ionic bond and it just depends on you know what electronegativity values you're dealing with what what type of chemical reaction that you're working with so we could consider this to be an ionic bond let's go ahead and do an example of a compound that we know for sure is ionic sodium chloride of course Bede the famous example so for for the to start with I'm going to pretend like there's an a covalent bond between the sodium and the chlorine all right so I'm going to say there's a covalent bond to start with and we'll put in our electrons and we know that this bond consists of two electrons like that let's look at the differences in electronegativity between sodium and chlorine all right so I'm going to go back up here I'm going to find sodium which has a value of 0.9 and chlorine which is a value of 3 so 0.9 for sodium and three four chlorines so sodium's value is 0.9 chlorines is three that's a large difference in electronegativity that's a difference of 2.1 and so chlorine is much more electronegative than sodium and it turns out it's so much more electronegative that's no longer going to share electrons with sodium it's going to steal those electrons all right so when I redraw it here I'm going to show chlorine being surrounded by eight electrons so these two electrons in red let me go ahead and show them these two electrons in red here between the sodium and the chlorine since chlorine is so much more electronegative it's going to it's going to attract those those two electrons in red so strongly that it completely steals them right so those two electrons in red are going to be stolen by the chlorine like that and so the sodium is left over here and so chlorine has an extra electron right which gives it a negative 1 formal charge so we're no longer talking about partial charges here chlorine gets a full negative 1 formal charge sodium lost an electron so it ends up with a positive formal charge like that and so we know this is this is an ionic bond between these two ions right so this represents an ionic bond and so the the difference in electronegativity is somewhere between 1.5 and 2.1 between a polar covalent bond and an ionic bond so most textbooks will see approximately somewhere around 1.7 so if you're higher than 1.7 generally considered to be mostly an ionic bond lower than 1.7 in the polar covalent range but that's not that that doesn't always have to be the case right so so we'll come back now to the example between carbon and lithium right so if we if we go back up here to carbon and lithium here we treat it like a polar covalent bond but sometimes you might want to treat the the bond in red as being an ionic bond all right so let's go ahead and and draw a picture of carbon and lithium where we're treating we're treating it as an ionic bond right so if carbon is more electronegative than lithium carbon is going to steal the two electrons in red right so I'll go ahead and show the electrons in red have now moved on to the carbon atom so it's no longer sharing it with the lithium carbon has stolen those electrons and lithium is over here so lithium lost one of its electrons giving it a +1 formal charge Carbon gained an electron giving it a negative 1 formal charge and so here we're treating it like an ionic bond full full formal charges here and this is useful for some organic chemistry reactions and so what I'm trying to point out here is you know these divisions you know 1.7 it's not absolute it's a relative thing you could draw the the the dot structure above and would be considered to be correct right you could draw it like this or you could treat it like an ionic bond down here since it's relatively close to the it to the cutoff so this is an overview of electronegativity and even though we've been dealing with numbers in this videos in future videos we don't care so much about the numbers we care about the relative differences in electronegativity so it's important to understand you know something as simple as oxygen is more electronegative than carbon and that's going to help you when you're doing organic chemistry mechanisms