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Identifying nucleophilic and electrophilic centers

Video transcript
- [Instructor] Nucleophiles and electrophiles are extremely important in organic chemistry mechanisms. So, first let's look at a nucleophile. The word nucleophile means nucleus-loving and since the nucleus is positively charged you can think about a nucleophile as being negatively charged because opposite charges attract. So a nucleophile could have a full negative charge which would be attracted to the positive charge of a nucleus or it could have a partial negative charge or you could just think about a nucleophile as having a region of high electron density. So let's look at some examples of nucleophiles. First let's start with the ethoxide anion here and the ethoxide anion has an oxygen with a full negative charge, so obviously that is a nucleophile and the oxygen is the nucleophilic center of ethoxide. Next let's look at ethanol. Well ethanol doesn't have a full negative charge but we know that oxygen is more electronegative than hydrogen so oxygen is going to pull the electrons in this bond closer to itself, giving it a partial negative charge, so this oxygen is the nucleophilic center of ethanol. Now the ethoxide anion is going to be a better nucleophile than ethanol because it has a full negative formal charge on the oxygen as opposed to only a partial negative. Next lets look at methyllithium. Let's think about the electronegativity difference between carbon and lithium. Carbon is more electronegative than lithium so the two electrons in this bond are pulled closer to the carbon, giving the carbon a partial negative charge, and so the carbon is the nucleophilic center of methyllithium. Since lithium is losing some electron density we could draw a partial positive charge here on lithium and here I've drawn it as a covalent bond but really you could also show it as being an ionic bond. So the difference in electronegativity is so great that I could show both of those electrons being on this carbon, so let me go ahead and put in the hydrogens here. Since carbon is more electronegative than lithium I could take these two electrons in magenta, and I could put them both on the carbon, which would give the carbon a negative one formal charge so this carbon with a negative one formal charge would be the nucleophilic center. I took an electron away from lithium giving it a plus one formal charge here, so here I've represented it as an ionic bond, here a little more covalent character, but this picture is useful because this is called a carbanion, let me write this in here, so a carbanion which just means a negative charge on a carbon and carbanions are excellent nucleophiles. Finally, let's look at cyclohexene and cyclohexene we know has a pi bond. So let me show the pi bond here, and pi bonds are regions of high electron density so this pi bond can act like a nucleophile in an organic chemistry mechanism. Now let's look at electrophiles. So an electrophile is electron-loving and since electrons are negatively charged we're gonna think about an electrophile as having a region of low electron density so it could have a full positive charge on it because a positive charge would be attracted to electrons, or you could be talking about a partial positive charge. So first let's look at this compound. We know that chlorine is more electronegative than carbon. So chlorine is going to withdraw some electron density and if chlorine is withdrawing electron density away from this carbon, this carbon is partially positive. So this carbon is the electrophilic center of this compound. Next let's look at acetone. So oxygen is more electronegative than carbon so oxygen is going to withdraw some electron density away from this carbon here and this carbon would be partially positive, so this carbon is the electrophilic portion of this compound. Next let's look at a carbocation where there's a full positive charge on this carbon so this carbon has only three bonds to it which gives it a full positive charge. Obviously a full positive charge is going to love electrons. Opposite charges attract, so this carbon is the electrophilic portion of this ion. And finally let's look at this compound, right. We know that oxygen is more electronegative than carbon so oxygen withdraws some electron density away from this carbon and we could even draw a resonance structure here, so let me take these pi electrons and move them out onto the oxygen, so let's draw a resonance structure so I put in my double bond. Now if I'm showing those pi electrons moving off onto the oxygen I would need three lone pairs of electrons on that top oxygen giving it a negative one formal charge. I took a bond away from this carbon in magenta which is this carbon which gives it a plus one formal charge, so that's one of the possible resonance structures that you can draw and of course we know the carbon in magenta is an electrophilic center, but I could draw another resonance structure so let me go ahead and do that, put in my brackets over here. I could take these pi electrons, I'll show it on this one actually, these pi electrons and move them over to here, so let's draw the resulting resonance structure. So I'd have a double bond here now an oxygen with a negative one formal charge, so let me put that in here, draw in the hydrogen, put in my brackets, and I removed a bond, we took a bond away from, let me use blue for this, from this carbon, so this carbon now has a plus one formal charge, so the carbon in blue is this carbon over here, so let me draw in a plus one formal charge, so that is also electrophilic, right, a full positive charge is going to be attracted to a negative charge, so this compound actually has, this compound actually has two electrophilic centers, so this carbon here and also this carbon.