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Identifying nucleophilic and electrophilic centers

Video transcript

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 a 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 as a full negative formal charge on the oxygen as opposed to only a partial negative next let's look at methyl lithium let's think about the electronegativity difference between a 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 methyl lithium 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 any hydrogen's 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 1 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 1 formal charge here so here I've represent represented it as an ionic bond here a little more covalent character but this this picture is useful because this is called a carb anion let me write this in here so a carb anion which just means a negative charge on a carbon and carbon ions 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 electrophile so an electrophile is electron loving and since electrons are negatively charged we're going to think about an electrophile as having a region of low electron density so it could have a full positive charge on it so the positive charge will be attracted to electron or you could be talking about a partial positive charge the 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 of 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 next let's look at a carbo-cation 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 1 formal charge I took a bond away from this carbon in magenta which is this carbon which gives it a plus 1 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 here so let me go ahead and do that put in my brackets over here I can take these pi electrons 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 have a double bond here now an oxygen with a negative 1 formal charge let me put that in here draw in the hydrogen put in my bracket 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 1 formal charge so the carbon in blue is this carbon over here let me draw in a plus 1 or formal charge so that is also electrophilic right a full positive charge is going to be attracted it to a negative charge so this compound actually has this compound has to electrophilic Center so this carbon here and also this carbon