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when you are trying to determine between a substitution and an elimination reaction it's important to consider the function of the reagent does your reagent function as a nucleophile or does it function as a base so first let's look at nucleophile let's consider the idea of charge we know that water can function as a nucleophile we have a region of high electron density around the oxygen the oxygen is partially negative because oxygen is more electronegative than hydrogen so these hydrogen's are partially positive so water can function as a nucleophile however it is a weak nucleophile because we do not have as high a region of electron density as we would with the hydroxide ion so the hydroxide ion is a strong nucleophile it has a full negative charge on the oxygen instead of only a partial negative charge next let's consider the polarizability of the nucleophile let's compare the hydroxide ion with hydrogen sulfide so we've already seen the hydroxide ion is a strong nucleophile with its negative charge on the oxygen the hydrogen sulfide also turns out to be a strong nucleophile even though it doesn't have a negative charge on the sulfur and that's because of the concept of polarizability which is related to the size of the atom the distance of electrons from the nucleus sulfur is a larger atom than oxygen and we have several electrons that are far away from the nucleus and since those electrons are far away from the nucleus the nucleus doesn't have as much of a pull on them and it's easier to polarize those electrons so it's easier for let's say a pair of these electrons to function as a nucleophile to get closer to an electrophile and so that's the reason why hydrogen sulfide is a strong nucleophile hydroxide we've already seen is a strong nucleophile the electrons are closer to the nucleus so the nucleus has a stronger pull on them so it's not as polarizable but it still turns out to be a strong nucleophile because it's negative charge when you get to something like the hydride ion this is very small we know that hydrogen is the smallest atom so these two electrons are pretty close to the nucleus so the nucleus is a very strong pull on them so because this electron cloud is so close to the nucleus it's not polarizable even though it has a negative charge on it so this is not this does not function as a nucleophile so the hydride ion can't function as a nucleophile because it's not polarizable when you're trying to figure out the nature of the reagent there are four different categories and you want to assign your reagent to one of those four categories the first one is where your reagent acts only as a nucleophile and not as a base and a good example of that would be the chloride anion so can act as a nucleophile because we have a negative one a charge here we have a region of high electron density but it can't act as a base and think about why the conjugate acid to the chloride anion would be HCl just add an H+ to CL minus and you get HCl and we know that HCl is a strong acid and we also know that stronger the acid the weaker the conjugate base so the chloride anion is a very weak base and that's why it's the only going to function as a nucleophile in our reactions so the same idea for the bromide anion and the iodide anion and then we also have our sulfur nucleophiles which we just saw the this hydrogen sulfide here is a strong nucleophile because of the polarizability of sulfur but these are also only going to function as a nucleophile in our reactions and that's because the conjugate acids are fairly acidic so the same reason we talked about over here the second category is when your reagents only functions as a base and not a nucleophile an example of that would be the hydride ion we've already seen that why the hydride ion does not function as a nucleophile but now let's talk about why it's a strong base if you think about the conjugate acid - H - just add an H+ and you of course get h2 we know that h2 is a very stable molecule which makes it a very weak acid and the weaker the acid the stronger the conjugate base which makes the hydride anion a very strong base and if you see it in a reaction think based only as the reagent you would get the hydride ion for something like sodium hydride so NH would be your source another example is this molecule which has the abbreviation D B n so DB n functions as a base only and not a nucleophile you might think a lone pair of electrons on nitrogen could function as a nucleophile but not when you have not when you have this fused ring system here that would be too bulky and prevents this from acting as a nucleophile it does act as a base though and let's figure out which nitrogen gets protonated is it this sp3 hybridized nitrogen or this sp2 hybridized nitrogen it turns out to be the sp2 hybridized nitrogen and let's look at why so I'm going to make this lone pair of electrons magenta and that lone pair of electrons is going to pick up a proton and form a bond so that lone pair turns into this bond here and now this nitrogen would have a +1 formal charge so plus one formal charge on this nitrogen this is the conjugate acid to dbn and this conjugate acid is resonance stabilized I could push these electrons into here and then push these electrons off onto the nitrogen and it lets us follow those electrons let's make these electrons red so this lone pair of moves into here to form a double bond and then let's make these electrons in here blue the blue electrons come off onto this nitrogen and we still have a bond to our hydrogen in here which moves the formal charge to this other nitrogen so this nitrogen now has a +1 formal charge so our conjugate acid is resonance stabilized the positive charge is delocalized over to nitrogen and because our conjugate acid is resonance stabilized it's not very acidic which means that the conjugate base is strong and that's why this is a strong conjugate base even though it's a neutral molecule so protonation occurs at the sp2 hybridized nitrogen and not the sp3 if he tried proton the sp3 you wouldn't be able to delocalized the positive charge over both nitrogen so there's a similar molecule to DB n which is abbreviated DB u and an ax in the same way only as a base in a reaction our third category is where our reagent is a strong nucleophile and a strong base and a good example of that is the hydroxide ion we've already talked about why the hydroxide ion is a strong nucleophile we notice from experience that hydroxide is a strong base so something like sodium hydroxide is used all the time in general chemistry if we replace the hydrogen with an alkyl group we form an alkoxide ion which functions in a similar way to the hydroxide ion so they're both examples of strong nucleophiles strong bases our fourth and last category is weak nucleophile weak base and the water molecule we know is a weak nucleophile it does not have a negative 1 formal charge on the oxygen and water of course is a weak base so the conjugate acid would be h3o plus right just add h plus 2 h2o and you get h3o plus and we know the hydronium ion is fairly acidic so it would have a weak conjugate base if you replace one of the hydrogen's with an alkyl group then you form an alcohol which is also a weak nucleophile and a weak base