so we've gone over PKA and we've seen that PKA that are lower are going to have more of a tendency to act as an acid than substances with higher PKA right so if you have compare between a pka of five and a pka of 15 the one that's more likely to act as the acid is the one with the lower pka of five right so why why does a certain substance have more of a tendency to act like an acid and and another substance has much less of a tendency to act as an acid right that's the question we're going to be asking ourselves today so why is an acid an acid basically and so what we're going to be doing is we're going to be looking at not just the acids but its conjugate base and the reason why this is important is because the strength of an acid is determined by the stability of the conjugate base and this is because the more stable the conjugate base is the stronger the acid is going to be for example if we look at the relative acidities of elements in group two or sorry row two on the periodic table we can see methane ammonia water and hydrofluoric acid now this is going to be the strongest acid out of this Bunch well why is that the case right we're just going across the row and adding hydrogen's here why is this the strongest acid and the reason is because its base right fluoride the fluoride ion is going to be the most stable and a stable base is going to readily bear the electrons that it used to share with that hydrogen with that proton so we could say here that stable bases are weak bases right they're okay having extra electrons each of these after it loses a proton has that negative one charge but fluide is most okay with that negative one charge and this methyl anion is the least okay with that negative charge and for this particular case what it has to do with is electro negativity and we can see that on the next slide so when we compare the electr negativities of carbon nitrogen oxygen and Florine we can see that Florine is the most electronegative atom and so therefore because Florine is the most electron netive it's going to be the most okay with having those extra electrons remember that electro negativity means like electron hungry right pulling those electrons towards itself and so Florine being the most electronegative the most okay with extra electrons the fluoride ion is going to be the most stable conjugate base followed by the hydroxide ion the one with oxygen followed by the amide ion the one with nitrogen followed by the methyl anion the one with carbon the least electronegative out of that bunch and the least okay with having that negative charge and so because this is the least electr negative it's going to be the least stable base or you could say the most unstable and so that's why we have hydrochloric acid being the strongest acid because its base the fluoride ion is the most stable because of electro negativity the most Electro negative the most okay with that negative charge and again oxygen is not as electronegative as fluoride it's the Florine so hydroxide is not as stable as the fluoride ion so it's water is going to be more acidic sorry less acidic excuse me less acidic than hydrofor acid because it has a less stable conjugate base it's more likely its base is more likely to grab a hydrogen than the base of hydrofluoric acid than fluoride okay so one thing I want you to memorize and this is going to be important later is that saying that that I said on the previous slide right the stronger the acid the weaker the conjugate base so strong acids have weak conjugate bases and weak acids have very strong conjugate bases it has a strong tendency to grab electrons sorry to grab protons because it's not happy with the proton with the with having a negative charge right because stable bases are weak bases fluide is the most stable based on electr negativity most okay with having that negative charge and the methyl annion is going to be the strongest base and the least acidic because it's the least okay with having those electrons it's the least electr negative out of that Bunch so because they're all in the second row we have similarly sized atoms and the main difference here then is the electr negativity so the most electr negative of similarly sized ions is going to be the most stable base and therefore it's conjugate acid the one where a hydrogen is added to it is going to be the most acidic okay so this is the most stable base and therefore this is the strongest acid this one is the least stable base and therefore this is the least acidic the weakest of these two acids so we can ask ourselves why are alcohols stronger acids than amines if we compare the pka of alcohols it has a pka of about 15 whereas the pka of amines are around 40 and the reason is because of the electr negativity so comparing the pka values of the alcohols and amines we can see that more electronegative atoms are going to be stronger acids because it's a hydrogen coming off of this oxygen here and a hydrogen coming off of this nitrogen here so because oxygen is more electronegative than nitrogen alcohols are more acidic than amines looking at the base after a hydrogen has been lost we have ch3o minus and ch3 NH H minus okay so comparing the stabilities of these bases this one is the most stable because it's the most okay with having this negative charge whereas this nitrogen is the least stable here because it's not is okay with having that negative charge and again this is due to electr negativity because oxygen is more electr negative it more readily bears that negative charge than the nitrogen does we can also say the same thing for protonated alcohols versus protonated amines again a similar concept here is because that the oxygen is more electronegative than the nitrogen the base of the protonated alcohol right which is ch3oh is going to be a more stable base than the base of the protonated methylamine the conjugate base of the protonated methylamine which is ch3 nh2 okay so we are getting some extra electrons here a hydrogen's leaving each of these grab a lone pair have a lone pair left and so the oxygen again handles electrons better even though we don't have a negative charge it still handles those electrons better because it's more electronegative than the nitrogen does hence the difference in these PKA values other things we have affecting um the acidity related to electro negativity is actually hybridization so hybridization affects electr negativity and we've already seen that Electro negativity affects the acidity of a compound so hybridization actually affects whether something is going to be more acidic or not and so what we can say is that an SP hybridized atom is more electr negative than that same atom that is sp3 hybridized that's the least electronegative and the SP2 hybridization is somewhere in the middle there so why is that the case right remember we have to consider the bases and considering the bases here after a hydrogen comes off what we see is that the difference in these bases is the difference in the amount of negative charge so an SP hybridized atom because it's closer to the nucleus actually bears that negative charge a little bit better than one that's sp3 hybridized because the negative charge is on average aage farther away from the nucleus okay so here electr negativity right pulling electrons towards itself because we have closer to the nucleus here again we have a more stable um conjugate base and so the pka notice of this triple bonded carbon is actually about 25 and that's fairly low that's even lower than an Aman so this is actually actually um it's not super strong it's not a super strong acid right but it's actually a stronger acid than an amine is so please remember these approximate PKS as well at for triply bonded carbons about 25 at about 45 for double bonded carbons SP2 hybridized so SP SP2 and about 60 or greater for sp3 hybridized carbons right so to summarize this slide UL Ely what we're saying is that because this is sp hybridized the electrons are pulled closer to the nucleus and it stabilizes this carbon anion it stabilizes the conjugate base and because the base is more stabilized this makes the acid quote unquote a stronger acid here because those electrons are not as close to the nucleus right it's not as stabilized and we have a very weak acid because we have a very unstable conjugate base so we've talked about electr negativity we've talked about hybridization and the next thing we're going to talk about is size so size is going to affect PKA values when we look at these halogens what we see is a difference in the size of those ions because of where the electrons are located when the veence electrons are located iodide is the largest with the veence electrons being found in the fifth principal energy level and fluoride is the smallest with its veence electrons being found in the second principal energy level so when we're comparing atoms that are very very different in size the size is actually more important than the electro negativity in determining how stable that conjugate bases in other words we can say how well it handles that negative charge so ult Ely what's happening is that as we go down a column on the periodic table the atoms get bigger right we see the atoms getting bigger here and what happens is that the stability of the anion increases so the iodide ion is the most stable the fluoride ion is the least stable the reason is because we have a very big atom and there is more space to disperse that negative charge over okay so in our electrons here that are in those five sp3 the fifth principal energy level sp3 hybridized orbitals versus fluoride which are in the second principal energy level in sp3 hybridized orbitals these bigger orbitals give that negative charge more room to spread out so as we have an increase in size we have an increase in stability as well because its negative charge is spread out over a larger volume in other words we could say that its electron density is going to decrease okay so ultimately because this is the most stable base this would be the strongest acid so out of these halogens and their acids out of these hydrogen halides hydrochloric acid would be the weakest because it has the least stable conjugate base and hydrogen iodide would be the most stable the strongest acid because it has the most stable conjugate base okay so larger sized atoms are going to be stronger acids When comparing atoms of different sizes so we can see this reflected here in some of the pka values this is related to electro negativity right as the electro negativity increases the stability of the base increases and the pka decreases it becomes more acidic as we go across the row but as we go down a column what we see is the acidity increasing because the stability of that base increases as well because again going down the column what's more important is size and spread spreading out that negative charge and so these are for the acids but what you can see is even after that hydrogen is lost after this blue positive charge is lost you see more of a negative charge here that would be more throughout the molecule and less of a negative charge in these bigger atoms so stronger acids as we go down the column and stronger acids as we go across the row here we can also see this reflected in H2O and H2S so hydrogen sulfide would be a stronger acid than water because again this sulfur atom is bigger so when a hydrogen is lost there's more room for those electrons to spread out in the veent Shell of the sulfur atom the next thing to concert ourself with here and understand the effect that a substituent has on acid and so this is an effect called inductive electron withdrawal so ultimately what you can see here is we have a carboxilic acid and the acidic hydrogen is this one on the oxygen that's the one that's going to come off when we look at the pka we see that acetic acid has a pka of 4.76 and then when we attach an atom to a group that is not bound to this acidic hydrogen the one that's going to come off ultimately what we see here here is the acidity is actually going down so we compare 4.76 without anything on it to let's say fluoride being on it and we have a pka of 2.76 so the acidity increases when we have something bound to it that's pretty electr negative so what's happening here like why is this guy more acidic than this guy the Florine isn't actually attached to the hydrogen that's coming up off so why does it matter and so it does matter because this fluoride or these other atoms are electronegative they pull electrons towards themselves and so ultimately what we have here is that the electrons don't leave their bonds okay they're still between these two atoms but they're going to be pulled like this this let's say this Florine here let's take that for an example is pretty Electro negative so it's going to pull those electrons towards itself that's going to create somewhat of a positive charge on this carbon and that carbon's not going to be super happy with the positive charge so it's going to pull some of those electrons towards it from the other way and then that carbon's going to have somewhat of a positive charge and it's not going to be happy right so it's going to pull some of those electrons in the other Bond towards itself right so then ultimately when we get to the oxygen the amount of electrons on it is isn't a full negative charge anymore when that hydrogen leaves so when the hydrogen leaves we don't have a full negative one charge on It ultimately what we have is just somewhat of a negative charge right because those electrons are being pulled through those Sigma bonds towards the towards the most electronegative atom and so we have this Effect called inductive electron withdrawal that dep depends on the electro negativity of the attached atom the more Electro negative that attached atom is the more it's going to pull those electrons towards itself and the more it's going to relieve the conjugate base so whenever that hydrogen leaves instead of having that full negative one charge it's going to have only somewhat of a negative one charge right so it's going to be easier for that hydrogen to leave when those electrons are somewhat pulled away from it so you could kind of say it takes the pressure off of that oxygen hydrogen bond and makes it easier for the hydrogen to leave okay so summarize this inductive electron withdrawal is when you have an electronegative atom attached somewhere in the molecule that electronegative atom is electron hungry it pulls the those electrons towards itself through Sigma bonds kind of pulls each electron in the sigma Bond towards it and then ultimately what it does is it makes it easier for that hydrogen to leave so it stabilizes the conjugate base by decreasing the amount of electron density on the atom that is remaining okay so that's why acetic acid is actually less acidic than acetic acid with a fluoride attached to it now this effect does depend on distance right the farther away that electronegative atom is the less acidic it's going to be and the less that effect is felt so if the electro negative atom is closer to where that hydrogen is being removed from we have a greater effect than if that atom is farther away right you you the the pull is less because it has to go through all of these bonds to get to that oxygen so we can now say why a carboxilic acid is stronger than an alcohol one of those reasons is what we just talked about that's a inductive electron withdrawal so we have here no inductive electron withdrawal happening in an alcohol that oxygen when the hydrogen leaves has to Bear the full brunt of that negative charge there's nothing to pull that charge away nothing to relieve it but here when we look at a carboxilic acid we have this carbon oxygen double bond and that can pull those electrons away kind of towards the itself over here which decreases the amount of electron density on this oxygen and makes it easier for that hydrogen to leave right the decreased amount of electron density stabilizes that oxygen and makes it so that it doesn't feel the full extent of that negative charge it makes it a little bit less okay so a carboxilic acid is stronger than an alcohol for one reason of inductive electron withdrawal and the second reason that we're going to look at on the next slide is because of something called electron delocalization so resonance and when we look at resonance ultimately what we're saying here is that when we lose a hydrogen on the alcohol there is no other place for those electrons to go right they're stuck on this oxygen they can't move because there's no other Lewis structure that we could write that would put these electrons in various places right we see according to the electron density map those electrons being stuck on that oxygen but let's look at the the carboxilic acid and what you can see here is that when that hydrogen is lost we have those electrons now being able to spread out and so we can write two equivalent Lewis structures for the um conjugate base of a carboxilic acid and ultimately remember what that means is the structure is neither of these two but it's somewhere in between those two and so we can draw this resonance High hybrid and what you can see here is that negative charge is no longer exclusively on one atom that negative charge is spread out over both of those oxygen atoms so both of those oxygen atoms have 1/ half of the charge of the oxygen atom in the alcohol and so when we look at this electron density map what you see is not as much negative charge because it spread out over two oxygen atoms and that stabilizes the molecule keep in mind and this is going to be true for multiple things we look at this semester that anytime you can spread out charge it's going to stabilize a molecule okay so anytime you can have delocalization anytime you can have resonance anytime you can spread out that charge and stabilize a molecule it's going to be more acidic so a carboxilic is stronger than an alcohol because of inductive electron withdrawal pulling those electrons away and then also this delocalization where the negative charge is spread out between these two oxygen atoms and this is probably more of that important effect than the inductive electron withdrawal so to summarize we talked about five factors that affect well why we call acids acids why some things are easy to lose hydrogens from and other things are much more difficult to lose hydrogens from one of those factors was electro negativity right the more electronegative atoms better handled that negative charge and so more electronegative atoms were more acidic we looked at size and we saw that larger atoms had that electron density spread out over a larger area after the Hy was lost and so that size plays a role in acid strength hybridization matters because SP hybridized atoms are going to be more electr negative than sp3 hybridized atoms okay so hybridization due to electr negativity affects acidity of compounds inductive electron withdrawal like pulling those um electrons away stabilizes the conjugate base right by decreasing the amount of electron density and electron delocalization also stabilizes the conjugate base by spreading out that negative charge that's resulting from a hydrogen being lost right it spreads out that negative charge and increases the stability of the conjugate base which then increases acidity so you will need to know all of these five factors that affect acid strength and be able to explain them okay they're going to be very important in understanding why acids behave as acids because now we understand these five factors and how they affect acidity by understanding how they affect the stability of the conjugate base okay okay because a more stable conjugate base means it's happy it's okay being how it is and thus that conjugate acid right that acid will be a stronger acid will have a greater acidity we'll work through these inclass problems in the next class period please let me know if you guys have any questions