hello class we are going to continue our discussion on pkas and acid-base chemistry here and the first thing or what I want to talk about here is let's say we have ethanol and we compare that to isopropyl alcohol like that now we've learned that we have PKA tables that will tell us the pka of these alcohols however the tables in your textbook are very limited in scope because there are who knows like just so many molecules out there that you cannot have a PKA table for every single molecule out there and so how are you going to know what a pka of a acid is when it's not on a table one way you can do that is you could go to the table and find let's say this is the molecule that we want to find the pka for right there but when we go to our table that molecule is not there so how can we get a rough estimate of what the pka of this alcohol is well what we do is we go to our table and try to find a molecule that resembles the same functional group so we notice that this is an alcohol so we will go to our PKA tables and find an alcohol and then we will try to find an alcohol that the structure resembles our structure of resembles the molecule of Interest and so we go to the pka table and we see hey ethanol looks very similar to isopropanol clearly they're different but similar so we go to the pka table and we find that this guy has a pka of 16. and so we can look at this molecule and say it is approximately 16. and so that is what you're going to have to be able to do when you go and you see a molecule that's not on the pka table you need to find something that's very similar and then you can say it's roughly 16. when we actually calculate or do the experiment to figure out the pka of isopropanol it is 16.5 but will you know 16.5 no you won't because it's not on your PKA table so that's one way to do it another example would be let's say we have benzoic acid okay there's our benzoic acid what is the pka of that guy is not in the bka table so we've got to go to the table and find something that looks similar to this and so when I look at this molecule what functional group do I have a carboxylic acid so you're going to go and find a carboxylic acid and the one that may pop up on your table is acetic acid and acetic acid looking at my notes here has a pka of 4.75 so you could say benzoic acid has a pka of approximately 4.75 do you see what we're trying to do here okay but then this is an approximation but you're going to run into situations where this strategy does not work uh so for example you could have a molecule that looks like this what if we had a molecule that looked like this okay so that's TFA trifluora no no that's not TFA so when we look at this molecule we're like hey what is the pka of this and we look at the molecule and we say hey the functional group here is a carboxylic acid so we would be like hey go to our PKA tables and and we'll find hey acetic acid it has the same functional group as this and then we're like okay it's the pka table says that's 4.75 so we would approximate this to be 4.75 but this is not a good approximation because the real answer for this guy the pka is zero point seven seven do you see how they're not similar at all well why does this one not work well because we have different atoms here that uh they're not carbon and hydrogens typically when you have an acetic acid or sorry a carboxylic acid assorts the alkyl chain doesn't change it too drastically but the moment you start adding electronegative atoms like chlorine fluorine or anything it's going to mess it up so what we have to do now is the pka table is very very limited so the better way to approach figuring out what a PKA is or a rough estimate of a pka of acids is to learn some guiding principles and it's these guiding principles that will help you to compare if you had let's say acetic acid versus this guy the pka tables aren't going to help you very much okay because these other atoms right here so I'm going to teach you these guiding principles that won't actually give you a a hard fast number of what the pka is for this molecule but knowing the principles what you can do is look at these two molecules and say this one is more acidic than that one you don't have a PKA table so we're going to learn principles that we can apply when we don't have a periodic table or sorry a PKA table or when the pka table doesn't help us we'll use guiding principles okay so what we're going to take a look at is those principles okay so let me clean up this board and we can get started okay so here are the principles that we are going to discuss and these principles are going to be able to help you determine when you look at this Amine and this Amine which one is the most acidic and we're going to use this acronym here called the cardinal rule to help us figure out which one of these is the most acidic so what we have here I'm not going to write it out I'll just talk about it so the cardinal rule stands for it the principles that we need to learn and apply to situations like this C stands for charge a stands for atoms r d so we'll that's one one we put those two together okay our R is for resonance and D is for delocalization okay and the i n right there stands for inductive effects so you learn about resonance in gen chem you've learned it in my class you've learned about induction in gen chem and you're going to relearn it with me but we're going to apply these principles to help us figure out what's going on so we're going to start with the charges all right so there's this general idea General principle that when you have let me get to it here so I can say it accurately so here's the principle these protons right here the proton is more acidic when it is attached to an atom that has a positive has a Char positive charge on it so you see here's this hydrogen it's attached to a nitrogen which has a positive charge this hydrogen is attached to a nitrogen which is neutral So based off of that principle this molecule right here is going to be more acidic because the hydrogen is attached to an atom with a positive charge now when we go and look at the look up the pkas here we will see that the pka of this amine is 36 and the ammonium right here is 9.4 so those are the pka values that we can find on the internet and we can see that this is clearly the most acidic so what is it about charge that determines if something is acidic or not well we can look at it energetically here so I'm going to draw an energy diagram here and let's take a look at keep this all color coded here when you have reactants that are neutral they're generally low in energy here so we will say that's our NH3 plus water all right generally lower in energy and when you have a reaction here with a positive charge somewhere in the reaction that's going to make it higher in energy and so that's where our nh4 is going to be plus water so it's higher in energies that's our y-axis here higher in energy and so what's going to happen is that when you do this reaction here do you see what mechanism we're using our Waters the base the ammonia is our acid and so what's going on here uh we're it's a proton transfer reaction proton transfer mechanism now what happens is when this reacts it's going to have an energy activation here and be let's redraw this here we'll look something like this and so over here that would be our products right there and let's see here we want to get this right all right and that would be our nh2 minus plus h3o Plus hi so if as when we look at these energy diagrams here what can we see on the reactant side right here we can see that the positively charged species are higher in energy on the pr on the product side on the right we can see that this in Orange has a positive and a negative so two charges that's why it's higher in energy than this one in pink because this one has a neutral and a positive so it's slightly lower in energy and so the reason why this guy has a a smaller PKA or is more acidic is because look at the energy diagram here look at that energy of activation right here the energy of activation for the pink line is only that tall so it can go over that Hill a whole lot easier than if you take a look at the orange one where you and you can see that it is from there to there a much bigger Hill to climb and so this proton is influenced by the energetics of the system this proton's like hey yeah it's easy to take my proton take it take it it's fine whereas this guy's like I like it I'm low in energy I'm okay with it please don't take it and it has a much harder time giving it up because it has such a high energy of activation it just takes two it takes a lot of energy to rip that proton off so that's the first thing that we need to understand is that charged species are going to be higher in energy right and when you look at a positively charged atom in the reactants we can see it's going to be higher in energy easier to rip off the proton and hence makes it more acidic okay so let's take a look at a different idea now so what we have here are some molecules here with their corresponding pkas and you can see methane right here has the largest PKA and hydrofluoric acid has the smallest PKA so that makes hydrofluoric acid the stronger acid now we could condense this to just looking at the these atoms right here we could look at it like this when you go to your periodic table notice that this is the order in Period two you see carbon nitrogen oxygen fluorine in that way so what we can do is make this general idea here when looking at these molecules here that as you go further to the right you're going to see that the acidity increases so pay very close attention what are we talking about we are comparing the acidity of this proton and that proton and as you go further to the right on the periodic table when you're comparing acids in the same period you can see carbon nitrogen oxygen and fluorine are all in the same period you can make this General principle here or general statement foreign at a deeper level why this is the case you can memorize Trends but if you can learn principles then that is going to save you a lot of hassle so what if we have this I'm going to write out two reactions on the board and then I will come back so what we're going to take a look at is we're going to compare methane and ammonia and look at their pkas well we already know their pkas we already know ammonia is more acidic but we want to understand why that's the case so what you can do is you can look at this reaction and this reaction in the the reactants and we can see they're all neutral so we can draw a energy right there and we can say hey it's that's going to be both of them so we have NH3 plus water and and then we have the methane so what I'm trying to demonstrate here on this energy diagram is that because both of these reactions here are neutral in their reactants they generally have the same energy level yes this orange one is above and this one's below that's not what I'm saying both of these reactions are on this green energy level because they're neutral and yes we are making an assumption but it's okay to make this essential assumption that when you have different species but if they're all neutral they relatively have the same energy okay now if we expand out our table here expand that out when we look at the reaction of ammonia what do we see we now have positive um and negative we have charged species on the product side but whenever you have a Charged species system here they're going to be a different energy levels only when they're neutral do we assume they are at the same energy level but when they are at when they are different molecules with charges there's going to be a difference and what we see here is the nitrogen is going to be about here so that's our nh2 minus plus water and then this is going to be right here ch3 minus plus water not water hydronium there so what we have going on here is energy diagram that kind of looks like this and then the blue curve like that so what do we see in this energy diagram we can see that the conjugate base for ammonia is lower in energy than the conjugate base of methane so you can see that ammonia here is going to be more acidic more willing to give up its proton because look at the orange curve has a lower energy of Activation so it's easier to get rid of that proton but then it begs the question here how did I know this conjugate base is higher in energy than this conjugate base how did I know that okay well I will get to that in a minute but let me make one statement here that you need to write down because it's really important when you are comparing two assets and you want to know which one's the most acidic what you do is you figure out their conjugate basis so here we have our ammonia and our methane I draw out what their conjugate bases are okay and the conjugate base that is the most stable is going to give you the most acidic molecule so when I look at the energy diagram here comparing these two conjugate bases I find that this guy right here in Orange is lower in energy so it's a more stable conjugate base than this one so that means because this conjugate base right here is more stable than that conjugate base makes the acid right here more acidic so let me read it to you one more time so when comparing two uncharged acids the stronger acid is the one who's negatively charged conjugate base is more stable that is a true statement now now let's talk about why and how do we I know that these two have different energy levels even though they have the same charge the answer to how do I know this guy is higher in energy than this guy comes to is because of electronegativities so on the periodic table what is the most electronegative atom it's in the top right corner it's fluorine right and so what we have going on here is that when you look at the conjugate bases from the three at four acids that we're looking at the atoms here the electronegativity increases as we go from left to right so this is more electronegative as we go towards the flooring and okay so what so what if the fluorine is more electronegative than the carbon well it comes down to stability if this atom is more electronegative it can endure or it likes a negative charge better than a atom that has a smaller electronegativity so because fluorine has a higher electronegativity it can tolerate the negative charge and if it can tolerate the negative charge then it's going to be lower in energy and that's what exactly what we're seeing here when we compare ammonia and methane we look at their conjugate bases and we look at how stable their conjugate bases are by invoking electronegativity nitrogen is more electronegative than carbon hence this conjugate base is lower in energy more stable which then makes the acid more acidic isn't that pretty cool so what we can also say when we go from left to the right is that these anions are more stable pretty Slack so that right there is the guiding principle all right next I want to show you how to do this analysis when acids are positively charged yay so what we're doing here is we're comparing two acids our hydronium and ammonium ammonium sorry you can see they're both acids they both have a positive charge so which one between these two is the most acidic well here's the the statement on how to do that When comparing two positively charged acids the stronger acid is the one that is less stable so now what we're doing is we're looking at the reactant side and finding out which ones the less stable so if we take a look at hydronium okay and we'll just put hydronium right here okay that's our h3o hydronium on the reactant side now in compare now where would I put ammonium where would that be would it be higher in energy than hydronium or lower well we use the same principles of electronegativity we can see if we look at the trans carbon nitrogen oxygen fluorine it's more electronegative to the right so we can see that the oxygen is more electronegative than the nitrogen atom so that's what we're comparing this oxygen and that nitrogen I know oxygen is more electronegative so electronegativity means it can and likes negative charge what do we have here we have a positive charge so because oxygen is more electronegative but has a positive charge it's going to be more uncomfortable more electronegative atoms don't like positive charges so because nitrogen is less electronegative than oxygen it can tolerate that positive charge a little bit better so we can expect that the ammonium nh4 plus is going to be lower in energy than hydronium and then we look at the products here okay and we can see that both have hydroniums so that kind of like just cancels each other out right so now we're just comparing this species and that species and remember when we're looking at molecules that are both neutral okay they have the relatively the same energy right there so that is going to be the products for both reactions so if we look at the energy diagram here we can I'm going to extend this energy diagram to right here because of how I drew it here what happens now is we can have something like that all right am I doing the color code right yeah got to make sure that's right and then I'll extend the energy little diagram over there okay and then this could be something similar to that like that okay now you're looking at that and how can I say hey which one's going to give up its proton easier well I look at the energy of activation here and when you compare these right here which arrow here is bigger the blue one so that means ammonia here is going to uh take a longer time to react and it's not as reactive so what can we say here when you have acids that are positively charged it is the acid that is the least stable that is going to be the most acidic molecule all right and that principle that is being taught here is due to an electronegativity argument oxygen is more electronegative hence when it has a positive charge it is that much more unstable and unhappy than a less electronegative nitrogen atom with the positive charge right now let's take a look previously we were comparing acids in the same period but now we are going to take a look at acids down the same group so that's what we have here what we have is hydrochloric acid and hydrofluoric acid and we're going to make that comparison so they're all these two acids right here are in the same group now proton transfer mechanism is the same so what's the rule here how can we determine which one is more acidic well we can see that they're neutral acids and so the rule says look at the stability of the conjugate base and the more stable the conjugate base makes the acid more stable or sorry more acidic so let me resay that so there's no confusion when you're comparing these two acids how do you know which one's the most acidic you go to the conjugate bases and you say hey you have to make a decision which one of these conjugate bases is the most stable if you say hey the chloride anion is the most stable one then that makes hydrochloric acid the most acidic or if you said fluoride is the more stable conjugate base then that would mean hydrofluoric acid is the stronger acid so how do we do this well what did we do when we were going in the same period we used an electronegativity argument right so which one's more electronegative fluorine or chlorine you would say fluorine is more electronegative so it can stabilize the negative charge better so you would say hey hydrofluoric acid is the stronger acid so let's pull out the pkas from a book so the pka for hydrofluoric acid is 3.2 the pka for hydrochloric acid is Let's uh negative seven negative seven is a smaller number than 3.2 so that means hydrochloric acid is a stronger acid than hydrofluoric acid so now right now you should be like what's going on here you teach me a rule and then you violate it in the next concept here okay so I can see that this is a little confusing but we when I first analyzed this I was using electronegativity argument I failed to look at the size argument so when we have the fluoride anion right here It's relatively small but when you look at uh a chloride it is larger and so now we have a size argument that we have to uh rationalize here whenever you have a negative charge that's localized in a small space it's going to be higher in energy whereas with a chloride anion the charge has a much larger volume to move around in and so that ability to move around lowers the energy of the anion it's kind of analogous to have you ever tried to uh or have you ever babysat a toddler and you want the toddler to stay in one room in an enclosed space they they don't like that they need freedom and space to move around and they're generally more happy when they can move around same thing with the atoms here when you have charges if it's a fluoride the charge is more localized and concentrated whereas in a larger atom the charge is delocalized or diffused away it's just more space so the idea here then is when you're going down these same group on the periodic table it's not an electronegativity argument the size argument trumps the electronegativity argument okay so let me conclude with reading the rule one more time the stability of an anion tends to increase when the negative charge is is on an atom farther down a column of the periodic table so if we change this right here what if I change this guy to um BR hydrobromic acid we'll change that guy now which one is the stronger acid hydrobromic acid or hydrochloric acid well the bromide right here is going to be what larger than the chlorine so we could see something that looks like this and so that would make the hydrobromic acid more acidic and the exact PKA on hydrobromic acid let's see if it shows here um PK of negative nine so that's two orders of magnitude uh more acidic than hydrochloric acid so that's pretty significant okay let's keep moving along now let's take a look at uh when you look at an alkane alkene and alkyne and what I'm going to do is I'm just going to draw an arrow down here to show the conjugate base okay so this one now you can notice that what am I doing I'm abstracting off one proton from these so the general rule still applies when we're looking at the acidity of these guys and that is more stable the conjugate base means the stronger acid so when we look at these conjugate bases here we can say that this conjugate base is the most stable amongst the three and we can say that because we have the answer already before us the pka is 25. for this alkyne so that means this hydrogen is the most acidic compared to these two so the pka values just tells us that but I'm teaching The Guiding principles here not by just looking at numbers why is this conjugate base here more stable than these two well when we take a look at the cardinal rule right cardinal well let's just worry about these first three letters here we can see charge they're all the same charge and they're all the same atom so the atoms don't uh change so how can we determine this guy's the most stable well it has to go back to electronegativity argument okay so electronegativity but when we look each atom here is a carbon okay so how can I use an electronegativity argument here well it's because it's a little bit different it's called effective electronegativity an elect effective electronegativity deals with when you're looking at carbons and what is the hybridization state of that carbon three two what happens is the SP is more electronegative than the SP2 which is more electronegative than the sp3 so that's saying the SP is more electronegative but we don't use that word just electronegative we have to say effective electronegativity and since this conjugate conjugate base came from a SP that means this negative charge is going to be more stabilized on a more electronegative atom so this carbon here has a greater electronegativity than that one and the reason why that is the case is because when we look at this carbon here it is a SP and an SP has more s character than a sp3 an sp3 uh orbital looks more like a p orbital not exactly like a p orbital and then the SP orbital looks more like an S not exactly like an s but more so and so what that does is when you have more s character the charge is going to be more localized on those carbons so the charge is more localized so it's more electronegative it attracts the electrons whereas when you look at an sp3 the electron density is more dispersed and so it's less electronegative but when we're comparing the same atoms all three carbons we call it effective electronegativity so the take-home message is that the SP hybridized carbons have a higher electronegativity than sp3 which means it's going to stabilize the negative charge on the conjugate base and since that conjugate base is more stable than that conjugate base it makes the acid more acidic that's all there is to it okay okay so let's just review what the Cardinal rules we've been looking at uh charges we've been looking at assets with different atoms in them and now we have resonance and delocalization to look at so if we take acetic acid and compare that to ethanol so which one is the stronger acid well if you've memorized pkas then you already know the answer you know the pka of carboxylic acids are about five and alcohols are about 16. so that's a stronger acid but we are here to learn guiding principles that is super super important so what happens here that comes like this we do our proton transfer reaction and we're going to get that plus our hydronium all right and so we've got the same thing here like that and we have that many electrons one two three four so let's put them there okay plus the hydronium so the print the guiding principle is just the same we have to look at the conjugate bases and we already know this conjugate base is more stable than that conjugate base because of the pkas we know that but what's the guiding principle here well it has to do with resonance and delocalization we can take the carboxylate right here and draw a resonance structure now remember with resonance structures you have to draw that arrow that looks like that and so this lone tear hair here can come like this oh we gotta put those there as well and then we could have a resin structure that looks like this like so whereas can we look at this ethoxide and draw a resonance structure can we take these electrons and bring them in and the answer is no because you would form a Texas carbon so that's a No-No so what we have is resonance and resonance is going to stabilize the conjugate base why because the electrons or the charge is delocalized you see how the charge is right here now it's right there that charge is going around the molecule so that's what the d stands for is delocalized that chart is delocalized through resonance and that's a stabilizing effect whereas this negative charge is localized it does not move it stays on that oxygen atom and does not move through the molecule so that is a very very important principle resonance and delocalizations stabilize the conjugate base which then if the conjugate base is more stable makes the acid more acidic let's wrap up the cardinal rule by looking at the the induction effects so if we take an alcohol treat it with water we could have an acid-base process go like this give us our ethoxide okay plus our hydronium but then we could take a look at let's say let's just replace one of these hydrogens on this carbon with let's say a electronegative atom like fluorine there should be a plus there so we could have the proton transfer mechanism negatively charged there's our chlorine plus our hydronium so which one's going to be the most acidic principle is we look at the conjugate bases and figure out which one is the most stable now when I look at the cardinal rule I can see that the charge is the same I can see that the atoms are the same I can see that there is no resonance or delocalization those are localized charges but now we have the last one induction induction effects which means that we can take an electronegative atom and we know electronegative atoms are going to suck electron density towards itself so as the chlorine is sucking electron density the electrons in this Bond right here towards itself that will then make this carbon a little bit more positive charge so it's going to suck electron density from this guy and then it it's just a domino effect that this electronegative atom here is going to be pooling electron density towards itself and so as that does that it's going to stabilize that negative charge because it's almost kind of like delocalization almost where that negative charge is filling a pool towards the electronegative atom and so it's kind of moving and so it's stabilizing it and so since this conjugate base is the most stable compared to this one that makes this guy the stronger acid between the two inductive effects something that you saw in general chemistry now you can apply the same principle to um let's see here let me look at the example now let's take a look at this example you see what I did with these molecules is I just added an extra hydrogen when I added that extra hydrogen on the alcohol group I generated what a positive charge now the rules are a little differently now when we're looking at acids that um have a positive charge we're trying to find the acid that is the least stable and the least stable acid is the one that's going to be the most acidic and so we can look at these two molecules here like which one's the most acidic one well what's the difference between the two there's obviously one difference and that is the chlorine atom so what does that chlorine atom do well it has induction right so it's going to pull the electron density towards itself so is that chlorine a stabilizing effect to that positive charge or a destabilizing well when we were looking at it when it was when we were looking at this molecule like that it was stabilizing because it was taking that negative charge and moving the electrons inductively towards the electronegative atom that's a good thing but now in this situation it's positive and so very little electrons so now very little electrons already very very uncomfortable due to the positive charges now trying to be yanked towards the chlorine and so that inductive effect now is destabilizing this positive charge because there's very few electrons over here and this guy's trying to suck electron density towards itself when there's nothing to give and so in in this example the inductive effects is destabilizing so when we compare these two acids this one's more destabilized or the most unstable so that makes that guy the most acidic in that example no no yeah okay so if you have any so that's the rules for the cardinal rule so if you guys have any questions or concerns please feel free to reach out oh before we before I end this video what's going to happen now is all these examples that I showed you we were looking at examples where we're only looking at one of the principles in the cardinal rule but now what you're going to have to do is look at an array of molecules and use all of these rules at the same time and that's what we will discuss later in class