3.2 Ranking Acids and Bases

You see, okay, so initially I didn't want to clutter the board, but in addition to comparing these beautiful

bases

now we're going to compare their conjugate

acids

and again the key is that the rules are for the

bases

and we just jump right into comparing the bases, but If you had been asked to compare any of these pairs of conjugated

acids

first, well, the first thing we need to realize is, oh, they're asking me to compare acids. I don't compare acids, I would take out their conjugate bases and then compare them and fortunately we can now take advantage of the fact that we have Done that, we are now ready to compare them because we have already compared the conjugate bases and in this case, in the first example of these two conjugate acids, I would again extract their conjugate bases and realize that this is the most stable conjugate base the most stable conjugate base is the weakest conjugate base and the most stable and weakest conjugate base comes from the strongest conjugate acid so now we are surrounding the stronger conjugate acid the same in the next example we already discovered again By comparing these two acids, we already discovered the relative basicities of the conjugate bases, so chlorine was closer, which had a most stabilizing influence by induction and the most stable base is the weakest base and the weakest base comes from the strongest conjugate acid. next comparison again we are comparing these two acids, the first thing you do is draw these two conjugate bases and here the fluorine was more electronegative than the chlorine, so the fluorine will move more electrons away from the oxygens, making them less negative and more stable and more stable means weaker and weaker base.

The base again comes from the stronger conjugate acid and finally, in the last example, we found that two fluorines were more stabilizing than one fluorine, so this was the most stable base. The most stable base once again comes from the strongest conjugate acid, so you're obviously very lucky. that we already had the conjugate bases on the board and we already had them classified, but if we hadn't, that would have been the first thing we would have to do, so the last rule in our mnemonic is orbital, so either here and a time. Again, you wouldn't just start using this rule, you'd first have to check that no distinction is made between the rest of the rules and the comparison you're making, and again, here it's about orbitals if or for orbitals, but again.

It really refers to a difference in the hybridization of your basic atom, so if we take a look here to compare these three beautiful bases, the first thing we do is say do they all have the same charge, yes, one negative, one negative, one Negative, okay, no difference. So we move on to talking about the basic atom and it's carbon compared to carbon compared to carbon. Well, the atom rule doesn't help us either, so we move on to resonance and it turns out that none of these are stabilized by resonance, so if you look, you could go, chad, wait a minute, charge like this guy, well, another one Maybe when your single electron pair is exactly one bond away from the pi electrons, i.e. as allylic or benzylic, that's when you can expect resonance, but here it's not one bond away. right where the pi electrons are, so there is no resonance in any of these, so let's move on to induction, are there electronegative atoms in the area to help stabilize this?

No, not in any of the cases, so we finally move on to the orbitals and say is there a difference in the hybridization of the basic atom in all these cases and in fact there is a difference, so this carbon here has only two domains of electrons, it's bonded to an atom and it has a lone pair, so it's going to have sp hybridization, so this carbon here is bonded to two atoms, a carbon and a hydrogen, maybe I'll attract that hydrogen to do this a little bit more obvious, so it's bonded to two atoms, a lone pair, three sp2 hybridized electron domains and finally this one here is bonded to three.

The atoms, one carbon, two hydrogens and it has a lone pair, four domains, will have sp3 hybridization, so there is a difference in the hybridization and it turns out that that is something significant now if we look at the difference between sp, sp2 and sp3 . so your sp is 50 s 50 p we would say 50 s character and it probably looks like this your sp2 is now just 33 percent s one third s two thirds p and it looks more like a p orbital now just 33 s character it's a little bit longer and the electrons will be a little further from the nucleus at the node than they would be if they weren't sp hybrid orbitals, so having a lone pair and an sp compared to having a lone pair in an sp2 are on average, it will be closer of the nucleus in an sp hybrid than in an sp2 hybrid orbital and finally in an sp3 hybrid orbital it is elongated even more, so the electrons on average will be even further from the nucleus and will have an even higher and less stable energy in an sp3, so the idea then is that your single pair will be more stable on an sp hybridized carbon compared to an sp2 or an sp3 and if it is more stable here, that makes it the weakest base that Put the number three in this case and then here on the sp3 hybridized atom, that lone pair will be the least stable, which in our case will mean the strongest gap and then this guy is in the middle and that's our basicity trend. the orbital rule really is a difference in hybridization, so if you were asked to compare conjugate acids well again, you would realize that I am not comparing acids and the first thing you need to do is extract all the conjugate bases, follow the rules and obtain this relative basicity which would then allow you to obtain exactly the opposite trend in acidity for the conjugate acids, so that the most stable and weakest conjugate base has the strongest conjugate acid, etc., this rule does not arise. very often, but it is something that you will probably see at least once or twice between your assignments and your exams, so now we have completely completed the mnemonic and we have gone step by step and you know, I knew what comparisons would come because we were going in order, so now I want to work three examples and with these three examples we are going to do specifically comparing the acidity and we are going to take into account the pkas that are supposed to To know as well as all these beautiful rules, then we have three different comparisons here and the first it's going to be this beautiful species versus this one and the first thing you need to do is keep in mind which pkas you know well.

It's a carboxylic acid and its pka ​​is in the range of four to five, usually pretty typical, whereas this is an alcohol and its pka ​​is usually in the range of 16. Now the truth is, this one has chlorine in it. which will inductively stabilize the conjugate base which will ultimately make the conjugate base weaker and this acid a stronger acid, but 16 versus four to five is not going to go down as much, it's not a possibility, so in this case technically you could say, yeah, there's my strongest acid, I circled it, life is good, so if you didn't know, remember your pkas or you didn't categorize them correctly again, you can always just draw their conjugate bases and use your reo mnemonic here, so in this case check and say if they have the same charge, yes, a negative one to compare their bases, then I would say okay, Adam's rule, the auction is the base and the auction is the base, it's a tie.

Adam's rule didn't help me, so I moved on to the MRI. rule and we would see that the one on the left has resonance and the one on the right has no resonance and then the one with resonance is more stable and a more stable base is a weaker base and a weaker base comes from the stronger conjugate acid the rules would have taken us to the same great place, moving on to the next one here and this is one that I mentioned before, so we have a carboxylic acid here and again the pka of a typical carboxylic acid is in the four to range five and this is a phenol and The pka of a typical phenol is in the range 10, so we can say that yes, we have our strongest acid right here because we memorized some pkas.

This is the complicated one if you hadn't memorized those pkas. You had a hard time with this one until you found it somewhere along the way and we saw a carboxylic acid, it has two resonant structures, it shares the negative charge on two oxygen atoms here, but the phenol we haven't actually drawn. specifically, but the conjugate base actually shares the negative charge between the auction and then three of the carbon atoms in the ring result. I'm going to take them out, but I'm not actually going to show them how to get there. I'll leave that to you for now and finally one more, you know, I'm going to be a little nicer than I said I was going to be like this and here the lone pair goes down, the pair goes out to that atom, here goes the lone pair. in forming a pi bond the pi bond becomes a lone pair that brings you here here the lone pair becomes a pi bond the pi bond becomes a lone pair that brings you here well so the question you must ask yourself many students are wrong because here They are all more resonant structures, more resonance, more stable base, more stable base, weaker base, the weaker base comes from the stronger acid, but we saw that that was not the case at all, but we are not comparing apples to apples here, so you can't just compare the absolute number of resonance structures unless the negative charge is simply shared on the same atom each time, that would work, but here sharing the negative charge in two options compared to share the negative charge on one share and three carbons. and there's no intuitive way to say it, well I have a hunch so it all comes down to having seen this example before or just knowing those very useful pkas, in this case it turns out that sharing it across two auctions was more stable. the weakest base that came from our strongest acid, finally, this last one is a super complicated example, but you might remember that the pka here and the pka of a terminal alkyne here was around 26.

So we didn't cover the one on this one. guy. pka explicitly, but we covered a very similar molecule and that similar molecule was ammonia and we said the pka of ammonium was around 38. So if you deprotonate neutral nitrogen, ammonia or neutral nitrogen into an amine, it will be around a pka of 38. and I can see that my lowest pka is definitely this guy and he's going to be our strongest acid and I mentioned this one on purpose because it's a complicated example, if you try to follow the mnemonic here, you'll be screwed. and let's see why, if we draw those conjugate bases, as was said at the beginning of this lesson, there is no perfect set of rules because there are always exceptions like this, so if we look here and start with a charge like minus one, minus one life. okay, so we move on to the atom, we're like carbon versus nitrogen, well carbon and nitrogen are in the same period, which means it's going to be about electronegativity and since nitrogen is more electronegative, we would think oh, more electronegative, more stable, more stable base. weakest base the weakest base comes from the strongest acid and we got the complete opposite determination of our pka values ​​that we knew, so what's going on here turns out so good that this is usually the order of priority as far as you know what distinguishes two bases. and compare them and order the importance, however, there are two differences here: it is a difference in the atom, but it is also a difference in the hybridization of the orbitals, so it is not just a matter of comparing carbon with nitrogen and that is the The only difference is an sp hybridized carbon versus an sp3. hybridized nitrogen and it turns out that sp so is more stable, sp carbon is more stable than sp3 nitrogen.

Now there's no intuitive way to know, but since we knew about pkas, this exception didn't affect us. our rules, so like I said, there are usually two or three comparisons you can make where the rules will lead you astray, but if you know all your pkas, those can't make you feel good, this is asked in the bases, it classifies them and, again, the relative is understood. Acidity and basically basicity are really going to be key to understanding relative reactivity. When we start talking about chemical reactions later, I can't emphasize how important this is to get a good practical understanding of how the world works from an organic chemistry perspective, but how chemistry works.

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