3.2 Ranking Acids and Bases | Organic Chemistry
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- čas přidán 27. 07. 2024
- Chad gives a comprehensive lesson on how to rank acids and bases. This is presented in the context of the ARIO mnemonic which stands for Atom, Resonance, Induction, and Orbitals which helps students remember the most important factors affecting acidity and basicity in the most common order of importance. The Atom Rule for ranking acids and bases shows how the size and electronegativity of the basic atom affect its strength as a base. Of second importance is the effect of resonance on acidity and basicity as resonance can stabilize a base resulting in a weaker base. Typically of third importance is the effect of induction on acidity and basicity. Electron-withdrawing groups such as electronegative atoms near the atom acting as a base will help stabilize it resulting in a weaker base. Finally, the Orbital Rule describes the role that the hybridization of the atom acting as a base affects its basicity. Generally and all else being equal the order of stability with regard to hybridization is sp / sp2 / sp3 and the order of base strength is the opposite: sp3 / sp2 / sp. Chad provides numerous examples of how to rank acids and bases to demonstrate how each rule is applied, and concludes the lesson with several additional examples of ranking acids.
If you want all my study guides, quizzes, and practice exams, check out my premium course at www.chadsprep.com/organic-che...
00:00 Lesson Introduction
00:33 Ranking Acids and Bases (ARIO mnemonic)
03:31 The Effect of Charge on Acidity and Basicity of Organic Compounds
06:10 The Atom Rule for Ranking Acids and Bases of Organic Compounds
12:27 How Resonance Affects Acidity and Basicity of Organic Compounds
16:20 How Induction Affects Acidity and Basicity of Organic Compounds
25:12 How Hybridization Affects Acidity and Basicity of Organic Compounds
29:09 Examples of Ranking Acids
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Can't tell you how many times your videos have made it all click for me! Love and appreciate these videos so much!
Awesome Sierra! Thanks for the feedback and so glad you're finding the helpful!
Hi, Chad! Thank you so much for your superrr helpful videos! You're the reason I survived Chem 2 (and now hopefully ochem)! Quick question though, what are the exceptions to the ARIO rule and how do you know the problem that you are working is an exception? Thanks again!
Thank you for videos Chad! They are great review for DAT :)
You're welcome Hyun and glad you're finding them helpful!
Huge help, as always! Thanks Chad! :)
You're welcome, Timothy - thank you!
Thank you so much, Sir. I've been reviewing organic chemistry now because online classes are really difficult atm and I am worrying that I'll graduate without learning anything. So thank you so much for this!!
Glad you found the channel, Miles - Happy Studying!
Thank you for this video. This helped give me hope for my exam tomorrow! It helped me start putting the pieces together!
Good luck on your exam, Madeline!
Excellent videos. I have only watched acid-base but will be going back and checking out the rest of the series. You teach well, and don't talk so slow I have to put you on 2x speed. Thanks!
Welcome aboard, Mcbaginns!
thank you so much! that was really helpful
You're welcome!
This has been the best video I've seen on acids and bases, your attention to detail and the way you articulate makes it easier to understand the concepts. Thank you sir!
You're welcome and Thank You!
I always watch your videos first before reading the textbook or the professor's slides since you make these difficult concepts EASY to understand. The O-chem textbooks, I find, confuse me even more if I don't watch your videos first. Thanks for all of your help!!
Figuring out what works best for you is a must - glad the videos are a part of your success. Happy Studying!
Man: I'm watching those videos because since I was 8 years old I wanted to understand the atom diagrams, but I never formally studied anything related to chemistry (I'm a software engineer), I'm amazed at how you explain things. Thank you very much for all the content.
You're welcome - glad you found us!
can you specify the list of all of the necessary pkas that are needed to be memorized? thank you
There seems to be no mention of electron density. How would you compare tertiary, secondary, primary alcohol if it weren't for electron density? Or is it accounted for by Induction?
I also think for the last example electronegativity didn't work because it's not a big difference between C and N.
Thank you so much ❤
You are very welcome.
Awesome brother!
Thanks!
Great vid. Thank you, Chad!
You're welcome.
What a great content sir ! I study pharmacy and for me your videos are great for understanding the basics of organic chemistry , thank you.
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You explain like a master! Thanks a bunch 🎉
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Thank you so much!!!! this saved me
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Amazing video, thank you so much!
You're welcome and Thank You.
Thank you so much this saved me before an Ochem midterm, got a 97% on it thanks to you!!
Way to go - Keep it up!
thank you so much 🤗
You are most welcome.
Hi Chad! Thank you so much for this video. I have shared it with so many of my classmates!
I have a quick question for you regarding induction. What happens if you have two molecules and one is 1 carbon longer with a fluorine atom attached and the other is one carbon shorter with a chlorine atom attached. Which will be the more stable base in this case? I would think the one with the fluorine attached even though it is further away but I am not entirely sure. I hope that makes sense.
Great question Julia! It turns out proximity has the greater effect. You can see this in comparing the following acids by their pKas:
3-fluoropropanoic acid pKa = 3.9
2-fluoropropanoic acid pKa = 2.7
2-chloropropanoic acid pKa = 2.8
2-bromopropanoic acid pKa = 3.0
2-iodopropanoic acid pKa = 3.1
I don't know that there was really a way of predicting this; it just so turns out that proximity has the larger impact. So if you were comparing conjugate bases as you were the conjugate bases having any halogen at position 2 are more stable than having a fluorine at position 3. Again, nothing intuitive about this, it's just the way it is.
Hope this helps and thanks for sharing!
hi im your classmate and i am grateful julia!
Doesn't the comparison between phenol and carboxylic acid show that Induction should be considered before resonance?
thank you!!!
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Thank you so much😭
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Thank you!
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THANK YOU
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THANKS THE VIDEO VIDEO WAS VERY HELPFUL
Glad to hear that!
excellent job chad
Thanks!
@@ChadsPrep you are welcome sir
@@Chem_crown 👍 👍 👍
I am preparing for JEE without any coaching this is the best video where I can find 👍🌝
Glad the channel is helping you.
How about when dealing with positively charged atoms
Doesn't change a thing. More negatively charged could mean -1 compared to neutral, but it could also mean neutral compared to +1. Ultimately, more negative means the same thing as less positive in that latter example. Hope this helps!
Quaternary amine looked more like the amine molecule than NH3 itself??
man, when I took O chem we weren't given any mnemonic for base ranking at all
u are a gem
Thank you
Life saver
😊
9:25 Exactly why do big atoms make weaker bonds to H^+? Is there a simple explanation for this?
Smaller atoms facilitate shorter bonds and shorter bonds are stronger bonds. Bases with stronger bonds will more tightly hold on to H+ ions and thus make poor acids since it is more difficult for them to break the bond and give up a proton.
Some people look at the bond to H for justification of acidity, but if you look at R-OH and R-NH2, you found R-OH to be the stronger acid (11:55), but one can see that the bond to H would likely be stronger in R-OH which would mean a weaker acid (i.e not likely to give up the H). How do you explain that? I guess the bond to H doesn't always apply?
Bond strength isn't a perfect correlation to acidity based upon how it is defined. Bond strength is measured as the energy to homolytically break a bond (homolytic cleavage). But homolytic cleavage results in two radicals--essentially you're splitting up the two electrons in the bond evenly so that both atoms get one.
But acid-base reactions involve heterolytic cleavage where the H doesn't get either of the electrons from the bond but the nonmetal it was bonded to ends up with both of those electrons. So an acid dissociate will end up with an H+ ion instead of an H radical and then some sort of nonmetal anion instead of a nonmetal radical. The stability of the nonmetal anion (conjugate base) then becomes the key to comparing different acids.
Hope this helps!
@@ChadsPrep I see, makes sense.
@@MrYahya0101 Excellent!
I think aromaticity should also be part of the rules because if a compound loses its aromaticity due to protonation then it would become a weak base.
how is SH a stronger acid than OH? i thought oxygen is more electronegative and is less willing to give up its proton therefore making it the stronger acid
Remember that an acid must be willing to give up a proton so OH is a base whereas SH is an acid
Sir you are saying that a more electronegative atom should be a stronger base as it forms a stronger bond to proton but you are also saying that a more electonegative atom could strongly attract the _ charge(lone pair) on it making the energy of electons lower as they come close to its nucleus so now he is a weaker base as he becomes less reactive . Sir how could we find exact answer???
Try to understand the concept that a *less stable base = more reactive = stronger base since it is less able to stabilize the negative charge density*.
So, a base with a negative charge on a *less* electronegative atom is a less stable, stronger base as the nucleus has less ability to draw electrons towards itself and stabilize the extra electron density. Conversely, a base with a negative charge on a *more* electronegative atom is a more stable, weaker base as the nucleus has greater ability to draw electrons towards itself and stabilize the extra electron density.
Shouldn't Induction play a bigger role than resonance? For example, phenol and ethanoic acid. Phenol has so much resonance, but it still has the stronger conjugate base because it's not as pulled as ethanoic acid.
I could be wrong but doesn't Induction give rise to resonance in many cases for example in carboxylic acid where Oxygen pulls electrons and therefore produces resonance. If induction produces resonance, then why should Resonance be considered as a separate criterion or prioritized over Induction?
Hello Youssef! Ok, where to begin. So first resonance and induction are different phenomenon, even when they both occur simultaneously. Resonance is the delocalization of pi electrons. We draw multiple resonance contributors to see where the electron density is for the resonance hybrid which is a better depiction of what the molecule looks like than any individual resonance contributor.
Now induction has to do with the 'pull' of electrons through the bonds due to differences in electronegativity. When you have an electronegative atom near an atom acting as a base it ultimately lowers the energy of the electrons on the basic atom by lower the amount of electron density on the atom.
For example, let's say we have an oxygen acting as a base in the following arrangement F-C-C-O. Now I'm not attempting to draw a complete structure and would obviously have to add some atoms to do so. But I think we can demonstrate what is going on here with induction. So the F is more electronegative than the C it is bonded to. So it will pull the shared electrons closer to itself and further from C. The result is a partial negative charge on F and a partial positive charge on C. But it doesn't stop there. This partially positive carbon is bonded to another carbon atom. And because the 1st C is partially positive it will pull the electrons shared in the bond with the 2nd C closer to itself. The result is that now the 2nd C is also a little bit partially positive (but not as much as the 1st it turns out). And now this C is bonded to the oxygen that is acting as the base. Because this 2nd carbon is now a little partially positive it will also pull a little electron density from the oxygen (at least relative to a molecule that had no F nearby). This lowers the electron density on the oxygen and makes it less negative/more positive which lowers the energy of the electrons on the oxygen making it a weaker base. And all of this has been accomplished through the bonds due to differences in electronegativity and has nothing to do with drawing resonance structures.
Now on to a couple of your other questions. As far as phenol and carboxylic acid, the difference here is a difference in the quality of the resonance structures rather than the quantity. This is an example I often go into great detail with my students. The phenoxide ion has 4 resonance structures whereas the carboxylate ion only has 2. But we need to examine further. The phenoxide ion shares the negative charge on 1 oxygen atom and 3 carbon atoms. The carboxylate shares the negative charge on 2 oxygen atoms. Now oxygen is more electronegative so we know we'd prefer a negative charge on an oxygen over a carbon but the question is is it better to share the negative charge on 1 oxygen and 3 carbons OR on 2 oxygens. Well it turns out it's more stable to share it on 2 oxygens. So even though the phenoxide ion has MORE resonance structures, the carboxylate ion has BETTER resonance structures.
It's kind of like trying to figure out who has a better chance of lifting a heavy weight off the ground: 1 body builder with 3 two-year olds working together OR 2 body builders working together. Not a perfect analogy but I think you'll get the idea.
Next, at one point you mentioned comparing the acidity of HF and HBr. But you need to realize that this has nothing to do with induction. Induction takes place when you have an electronegative atom nearby to the atom acting as the base. The more electronegative the neighboring atom the more stable it will make the atom acting as the base. For HF vs HBr the conjugate bases are F- and Br-. These are the atoms acting as the base. There are no neighboring atoms to even talk about and induction is completely irrelevant to the comparison. When you're actually comparing the atoms acting as the base both size and electronegativity come into play with size being more important. This is categorized under what I called the ATOM rule completely separate from the INDUCTION rule. And the atom acting as the base is generally more important than any electronegative atoms that might happen to be nearby. This is why the ATOM RULE gets priority over the INDUCTION rule. And while I'm discussing it we just know from experience that resonance usually has a more significant impact than induction which is why the RESONANCE rule gets higher priority than the INDUCTION rule as well.
However, I do mention in the video that there is no set of rules that works 100% of the time and I can come up with several examples that contradict the relative order of priorities of the various rules. In fact I present a couple in the video. But a set of rules like this will generally work more than 90% of the time when comparing acids and bases and has great explanatory power for most comparisons.
Finally, you also mentioned comparing primary, secondary, and tertiary alcohols. It turns out that this is a very special case that normal set of rules doesn't completely explain (or even mostly explain) It turns out it is a rare example where solubility plays a significant role. I actually cover that in chapter 12 on Alcohols in this lesson: czcams.com/video/MacZlzdt5nc/video.html
Hope this helps!
@@ChadsPrep Thank you very much! That was very helpful!
Looking at acetone (pka = -2.9) and the ester with R group as methyl group, whose name I don't know (Pka = 4.75) at www.studyorgo.com/blog/wp-content/uploads/2017/04/ammonia-water-1.png, the ester has more resonance on electronegative atoms, therefore it should have the weaker conjugate base right? But it seems to have the stronger conjugate base?
Hello Youssef! Be careful on this comparison. The one shortcoming on the ARIO mnemonic is that it doesn't factor in any difference in charge which is why some use the CARDIO mnemonic instead. The 'C' in CARDIO stands for charge and likely has the most significant impact as it is the first rule in the mnemonic. The idea is that a negative formal charge raises the energy of the electrons of a base making it a stronger base (and weaker conjugate acid), while a positive formal charge lowers the energy of the electrons of the base making it a weaker base (and stronger conjugate acid).
And the major difference here is a difference in charge. The protonated version of acetone you're looking at here has a positive charge and the conjugate base is neutral. The carboxylic acid (not an ester) you're comparing it too is neutral and the conjugate base is negatively charged.
Now if we compare the conjugate bases acetone is neutral while the carboxylate ion (conjugate base of carboxylic acid) is negatively charged. Based on charge considerations alone acetone should be the weaker base and the carboxylate ion the stronger base and this generally takes precedence over the ATOM rule or RESONANCE rule.
One caveat, I can definitely come up with exceptions to the CHARGE rule (just like I can with any of the rules). The CARDIO and ARIO rules are not 100% accurate in correctly leading us to the correct conclusions in any comparison we might make. But they do provide us with a pretty solid line of reasoning that will guide us to the right conclusions most of the time.
Hope this helps!
@@ChadsPrep Thank you very much!
9:36 I'm still trying to wrap my head around this.. re: "the bigger atoms ⚛️ don't make strong bonds with H". In summary, i'm getting that F would be a better base (vs the other halogens) cause it would more readily bond to H due to its smaller radius, and thus a stronger bond (between H and F) makes it harder to lose the H when it forms an acid.. is this correct?
Couldn't have said it better myself Alessandra!
@@ChadsPrep thx for all your informative videos. They're very helpful
@@alessandrac1940 You are most welcome - Happy Studying!
Sir in a period as we go from left to right the more electonegative atom like florine has the greastest ability to pull a proton towards it so it has to be a stronger base as you tell in a group that a smaller base make a shorter and stronger bond to a proton hence he is a stronger base but you are telling opposite thing in a period????
Yes, opposite in a period going from left to right as electronegativity increases due to greater number of protons and therefore greater pull of positive nucleus on negative valence electrons.
Sir well in a group we are also noticing electronegativity florine has the smallest size has the highest electronegativity in it's group so he is a stronger base and in a period the the smaller size of atom leads to a greater electronegativity hence he should be a stronger base but you are telling an opposite trend in a period why dear sir??????
I have explained above
2:22 "all about the base, we're gonna talk about your size". Sure meghan Trainor would love that phrase😅
She should come check out the channel and learn a few things.
you are a chem wizard
Thanks!
Sir why less electronegative base is a less stable base
The correct statement is that a base with negative charge on a less electronegative atom is a less stable base. Less stable base = stronger base i.e. more reactive. If we think about the definition of electronegativity, the ability of an atom to draw electrons towards itself, then less electronegative atoms have a lower capability of drawing electrons (negative charge density) towards themself and so are less stable at holding a negative charge.
Thank you so much sir.
@@user-vl1bk9jh4h You're welcome.
"If you follow the mnemonic here, you're going to get hosed" hahahahah (33:30)
😁😁😁
Sir why a large atom has high energy electrons?????
Can you please give the time in the video and ask this question more clearly 🙏
Sir this is about ranking of bases .
Can you write the time of the video, e.g. 8:30, thanks
3 :31 sir
Thanks for putting the time in! So if we have a larger atom then the outside (valence) electrons are further away from the nucleus as they are on the edge. Further distance from the nucleus corresponds to higher energy electrons because they are less 'tightly' held by the nucleus
You said Induction is purely electronegativity. F is more electronegative than Br, so you'd expect HF to be a stronger acid, but HBr is a stronger acid. Why?
The conjugate base F- is stronger than Br- since it is smaller, meaning that the bond length is shorter. If a conjugate base is stronger, then that acid must be weaker and vice versa
Lol, "I just wanted to cover my bases."
i love you
Glad the channel is helping you.
You wanted to cover your bases…pun intended? 😅
#DadJokeAlert
@@alessandrac1940 Cover my bases, drop the bases - all the bases!
Size is more important. Got it.
Hope you do well on your next quiz!
@@ChadsPrep Just did. I’m really happy. Your videos helped me out a lot.
@princegali2605 Excellent - glad the channel helped - Keep up the good work!
Sir l can't speak in English well . And l have distrubed you sir in sort of like l can't make my question understandable for you .sir if you don't understand my questions then you can simply ignore them but sir l will try my best to understand your lecture.😶
Your English is absolutely fine, just take your time when typing out the question - I know learning can be stressful, but I'll help if I can :)
@@ChadsPrep😶
Life is always stressful sir😢
we are here to help 🤗
Chad
and I wasn't referring to your name
@@tacticalmistress Got it!
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