Acidity & Basicity of Amines

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Video in TIB AV-Portal: Acidity & Basicity of Amines

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Acidity & Basicity of Amines
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21
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27
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2015
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English

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Abstract
This is the third (and final) quarter of the organic chemistry series. Topics covered include: Fundamental concepts relating to carbon compounds with emphasis on structural theory and the nature of chemical bonding, stereochemistry, reaction mechanisms, and spectroscopic, physical, and chemical properties of the principal classes of carbon compounds. Index of Topics: 01:04 - Famous Amines 03:13 - Reaction of an Amine as a Base 05:29 - Reaction of an Amine as an Acid 06:37 - Substituent Effects on Amine Acidity and Basicity 32:44 - Acidity & Basicity of Aromatic Heterocycles
Biosynthesis Acid Lecture/Conference Meeting/Interview Amination Aromaticity Electronic cigarette Chemistry
Barbiturate Resonance (chemistry) Cocaine Chloride Hydrochloric acid Chemistry Alpha particle Molecule Electron Substituent Lysergic acid diethylamide Induktiver Effekt Alkylation Chemical reaction MAO-Hemmer A Hydroxide Lone pair Acid Chemical compound Amination Ethylgruppe Biosynthesis Recreational drug use Species Activity (UML) Phenyl group Carboxylate Sunscreen Hydroxyl Morphine Wursthülle Methylgruppe Ammonium Beta sheet Elektronentransfer MAO-Hemmer Conjugated system Setzen <Verfahrenstechnik> Hydrocarboxylierung Alkane Stickstoffatom Diazepam Carbon (fiber) Protonation Computer animation Urea Proteinkinase A Base (chemistry) Ammonia
Substituent Induktiver Effekt Resonance (chemistry) Elektronenpaar Methylamin Methoxygruppe Nitroverbindungen Feed (film) Acid Computer animation Electron PH indicator Shear strength Amine Valence (chemistry) Chemical compound Proteinkinase A Delocalized electron Base (chemistry) Conjugated system
PH Stickstoffatom Density Hybridisierung <Chemie> Chemical reaction Kupplungsreaktion Atomic orbital Lone pair Computer animation Electron Magnetism Chemical compound Kernproteine
Hydrocarboxylierung Setzen <Verfahrenstechnik> Potenz <Homöopathie> Stickstoffatom Induktiver Effekt Resonance (chemistry) Carbonylverbindungen Chemical reaction Organ donation Atomic number Lone pair Acid Computer animation Electron Proteinkinase A Kernproteine Cobaltoxide Conjugated system Dipol <1,3->
Stickstoffatom Heterocyclic compound Carbon (fiber) Aromaticity Electronic cigarette Atomic orbital Lone pair Systemic therapy Acid Computer animation Electron Covalent bond Spring (hydrology) Chemical structure Base (chemistry) Isotopenmarkierung
Sense District Nitrogen fixation Heterocyclic compound Imidazol Aromaticity Ammonium Electron Midazolam Kernproteine Conjugated system Histidine Process (computing) Pyridine Biochemistry Stickstoffatom BET theory Protonation Chemical reaction Lone pair Systemic therapy Acid Systems biology Computer animation Chemical compound Proteinkinase A Base (chemistry) Cobaltoxide Ammonia
so we had to talk about acid face a acid based behavior of amines today so that's really that we only need to test on that on Friday so we're gonna we're going to do that a lot of acid-base behavior and then we're going to learn how to synthesize different amines okay so there's a lot of synthesis in this chapter so um you know I know it's I know you get really after you've taken that test you want to take about a week off don't recommend that because we got the final coming right around the corner I also recommend that you for practice to get you ready for the synthesis as the after your test is over then go back in chapter 18 and go to the end of the chapter in the synthesis section and just do some problems okay so that will remind you of all the aromatic chemistry because there's a lot of aromatic chemistry here yeah well I have extra office hours this week yeah how about 1 or 1 o'clock on Wednesday yeah ok we'll add an extra office hour this week all right so we're talking about last time
about beta finet allah means and you'll notice there's two types there's the beta phonetically meaning that has a no hydroxyl in the Alpha position that would be these guys all the way across and then there are some that have the hydroxyl in the Alpha position right these hydroxyl in the Alpha position no hydroxyl in the opposition we're going to learn how to synthesize both of those types and some other types of amine also all right on the next page we have some more means that are not betaf in ethyl means but are also so we have tricyclic antidepressants contain amines I soak our boxes it is Whitsitt which is an MAO inhibitor monoamine oxidase inhibitor we have nicotine we have valium we have xanax so you're seeing a lot of amines here and so that's why we're going to learn how to synthesize there's Seco barbra tall which is we're gonna actually do that this week in discussion we're gonna learn how to make those so this is our part bitch you it and so barbiturates if you look at this part of the molecule here this is made from an active methylene compound and then you just condense that with urea so we're gonna do that in discussion this week that's old chemistry then we have cocaine and what else do we have scroll down morphine and LSD okay so a lot of drugs that affect the brain have nitrogen's in them so neuroleptic is what they call drugs that affect the brain so important thing for you to be familiar with and and and and a good target for us to learn how to synthesize some of these things alright so right
now we're gonna focus for the rest of today's lecture on acid-base behavior of amines and we know that amine can act as a base or starting off real simple here okay so we mixing it mean with the carboxylic acid it makes sense that we're going to have a proton transfer methyl ammonium ion is what we get plus our conjugate base and so in order to see the direction of this equilibrium we look for a conjugate acid-base pair here's our conjugate acid-base pair the one with the extra proton is the acid and then we do the same thing over here we should have if we do that we should have one acid on each side of the equation and this would be PKA about five and this is PKA about ten so the reaction highly favored to the right so we'll have just a little arrow going in the opposite direction here so equilibrium favored to right and at equilibrium what do we have and we said we saw in carbonyl chemistry it's a good idea to figure out about how much we have at equilibrium of both species so we can make decisions about what's going to happen so wide equilibrium we take a difference in the pKa that's five so we would have one protonated carboxylic acid to every ten to the fifth deprotonated reactants to products so the whole first part of this chapter is chapter 2 chapter 2 review and then some of it is chapter 17 review where we have aromatic amines and what kind of acid-base behavior do they exhibit all right so that's remain reaction of an amine as a base we can also have reaction of amine as an acid so mixing an ammonium ion with hydroxide ion we're gonna do we're going to we're gonna do protonate could have that plus h2o we can also if we could take an amine that is not already protonated okay not an ammonium I but an amine that's not already protonated if we treat it with a really strong base then we can remove that proton also so here's two examples of an amine acting as an acid now remember this is how we made Lda all right so we're going to move on to talk about its substituent effects and how substituents affect acidity and basicity we have the effective alkyl substitution we have inductive effects we have resonance effects there's also solvent effects but in this textbook they're not talking about solvent effects so we're going to leave that out also alright so test your knowledge question why which is the most acidic compound what do you think first one second one third one all right so this you're saying this one's I heard first most and if this would be least let's see why that is and see if that's right and see why that is a good idea when we're trying to figure out acidity and basicity is to look at the conjugate base and when we look at the conjugate base we're looking for anything that might stabilize it maybe destabilize it and those are the factors that we're looking for so conjugate base here would be ammonia conjugate base here there's our conjugate base here and then our conjugate base here has two alkyl groups bonded to nitrogen alright so our alkyl groups electron donating or electron withdrawing their electron donating so what is that going to do to the stability here this one has one methyl group donating electrons to the nitrogen this one has two electron donating groups which is the most electron rich nitrogen first one second one third one third ones most electron rich okay so this one has has two electron donating edy Ethel's this one has one electron donating ethyl so the one electron-donating Athol makes nitrogen more electron rich than ammonia this one has two electron donating Ethel's so therefore it's going to be even more electron rich so this one is less stable than ammonia and the third one is even less even even less stable remember things that we do to stabilize the conjugate base to make the acid more acidic so even the second case in the third case when it's when it's less stable conjugate base that means the acids going to be less acidic so here the conjugate acid is least acidic and it's easy for students to get confused when we start going through this logic here just remember the strongest acids have the weakest conjugate bases so hydrochloric acid PKA -7 chloride is extremely weak base so if we have so that's how that's that you can always go back to that example if you get confused when we're talking about this alright how about this next one which is what you mean is the most basic first one or second one first I'm hearing more first one let's look let's look and see what we have here this methyl is electron donating group EDG for electron donating group that makes nitrogen more electron rich this one over here this arrow ring is electron is electron withdrawing group in two ways or we should say arrow ring stabilizes the base in two ways number one is inductively okay so that nitrogen here this this is an S this is an sp2 carbon is more electronegative than an sp3 so we're going to pull electron density towards that carbon so sp2 carbon more electronegative and sp3 so we have a mild inductive effect to up from that adjacent phenyl ring we also have it stabilized by resonance so if we draw this ring here that lone pair on nitrogen could be delocalised onto the ring so this is also stabilized
so it's not only stabilized by inductive effects it's also stabilized by resonance okay we can go here so remember all those resonance structures we drew in chapter 18 that's why I want you to go back and review some of that so you make sure that you're up to speed here because it's been a while since we've had that and it looks like we can draw one more so let's draw one more resonance structure we don't have any of this sort of delocalization in the first example for methylamine so what we say here is that electrons are delocalized and let's put brackets around that so the electron pair is delocalized over entire ring so therefore it's a more stable and therefore it's going to be less basic over here we don't have any stabilizes stabilization by resonance and we have a destabilizing inductive effect so this one's going to be more more reactive or another way of saying that is less stable and therefore it's going to be more basic so remember when we're going in the same row of the periodic table on basicity parallels nucleophilicity so we can make decisions based on basicity so notice in this one the first one we're asking about which one's more acidic here when we're asking which one's more basic when we asked about acidity we look at the conjugate base when we asked about basicity we just look at the base and see if there's anything stabilizing it there's other ways to do this you can look at the stability of the acid but if if for most compounds that we're dealing with this is going to be a little bit easier to do questions so far anybody all right now this is the same question which I mean is the most basic and we're talking about the same exact ones except now we're protonated and we're saying which one's more acidic okay so I can ask you either kind of question and I will on the final what you mean is more acidic so always look at the conjugate base it's conjugate bases more stable than the compounds more acidic we just looked at the conjugate base on the previous page so conjugate base more stable therefore this is more acidic so I want you to feel comfortable going back and forth I mean and you're sophisticated enough here at the end of 51c that you should be able to do that all right how about this next one here number four which one's more basic first one second one or third one third one most basic okay let's write that happens to be the right answer but you know maybe I wouldn't write it if it wasn't the right answer most basic what is once the least basic would you say least basic okay somebody knows what they're doing here okay so when we're talking about base to city we don't draw the conjugate acid we just look at the base strength and look for special stabilization now all three of these can be stabilized by resonance right boom when we move the electrons down just like we did up here all three can be stabilized by resonance okay so I'm not going to talk up I'm not going to draw resonance structures again so what I'm going to look at is these ones that have these substituents here and this nitro group let's draw out the nitro group I'm just going to draw right on top of it and we know that that's an electron withdrawing group we know that from chapter 18 so again with this a more indication that we really need to review chapter 18 to make sure that on top of things here and so what you notice is we do have the resonance structures where we put the pot the negative charge three places in the ring for all these but this one actually has another extra resonance structure here where we push electrons all the way on to the Nitro group so we get an extra resonance structure let me draw that red extra resonance structure here again I'm not going to draw all of them for each of these because they all have the same but this one has an extra one so what we have in this ring system is we have an electron a very strong electron donating group in a very strong electron withdrawing group or if though our para to each other and if you have that situation where you have in a really strong electron donating group a little really strong electron withdrawing group ortho or para to each other it makes for something that's very stable because one of the groups is pushing electrons and the other one is pulling them so it's it there if there it's feed the electron rich is feeding right into the electron poor and that makes for especially stable resonance so here we have extra especially stable resonance structure therefore this one is going to be the most stable and by most stable it means it's the least basic the methoxy group on the other hand is an electron donating group it's going to be putting electron density into the ring electron donating group makes the the ring more electron rich and increases electron density near there in the amine group okay so therefore this one is going to be the least stable or most basic questions on number four question number four anybody here you look at the pk's of the conjugate acids for these and you can see that's it looks like it's not scrolling right but we got the pka's of the conjugate acids here four point six PKA one and this one here is PKA five point three four so exactly like we predicted and that's always nice when we can use our predicting ability and we find out that we're actually right with what what we think so most acidic here therefore the conjugate base is the most stable just as we predicted this one here is least acidic therefore the conjugate base is least stable all right how about question 5 what are you thinking
first one second one third one most basic I heard second one very lightly anybody else want a second to secondary let me second the second one okay which nicer is the most basic definitely this is correct most basic this is due to hybridization effects so may not remember hybridization effects that again is back in chapter 2 so everything you learned all years coming back here right here in 51c so let's remind ourselves what hybridization effect is if we look at each of these compounds here the lone pair is in an sp2 orbital this lone pair right here is in an sp3 and this lone pairs and in sp and so that nitrogen there is SP SP hybridized and we know that a nitrogen that's SP hybridized is more electronegative than a nitrogen that's sp3 hybridized so that lone pair is going to be more stable on the on the SP nitrogen okay and so that's because when you hybridize when you make sp3 you combine a 1s and three P's so the sp3 orbital is going to look more like p orbital than an SP than an S orbital when you combined one s and one P and this this and this is example here of the SP it looks a lot more like an S than a P and we know what an S orbital looks like s orbital looks like this oh hi oh hi electron density around the nucleus so this is an S orbital and that we know when we have a p orbital electron density is on average further from the nucleus so if we have 1/2 s and 1/2 P those electrons are closer to the nucleus as we show here in the in this third picture here alright so for this one here electrons closer to the nucleus that means they're held more tightly we can approximate this if you didn't have I talked about this in 51 a if you didn't have me for 51 a what we can have we can approximate this is we know that there's the positively charged nucleus and we know there's electrons and we can approximate that by thinking about really strong magnets if you have a really strong magnet and you line them up so they're attracted when you hold them really far apart you can't feel any force pulling them close together as you get closer and closer and closer you feel them wanting to pull together and if it's a really strong magnet you probably are gonna have a hard time keeping them from getting coming in contact with each other so as these get closer to the nucleus they're held more tightly and that's going to make it more stable less available for reaction a couple ways is saying this less available for reaction more stable
over here electrons farther from the nucleus therefore more available for reaction less tightly held and therefore it's going to be less stable all right so that's why this is the most
basic is because there is less s character electrons less tightly held cause they're further away so this is going to be less stable therefore more basic alright and this is actually a very large effect especially when you get to SP hybridized atoms compare the PKS or the conjugate acid acids you do not need to memorize these numbers but this one here is PKA - 10 pairs PKA about 10 and this is PKA about 5 so a really pronounced effect especially when you get to SP hybridized atoms questions on number 5 anybody so I may have you on the test I may actually moist the most basic which is the least basic I may have you ranked ok so you want to be prepared for both types of questions what about number 6 which one's the most basic first one or second one what do you think hmm I'm hearing a lot of first that's correct alright so this one here more stable due to inductive and resonance effects let's look at inductive effect first so what do we know about a carbonyl is that an electron withdrawing group or electron donating group from chapter 18 we know that that's an electron withdrawing group so inductively is pulling electron density away making nitrogen less electron rich right the carbonyl is powerfully electron withdrawing this makes nitrogen less electron-rich and therefore more stable and more stable equals less basic so may not even thought about the inductive effect of the carbonyl group that is certainly there we also have a really powerful resonance effect and we know that Amin's are stabilized by resonance and that's really important for an am abhi cuz the nitrogen is pretty basic so it because it's pretty basic it likes to donate its electron density and so here we have here the a mid it's gonna want to donate electron density to the and remember remember this had a really high boiling point because of the fact that it has a very very large dipole moment due to this second resonance structure that I've drawn we of course can also have the one where we just push electrons onto oxygen and leave the nitrogen there so we do three resonance structures for this this is the very minor resonance structure but we do have resonance stabilization and so because that lone pair on nitrogen is delocalized it's going to be less available for reaction so therefore it's going to be more
stable and that equals less basic questions number six anybody alright so now we're gonna jump ahead to chapter 17 and talk about aromatic heterocycles and acidity and basicity I'm not a favorite of you guys but let's talk about that that may be something that you might need to review because you know the things that we learn at the end of the quarter are or aren't learned as well and that was at the very end of 51 B everyone was ready to go home for the holidays and so it's you know you may need to review it okay so when considering a city and basicity of aromatic heterocycles keep in mind if nitrogen is part of a double bond then the lone pair is not part of the aromatic PI system and the nitrogen the lone pair it's free to act as a base if instead the lone pair is actually remember we're e hybridized we put the lone pair into a p orbital and made that ring aromatic if that lone pairs making that ring aromatic it's not going to be it's not going to act as a base it would much rather stay aromatic than act as a base okay so again a lot of you will need to review that so so so what we're saying here is this nitrogen is part of a double bond therefore on this lone pair is not part of the aromatic side PI system and so this is what the structure looks like so this nitrogen here is part of a double bond it is sp2 hybridized and so here this lone pair is actually in an is actually perpendicular to the P orbitals in the ring it is that that lone pair is in an sp2 orbital and because that lone pair is not involved in aromaticity at all has nothing to do with that ring being aromatic we can use that nitrogen as a base and we're used to that we used nitrogen as a base a lot right we did that in Chapter nine maybe some other chapters sprinkled here and there so we know that the nitrogen is going to be basic so let's label that let's do one more label so therefore the nitrogen is free to act as a base all right in the second situation this was one of those ones where we had to take that lone pair on nitrogen and we had to put it into p orbital in order to make this ring aromatic remember we needed six PI electrons for M plus two pi so nitrogen is not part of a double bond or certainly before we rehydrate eyes it it's not part of a double bond and so therefore the lone pair is part of the aromatic PI system so you know this is really not like chapter chapter 17 chapter 17 you were deciding whether that was aromatic or not now we're telling you it's aromatic and saying okay the fact that it's aromatic what does that mean about that lone pair that lone pair is tied up making this ring aromatic so that lone pair is part of the PI system part of the aromatic PI system and know what that means is that if we protonate nitrogen that rings no longer gonna be aromatic that's not going to happen right we're not going to protonate nitrogen to make that ring I'm not aromatic and in fact you actually can protonate the spring it does not protonate on nitrogen it actually protonates on carbon we're not going to talk about that but if this is you you are not ever going to protonate on nitrogen here okay so therefore nitrogen is not free to act as a base alright and so this is going to
help us make decisions about these guys as acids and bases we have oxygen and
again might need to go back and review chapter 17 if oxygen is part of an aromatic ring only one lone pair is part of the aromatic PI system okay so one lone pair is part of the aromatic PI system and the other lone pair is not it is not part of the aromatic PI system so that's what it would look like here so this lone pair is part of the aromatic PI system does this lone pair you can protonate that lone pair this lone pair is perpendicular to the aromatic PI system and so that means that you can protonate that oxygen so you can protonate here all right so let's see how how we're going to use this to make decisions about acidity and basicity so if we look at parole for example do you present you do you expect parole to be very basic what do you think no well impaired is part of the aromatic PI system it's part of the automatic PI system therefore if you protonate you lose aromaticity not going to happen so that means we have no protonated parole you will not find this let's put a big X through that what about deprotonation of parole can we do protonate parole well if we do pronate parole it's going to look like this is that okay that's isoelectronic with fear in right so if we take base yeah we do protonate that's going to look like Tyrion and in fact we can remove that break that base this is PKA approximately 15 we can't remove that base and this is still aromatic looks like this okay so this is a p-orbital here this is an sp2 and looks like you're an alright so let's compare the pKa of parole with that of ammonia alright so you do the same thing with ammonia and let's compare that with deprotonating parole this one actually has a pKa of 36 and it turns out that parole is in here the lone pairs are in an sp2 orbital here in lone pairs are an sp2 orbital here the lone pairs are I'm sorry sp3 orbital what am I saying here the lone pairs are an sp3 let me fix that lone pairs are an sp3 orbital and above the lone pairs were in an sp2 it's more stable it's more stable to have lone pairs in an sp2 orbital alright okay so it's less stable conjugate base than above most stable conjugate base and above then therefore the mean is going to be less acidic so it makes sense that pearl is more acidic than ammonia because of the stability of the conjugate base all right so let's write let's write the answer to that down wise pearl more acidic than ammonia it has a more stable conjugate base and that's because the lone pair is in an sp2 orbital which means we have more s character therefore electrons are closer to the nucleus and therefore held more tightly therefore it's a more stable conjugate base so all of these things are true so gotta got a bet that if I'm spending a whole lecture talking about acidity and basicity that you're going to have questions on the final on acidity and basicity guaranteed where is my scrollbar why are you there we go questions what about pyridine do you expect pyridine to be very basic well gosh we used it as a base all throughout chapter nine at least right so yes we do I would lone pairs not part of their aromatic PI system therefore protonation does not disrupt aromaticity all right so when we protonate when you protonate parole which we've done a number of times this acid has a pKa of of 55.2 compare this with the sp3 counterpart which would be just ammonium ion and this one has a pKa of 9.0 pKa of about 10 and so we have a more acidic conjugate acid because the conjugate base is more stable why lone pairs are in an sp2 orbital rather than an sp3 for ammonium ion the lone pairs are in an sp3 orbital four for protonated pyridine pyridinium ions the lone pair that the lung for the conjugate base the lone pairs are in an sp2 overall it's going to be more stable to have those lone pairs in an sp2 orbital alright so this helps us when we have some of these heterocycles that you see in bio you'll see these appearing things like histidine things like that and we want to be able to predict city and basicity of these compounds so let's look at a midazolam reactions of imidazoles and ask for base are important in many of biological systems and now hopefully you're gonna be able to look at that and tell me what happens if we add acid to this where are we going to protonate top of bottom nitrogen top isn't that cool now you can do that that's awesome really this lone pair is not part of the PI system therefore you will protonate here the other lone pair is not available for reaction because this is part of the aromatic I system therefore will not protonate here so a pretty easy job right now to draw me to draw the conjugate base may ask you conjugate acid may ask you to do that on test who knows this is what you'll get when you add acid you'll protonate on that nitrogen yes lone pair is yes lone pair not let's fix that thank you so much for catching that because I would have had a lone pair not part of the oh I put the not enough lone pair a lone pair not how about that is that good don't paired all part of aromatic PI system we'll continue this next time
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