Carboxylic Acids

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Video in TIB AV-Portal: Carboxylic Acids

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Carboxylic Acids
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1
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27
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CC Attribution - ShareAlike 3.0 USA:
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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: 00:08 - Structure and Physical Properties of Carboxylic Acids 09:33 - Acidity of Carboxylic Acids 32:37 - Common Bases to Deprotonate Carboxylic Acids 36:48 - Oxidation of Aldehydes and 1° Alcohols 37:48 - Oxidative Cleavage of Alkenes & Alkynes: Ozonolysis
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Alkyne Physical chemistry Ionenbindung Alcohol Kohlenstoff-14 Chemical property Carboxylate Resonance (chemistry) Alkene Electron Solubility Hydrocarboxylierung Heterodimere Peroxide Hydrogen bond Carbon (fiber) Carbonylverbindungen Hybridisierung <Chemie> CHARGE syndrome Water Lone pair Hydrogen Acid Computer animation Functional group Azo coupling Chemical compound Covalent bond Cobaltoxide
Physical chemistry Molecule Water Hydrocarboxylierung Kohlenstoff-14 Computer animation Chemical property Carboxylate Carbon (fiber) Angular mil Cobaltoxide Solubility
Alcohol Ionenbindung Weakness Carboxylate Resonance (chemistry) Acetic acid Chlorine Alpha particle Electron Shear strength Hyperpolarisierung Acid dissociation constant Conjugated system Hydrocarboxylierung Substituent Induktiver Effekt Carbon (fiber) Carbonylverbindungen Erdrutsch Food additive Water Hydrogen Acid Computer animation Functional group Chemical compound Base (chemistry) Cobaltoxide Methanol
Sense District Acetate Phenyl group Benzoic acid Carboxylate Resonance (chemistry) Acetic acid Chloride Molecular beam Methylgruppe Chemistry Alpha particle Fluoride Nitroverbindungen Electron Acid dissociation constant Benzene Conjugated system Atom Process (computing) Substituent Stickstoffatom Induktiver Effekt Iodine Carbon (fiber) Electronegativity Hydroxide Food additive Acid Computer animation Functional group Azo coupling Chemical compound Amination Breed standard Bromine Base (chemistry) Cobaltoxide Isotopenmarkierung
Alkyne Alcohol Biosynthesis Carboxylate Chloride Hydrochloric acid Chemistry Ammonium Alkene Dichromate Acid dissociation constant Sea level Conjugated system Oxide Ozonolyse Asset Aldehyde Protonation Chemical reaction Electronic cigarette Wine tasting descriptors Water Acid Computer animation Base (chemistry) Extract Bottling line Ammonia
all right carboxylic acids carboxylic
acids can contain the carboxy group so this group right here is the carboxy group may or may not remember that from 51a and we have a couple shorthand ways to write carboxylic acids I want you to be familiar with both of them we can use co2 H and we can use COOH I normally choose this way to draw it I think if you write it as COOH there's a tendency for students to write it like it's a peroxide and not a carboxylic acid so those are the two shorthand way of writing it let's talk about structure and physical properties let's we definitely know that carboxylic acids are resonance stabilized so let's draw some resonance structures we're going all the way back here to 51a so we can certainly move electrons up on the oxygen and break that PI bond and really we can do that with all of the carbonyl compounds and we will be doing that with all of the carbonyl compounds that leaves us with carbon without an octet it's a it's a small resonance contributor but it is a resonance contributor and then the other thing we can have the electrons on oxygen come in and kick in and we can put a double bond between the carbon and oxygen that leaves us with a positive charge on oxygen and a lone pair so definitely resonance stabilized carboxylic acids are residence stabilized and if we draw the hybrid again all the way back in 51 a pretty much everything you've learned all year comes back and 51c so all right so we draw all the ones that aren't changing when I draw hybrids I usually leave lone pairs off and if I ask you to draw a hybrid you can leave the lone pairs off but you can see that we're going to have a partial bond here any bonds that change are dotted and we also have some partial charges if you look on the carbon of the carbonyl it has a positive charge in the middle resonance structure so you posit partial positive here the oxygen has a partial positive charge also because we see a positive charge on the third resonance structure and then certainly we have a partial negative charge on the on the carbonyl oxygen all right so that's our hybrid and so what we say is that this has double bond character here's our double bond here here's our double bond here so we have double bond character and so what does that mean that might that means that bond is shorter than a normal single bond but longer than a normal double bond so this bond is shorter than a typical carbon oxygen double bond or shorter than a typical on carbon oxygen single bond and then if we look at physical properties just comparing that well compare a carboxylic acid with an alcohol with a ketone and with an alkene just to see what we have here boiling point of the carboxylic acid is going to be the highest one seventeen point nine you do not need to memorize these numbers but I do want you to know the trends here eighty-two point three degrees for an alcohol fifty six point five degrees for the ketone and minus six point nine for the alkyne percent solubility in water infinite for all three for the first three compounds if in it solubility zero percent solubility for the alkyne so one of the questions is why do the carboxylic acids have such high boiling points relative to many other organic compounds and the reason is is they form very strong hydrogen bonded dimers so let's draw what one of those looks like they form strong hydrogen bonded dimers so let's draw it will draw one carboxylic acid here so there's one and the oxygen of the carbonyl is going to want to do a hydrogen bond with the hydrogen of the other carboxylic acid there's one hydrogen bond here's the other hydrogen bond okay so there's a hydrogen bond dimer that's going to dramatically increase the boiling point and then when we extend the length of the carbon chain as you would expect the
boiling points are going to go up so adding one carbon we have one forty one one more carbon one sixty three so as we increase molecular weight we're going to increase boiling point 186 here and 205 so predictable increase of boiling point as we increase the length of the carbon chain and then solubility as we would expect is going to go down so infinite here infinite here and then we start getting to a point where we have too many carbons and not enough oxygen so this would be 3 grams per hundred mils of water again numbers are not you don't memorize these numbers we just want to know trends and then we go down to one gram in a hundred mils of water alright so definitely a trend that we would expect that you would have predicted back in 51a why do we have this trend if we increase boiling point increase falling point why do we have increased boiling point because we have increased molecular weight and increasing Vander Waals forces and solubility goes down on the other hand as we increase molecular weights we decrease solubility because the the nonpolar hydrophobic portion of the molecule is getting larger so as the nonpolar hydrophobic portion gets larger when this gets larger the solubility decreases all right questions so far and structure and physical properties so no no great new news here nomenclature we have a handout and we have a podcast the podcast is not posted it will be under the practice link and I'll let you know when we're ready to do that you don't have to do nomenclature of carboxylic acids yet so what I like to do is do all of the carbonyl a nomenclature together and that will all be on the podcast and I'll let you know when that's going to be posted online it's already done I just need to post it and you guys will
be set all right so now we're to the
portion of the chapter acidity of carboxylic acids if you had me 451 a my notes are almost identical to this if you go back and take a look at the notes 451 a so so it's it really is a hundred percent review so of course when we treat a carboxylic acid with a base we're going to deprotonate the acidic hydrogen that's bonded to oxygen that gives you a carboxylate ion and whatever the base you use do we get the conjugate acids BH and it turns out that that carbonyl it greatly enhances the acidity of carboxylic acids so if we compare here we have PKA about five it's actually four point eight but PKA about five and for water and alcohols these are be a pKa of about 15 so something pretty close to 15 here so if you had my class 451 1851 B those P those eight PKS that you had to know rounded to the nearest five that's also going to come in in this quarter also we're gonna add a few things though to that just a few carbonyl compounds so I'll let you know when that's going to happen so why are they why are carboxylic acids more acidic one of the first things we we like to do and we're comparing acid strength is to look at the conjugate base that's that's usually a very helpful way to go so let's look at the conjugate bases and see if we can explain so we'll just we'll just pick methanol methanol would be very similar to water but let's pick methanol let's look at the conjugate base of the Tharks I died on and let's compare that with the conjugate base of acetic acid oops I don't want that protonated all right so we know that conjugate base of acetic acid we know that that is resonance stabilized so we can push electrons we can share that those electrons between two oxygens equally and that's because we get two equivalent energy resonance structures and when you have two equivalent red energy resonance structure that that typically makes the compound very stable so we have two equivalent energy resonance structures all right so what do we have do we have any special stabilization so so definitely the conjugate base of acetic acid carboxylate is resonance stabilized do we have resonance stabilization for methoxide ion anybody so what we can say is that on yeah there's no resonance structures electrons are localized electrons localized on oxygen and certainly over here the electrons are de-localized onto two oxygens it's always better to have delocalized electrons always better to have to localize electrons so we need to localize on to two oxygen it's also really powerful as we're gonna see coming up that D localizing electrons onto oxygen is very good when we can do localize on to oxygen and so what we can say here is that the conjugate base conjugate base is resonance stabilized and since the conjugate bases Mora residence is resident stabilize that means it's going to be more stable so we have a more stable conjugate base if we have a more stable conjugate base what does that mean about the the conjugate acid for that it's going to be it's going to be more acidic right so the acids more acidic so anything that we
can do to stabilize the conjugate base makes the acid more acidic super-strong acids have very very weak conjugate bases ok so just something to keep in mind all right so that's reason number one and reason number two is that we actually have an inductive effect going on here that's not probably not as obvious to you so let's draw out our carboxylic acid this is just another way to think about it now we know that the oxygen is more electronegative than the carbon so we already have some good polarization of this bond here partial positive here partial negative and so so that will be what we end up having here is electron density the carbon eel is actually an electron withdrawing group just like we've talked about in chapter 18 so it's pulling electron density away so the carbonyl group inductively draws electrons away so it is an electron withdrawing group which I abbreviate ewg alright so what does that do for our conjugate base here's our conjugate base carboxylate ion and so what we have here is we have pulling electron density away here is that going to make that more stable or less stable more stable so the the inductive effect of the carbonyl makes oxygen it's pulling some electron density away from oxygen makes oxygen less negative and therefore more stable so more stable conjugate base means the acid is going to be more acidic and it turns out that if we have substituents so that would be just plain acetic acid but if we have substituents attached to near nearby the car box carboxylic acid that it's also going to affect acidity so anything that stabilizes the negatively charged conjugate base is going to make the corresponding acid more acidic so if we look here we compare acetic acid with the carboxylic acid with a chlorine bonded in and what we're going to when we do the nomenclature you'll see that this is the Alpha position there's chlorine in the Alpha position and so what happens to the pKa there when we have a chlorine in the Alpha position this is 4.8 and this PKA moves up to or moves down to 2.9 so it becomes more acidic when we have a chlorine adjacent to the carboxylic acid and how we can explain this as how we usually explain things when we're comparing acidity we look at the conjugate base all right so this chlorine is inductively electron withdrawing and so what it's going to do is going to pull on that electron density its going to pull electron density away from oxygen just like this so the carboxylate is stabilized by the inductive effect of the chlorine substituent and that's just simply because it's making oxygen less negative unless negative oxygen is going to be a more stable so therefore the conjugate the conjugate base is more stable so here we have not only the inductive effect of the carbonyl we also have the deductive effect of the chlorine adding together therefore the conjugate base is more stable and therefore the acid is more acidic and definitely this next slide which this is the inductive effect of other substituents on the next page and definitely this is straight off of my 51a notes so let's take a look at this
and this is just kind of comparing different groups that are in the again the Alpha position so basically what we have here let's look at the conjugate base what this is going to do for us let's just repeat what we did on that on the previous page if we have an electron withdrawing group here ewg it's gonna pull electron density away and stabilize the conjugate base and if depending on how good that electron withdrawing group is is going to do a lot of stabilization it's going to do a little it's going to affect the acidity and so if you look here well we're comparing x 4.76 and so that's sort of our that's sort of our standard there that's acetic acid with no substituents and then if we substitute iodine here we said iodine bromine and this gives the electronegativity so as the electronegativity is getting larger the pKa is dropping okay so fluoride more electronegative than chloride and so it's going to have a more powerful electron withdrawing effect it's gonna pull electron density away it's gonna make oxygen even less negative so the more electronegative the electron withdrawing group the more acidic the compound so that makes good sense these guys we can't really give them an electronegativity because electronegativity is for a single atom but these are electron withdrawing groups okay so they're electron withdrawing groups and you can see on three point eight three so hydroxide not super good it's not even as good as iodine and we're used to thinking of hydroxide as an electron donating group in chapter 18 right it can be both if when it's in the Alpha position it can't do anything with resonance so it's a it's just simply an electron withdrawing group tonight also an electron-withdrawing group it's about the same as well it's about a little bit better than fluorine right so it's kind of nice to have a measure of that nitro is the most powerful of all 1.68 so you put a nitro group in this position here there should be a carbon there - you put a nitro group in this position and it's going to be even a stronger electron withdrawing effect methyl is actually the opposite that's electron donating and so this would be for this one here this is an electron donating group and let's go ahead I should fix this I'll let you fix this we forgot we dropped our carbon here so let's fix that first and then we'll do methyl so ch2 and this is ewg so I want to fix that there that's better right so we're going all the way here for the methyl that's the opposite that's an electron donating group it's not a very powerful electron-donating group but it's a slightly electron-donating so that's actually pushing electrons in the opposite direction it's it's very weak but you can kind of see pushing electrons in that direction what's then so that's it's actually making it less acidic than acetic acid not a very powerful effect but it is slightly electron donating all right so I'm summarize this let's label these guys here here we have increasing electronegativity increasing acidity and this probably makes a lot more sense to you now than it did even back in 51a because we've had so much more chemistry these guys are also electron withdrawing groups and wanna make a note of this nitro is the most powerful strongest so when I say we want to make a note of it that usually means I'm probably going to ask you a test question about it so just to give you that right you want to make a note of that so you know that like you cut out I don't know how they edit this but that way you cut out what I just said okay so that's the strongest electron withdrawing group and then this one actually has a higher PKA because it's an electron donating group it's not by much I agree but it's got a higher PKA just does give you the idea here higher PKA than acetic acid which is the unsubstituted version and that's because ch3 is slightly electron donating when we have electron density flowing towards oxygen it's going to make it more negative this makes oxygen less neg more negative more negative is there is less stable alright so therefore the conjugate to be on base is stronger and therefore the acid is weaker and we're going to do this again when we talk about amines we're going to spend a whole lecture talking about acidity and basicity of a beam so this is same same sort of thought process there and so it inductive effect decreases as the electron trying group gets further away so we we start off here this is 2.97 and we jump up to 4.01 and then we jump up to four point five nine and then we jump up to four point seven one so we're almost at four point seven six right we get there and so as this group gets further away its effect falls off pretty rapidly it's so big a big difference there all right let's do a couple more outside based examples how about this one which one do you think is more acidic first one or the second one first one not a profound difference but the first one is definitely more acidic for point two and this is PKA four point eight and so how do we explain this we're going to do the same thing we did previously look at the conjugate base it turns out that the the phenyl group the benzene ring is slightly electron withdrawing so if we compare benzoate ion with carboxylate acetate ion this is an sp3 Cart sp2 carbon right that's p2 carbon more electronegative than an sp3 so it actually an sp2 carbon is going to be more electronegative it's going to pull electron density way it's behaving just like an electron withdrawing group so inductive effect stabilizes the conjugate base and anything we do to stabilize the conjugate base is going to make the acid more acidic therefore the acid is more acidic all
right so might have some carboxylic acid acidity trends on midterm one pretty darn likely that's sort of a must okay all right let's do one more example which do you think is the most acidic first one second one third one I heard it's the first one out there all right PKA two point two three point four and four point two so the Nitro group makes a big difference it makes the biggest difference when it's the closest to the carboxylic acid the inductive group falls away rapidly as you get increasing distance so we're going to look at the conjugate base again here's our carboxylate and we'll put the nitro group in the ortho position and you can see that this nitro group very powerful electron withdrawing group more powerful than fluorine it's going to pull electron density away strong inductive effect the closer the better strong inductive effect stabilizes the conjugate base how does it do that it makes it less negative the oxygen less negative therefore the acid is more acidic over here with this one here the Nitro group is further away as as simple as that electron withdrawing group farther away therefore inductive effect is not as strong all right so this is going all the way back to chapter 2 on our acid base chapter so let's look at some common
bases used to deprotonate carboxylic acids you may run across these in the lab did you do an acid base extraction was that last quarter is that this quarter last quarter so you you encountered this some in lab last quarter it NASA could be deprotonated I based it as a conjugate acid with a higher PKA since pk's and many carboxylic acids are about five bases that have conjugate acids with PKA values higher than five are strong enough to deprotonate them so that's what you want to look for and so some typical bases these are some typical bases let's finish that sentence that are used to do protonated carboxylic acid all right and notice the notice the conjugate acids of those bases are all greater than five and that's what we want to look for notice pKa of the conjugate acid for all of these bases is greater than five could we use chloride ion to deprotonated carboxylic acid know because hydrochloric acid is a pKa of minus seven you have to be greater than five and it usually significantly greater than five so on let's show you let's show you why that works let's use ammonia the second one on this on this page let's use ammonia write out the acid-base equilibrium so we have that we have ammonia let's do arrow pushing let's finish writing out the acid-base equilibrium and then we'll level note the direction of the equilibrium so here's our carboxylate our conjugate acid is ammonium ion so to figure out the direction of equilibrium we look for conjugate acid-base pairs so here's our conjugate acid-base pair and the one that was the extra proton that's our acid and then here's our other conjugate acid-base pair and this one has the extra proton so this is our acid we should have one asset on each side of the equation and then we look up the pka's for those PKA about five and PKA if you had my class last quarter you would recognize these as one of the ones you needed to know round it to the nearest 10 PKA about 10 for ammonium ion alright so that means the equilibrium very highly favored to the right and one of the things that the other things that I do is kind of do an estimation of how much we have of each reactant and product we take the difference in PKA so that would be five and so at equilibrium we have about one reactant for every 10 to the fourth of products so that means that that's a really good base to choose to deprotonate if we tried to use chloride ion to do the same thing the equilibrium would be in the opposite direction okay so those are some common bases that we're going to use and in the last minute we'll talk about just some review reactions for how we talked about preparing carboxylic acids h2 co3 actually you can't put that in a bottle we get that from mixing cro3 and h2 so4 there's those reagents again or some people like dichromate I don't know I don't like this one as much CR 207 and h2 so4 can we use PCC for this no no PCC why does PCC um what does PCC give you with a primary alcohol aldehyde not a carboxylic acid so so we can start from a Nelda now primary alcohol we can also start from an aldehyde we will get a carboxylic acid we can also do ozonolysis ozonolysis of alkenes with a oxidative workup these are the things you forget over time reagents but if you understand the chemistry then that should be the thing that you don't forget over time if it's a if it's an alkyne we don't have an oxidative workup we just have a water workup again something that you're probably not going to remember and that's why you want to be doing those synthesis problems at the end of each chapter because it will remind you of these reagents it's time
to stop now we'll continue this on Wednesday
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