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Lecture 09. Addition to Sigma Star (σ*)

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OK so we've spent so far the 1st part of this class talking about addition
to empty Urals Koble kinds in reactions involved in Cobble Kanye incident and keep your wheels on carbon carbon wants to satisfy the temple that's why it's so fast added to cobble can not the charge that makes it fast it's the uptick rule makes it faster now administered prices .period is momentarily if I were to look at things that I want to attack the virus nucleophilic pair of electrons what what I want to attack by 1 0 at half price .period next would step that will come back to that and must talk about sigma star adding to Sigma store orbitals just looking at these energies you would predict that these
are the toughest reactions in organic chemistry In addition to
cobble can either going be the easiest everything can do that everything can and do without I'm but now it's about things that are the hottest reactions and all qualified abide by saying it depends on what kind of signal store or were talking about I'm going to sketch out a periodic table does roughly like this this is my view of the periodic table we try to put the 2nd row a little higher here and is 1 of those 2 that can only wonder conform bonds and that's hydrogen helium is not not interesting it's over there the hydrogen is in a totally different wrote no 1 considered 1st because when you look at Sigma store Wilson all created equal so you can imagine some sort of an and all of that the money to start off right here Of all dropping a nucleophile attacking us in store but to so have I have some nucleophile the displaces died in a concerted reaction the reason why this works is because we're adding to the Sigma store orbital for a carbon iodine bond drew the sketch
out that orbital on my sort of rough approximation for
what that orbital looks like and phasing in here notice these 2 interlopers of that war will have opposite phase and that's why there's no bond in the middle if you add this you break the bond so that's a typical listened to reaction but there's another type of Cygnus that's not slow but that's fast and it's consider that the alternative would be if I had a pair of electrons attacking a proton and I'm going to reuse different symbol here I'll use the symbol base to represent that people Sigma store orbital and this is this is a totally different role of the periodic table :colon is down here in the 2nd row protons in the 1st row and whereas these reactions tend to be very slow listen to reactions tend to be slow additions to this kind of Cygnus dawdle orbital tend to be very fast and furious spent some time talking about the signal store bolster to protons because they're fast and then we'll talk about making carbon-carbon bonds or bonds to carbon here so Gandhi 2 different I'm not going to claim that this is a good representation for an actual stink bombs dropped the same way they just you can see the face of the actual sinister is a slightly different chief Riccio but it's the same idea signature the orbital so here that the Sigma adding to a Sigma star for an age bond and here at Sigma Star carbon iodine OK so let's talk about addition to Proton and proton transfers and I think that's a talking about this because it just makes me feel lot because I told you on the 1st day the golden era pushing mechanism was to break mechanisms
down into elementary reactions status and so I'm gonna step aside from that just momentarily it was Manfred
Eigen was a German biophysicist who was interested in proton transfer steps to be look at any enzymatic reactions biological reactions he can most organic reactions proton transfers play a major role in that and what Manfred Eigen won the Nobel Prize for 1967 when he learned how to do Is he learned how to do ultra-fast reaction kinetics he learned how to measure reactions that goes so fast that nobody had previously been able arms to measure them before this illegal and show you the mechanism the discovered for our proton transfer reactions I'm going to draw a simple proton transfer reaction and I'm not
really intending Felicity diamond Philomena just wanted at 2 different Adams here politican sitter momentarily the mechanism for this approach transfer reactions wire
transfer proton tonight from nitrogen dioxide yet but this is a three-step mechanism to transfer the program that's what mankind of what Manfred Eigen show really disturbing stuff and otherwise when he showed is that the 1st step IN ANY proton transfer reaction is formation of a hydrogen bond that's the 1st step there's an energy barrier and a you form a stable hydrogen product looks like this and then there is a very fast bond vibration there's an equilibration amended this age at that I didn't do just this I apologize for this for me what I wanna do not wanna move overseeing can tell the behind reminders about 2 inches long the typical C H 0 wager on each Bonds about 1 Engstrom long she
can tell that this is not fully bonded the oxygen because it's close to the nitrogen that there's a 2nd step in this mechanism where this Proton vibrates forth you could
measure the frequency that and I are effective how fast it will have a stretch around 30 500 wave numbers that's the frequency so when I would redraw there's there's another extreme former this is now an ox only a mile on and this is now hydrogen bonded to the end minus and these are not residents structures the hydrogen atom has moved the atoms move it's not a resident structure so then an energy barrier for the very small but there's a manager and then in the last step you break the hydrogen bond and now the hydrogen atom transferred over to the oxygen now we already said do not use 0 pushing for data bonds or hydrogen bonds we can use this Ferrero pushing it if you try to take electrons of this bond there isn't really it doesn't
represent an orbital so we're stuck now How are we going to represent this using euro pushing an amendment is very simple there's a role for Arab
pressures and that means you draw H bonds draw hydrogen atom transfers as 1 mechanistic steps and change at the proton transfers not hydrogen atoms at all when I say I didn't have any radical this is another words when you draw the Aero pushing for this I want you to do what you've been doing from day 1 just take this long pair attacked the agent bond the the age and then break agent by that I want you to represent that every time you do that I want you to remember that it is a mechanism that is dependent on hydrogen bond formation and as soon as you forget that as soon as you forget that hydrogen bond formation is 1 part of this three-step mechanism so I wanted to draw like this but every time you do that I want you to remember involves H bond formation because if you forget that you're going to suddenly run into cases where reactions don't make sense anymore and when you remember that this is a three-step mechanism that's when you have something to fall back on OK so let's take a look at it on it's important fact about Proton transfers and it has to do with reaction geometry but but it was for the but so
when protons transfer back and forth between atoms they prefer linear geometries the we want and I feel like I need to justify this because you will have a tendency to do it through nonlinear geometries and I want to tell you why you shouldn't do that there's good evidence after the idea that the proton transfers and involve ideally transition states in the form of a city with a crystal structures involving hydrogen bonding from revived him bonding is
essential for Proton transfers if you look at hydrogen-bonding crystal structures they tend to be linear take a look at any protein crystal structure neutron diffraction structure where you can actually see the positions of the hydrogen atoms and you look at
hydrogen our bond geometries they tend to be linear 180 degrees In other words the hydrogen isn't like way over here it's right between the 2 atoms of the 180 degree bond angle between oxygen Proton and nitrogen or alcohol to oxygen and hydrogen bonds doesn't matter zillions of crystal structures and zillions of hydrogen bonds you can look at all of that and this is what you want to see the 2nd piece of information and I don't know how much harm this may not make much sense to you yet but when you take chemistry 202 you're going to learn about kinetic isotope effects what happens if I replaces Proton with a deuterium auditoriums more massive but twice the mass of proton
expect things to slow down everything she gets lower stress is massive it turns out that how much lower depends on the
geometry it's not quite as slow it's more than that for a typical on kinetic isotope effect for protons what you find is if you look at proton transfers for compounds that have deuterium instead of proton it was I got beat represent basin a to represent acid yes it slows down when you change of from out of Proton for a dude ranch but it slows most when the transition state Romania when he designed molecules where this has to be bent to redesign molecule that this has to be transferred in a linear fashion so that maybe it's like a five-member during it slows most when Proton transfers involved linear the government is right with linear transition
states and that's evidence that will indeed the preferred transition
state is a linear proton transfer those kinetic isotope effects don't worry about the meaning of this I don't regret that yeah he'll learn about getting camp 202 OK so everything we know about proton transfer suggests that Proton transfers wants to be linear and here's why this matters for you as someone who's is going to push heroes and mechanisms but the very frequently you're going to run into cases on like this where you have an intermediate that looks like this and you're going to want to somehow explain how this Proton get from here to over here maybe I'll draw on a methyl repairs of these oxygen is look a little bit different and somehow or another you have
to explain how the Proton going to get to the other oxygen otherwise you can't finish a and you will be tempted beyond belief to do this all look at this
because he's it's right there next door will come close that is assigned to and from the way that's not the mechanism based on everything we know about Proton transfers integrity to intermediate the mechanism is 2 steps toward a few under basic conditions drawl to represent based review under acidic conditions like a pitcher's verification from A-minus to represents the conjugate base of an asset and then pick up the proton burst but to generate an intermediate in other words ever trying to explain how that Proton moves over here we don't do it intramolecular that's not the way it works involve some initial proton transfer that's very fast proton transfers like this are fast here's my being here is my age and you go ahead and elegant that I've done right next to his own minus I can take it right off it's never faster transfers under realistic organic chemistry conditions it is never faster to reach over and have this longer somehow magically and up With the linear trajectory there's no way that that almighty spends time over here the only way you're going to get that Proton over there is if you draw a transition state with the word it's got a 90 degree angle between oxygen Proton oxygen and I can't I guess I can think of something less ideal feel but that is still not ideal it will be faster for some basic molecule to pluck off the Proton and we deliver so that's all I want you to draw proton transfers integrity intermediates through through intramolecular on Tuesday mechanisms cooking so why would I think this should be so fast while I believe that there would be such a fast reaction faster than that for Member during transition state
where I just reach over and grab I think there ought to be lousy in contrast I think I'd be fast to transfer
protons intermolecular the 4 of whom we know what we get wanted drawdown on the table for you worry ponder on this this simple equilibrium as a very the asset I want to look at the transfer of protons this is data that was formed was collected by again was pretty insightful this is meant to be a rate constant fears a rate constant for the backward reaction would be so I start off 2 things end up with do things and I'm just transferring a proton
back and forth between water and this a minor species and what I'll remind you of is that the
equilibrium constant for this reaction when you know the rate constants is equal to the board rate constant divided by the reverse rate constants you might have interpreted of equilibrium constants in terms of concentrations but there's a kinetic view of of equilibrium constants and you should recognize this equation this is the acid ionization equilibrium equation this equation that I just to appear as the basis for PKK Sony good drawing the table here right I consider some Proton transfers forth the water for various different acidic species make table here and outside on the structure of the acid normal right down the PKK and then all right down to rate constants that I did measured there's a scenic acid hydrogen sulfide too protons attached to sulfur and although at the CEO acetate you may have learned in near software Ganic chemistry courses that those ch is between the 2 carbonyl very acidic all drawn ammonium ion channels rather bond there final draw nitro methane Major Group strongly stabilize carbon lines next door on the furor made methamphetamine from nature thing you took advantage of that hope you didn't do that whatever I'm going to draw the PKK is here for the species and I could look this up in any PKA a table and I expect you to know these PTA's are approximately that help will talk about these more later ,comma all expect humanizing PCs and what I wanted to as I want to tabulate these forward and
reverse rate constants on for these different proton transfer so in other words up here in my equation if I replace HEA with hydrofluoric acid and I think about the transfer of
Proton for major where the water comes along an axis of base plots off Proton but I would find is the rate constant for water plucking off this Proton from hydrofluoric acid is about 10 dates and the these promoter Pesek and I'm not going to write the units in every case 100 thousand times per Moeller perceptive if everything was the present 1 molar concentration would happen 100 thousand times a 2nd itself fast water would pluck a proton off that feature that I look at a seedy guessing which of these 2 is more acidic a chair for overseas guests that Japan's
more a CD Gossett is less acidic but it's kind of comforting the city gas is
less acidic and transfers its proton more slowly to water get idea sulfide that's even less acidic than a CD gasoline it's kind of nice to see that it transfers its proton more slowly to water it's kind of comforting to see that I come back over here to this at the Lucille acetate and it's 10 to the minus 3 that way slower so I can see a Felicio I just look at the PGA conceded AFL-CIO acetate is less acidic it's now much so now I look at ammonia and it's actually 25 but others right tender the 1 to keep everything they stand for that doesn't make so much sense when I something like does it's getting faster again shall come back over here and look at nitro methane and now it's really slow and minus 8 ahead is a very slowly it's not easy for water to be premier nitro methane and stronger basis for that so what I want you notice it looks like for some of these there's this kind of correlation between acidity and the rate at which it donate a proton defends nominal the reverse rate constants how quickly because I had drawn and put a proton back on him too much you see is the rate constant here is about 10 the 11th this looks faster than diffusion control usually diffusion control actions were things collide max out at about 10 of the 19 but these were running in water so there's an advantage to transfer protons to water at a very fast 2nd order rate constant and it's fast because water is the sole of look at a seedy gasses water
plucks the proton water so sorry I join delivers a proton acetate very quickly a look over here just it's very quick
visa all diffusion controlled it can get any faster than at every single collision between a droning online and fluoride minus results in bond formation every single collision between high droning animosity minus resulted in a bond formation so now I come back down to the cellar Felicio acetate and well that's not diffusion controlled something is we're here but can ammonium ion well that's about diffusion controlled and I'll come down the nitro methane and kind of trouble there to outliers here where the outliers that don't seem to make sense then what is the scene here
I drawing and really delivers a proton back to the United defeated withdrawal rates except for this case and these were also the cases where I suddenly my
monotonic decrees that seem to match didn't work so well so this is kind of an exception for some weird thing going on here and there some weird things going on right here with these 2 holidays too different from the other 2 cases for the rest of the cases in the table yes transferring a pro from carbon atoms the general rule as we can construct a general rule except for these 2 cases so let's construct general I think I have enough room there swimming ,comma users like the general rule is that Proton transfers to and from carbon you are
generally faster both to carpet start to and from the federal atoms are generally faster oxygen nitrogen fluoride sulfur those
types of proton transfers or fast but for the most part of the world I think this is the federal Proton transfers to and from Hadow items fast and then only in a lot of these cases the diffusion controlled for more that's why I never worry about using drawing of proton transfers to and from oxygen for example up above that's why I'm so competent that it's faster to Tuesday get mechanism because Proton transfers tend to be fast itself it was the 1st of the a similar worry if you end up drawing out a mechanism that is many many steps because of proton transfers because those are fast it's not the number of steps in your mechanism that makes a mechanism correct you can have very long completely valid mechanisms as long as you have lots of proton transfers and don't worry about it but 1 mechanism that doesn't always doesn't involve pretend France's maybe there was a shorter mechanism and you didn't see it could never be afraid to draw proton transfer steps the generally fast the exception is Proton transfers to and from carbon it was was the "quotation mark transfers
to and from Karbala usually slow so I come back over here and I
look at the peak for ammonium and this this carbon acid rain here they have about the same PKK the same people but you look at proton transfers to and from this Carbonado morality 10 thousand times slower than from nitrogen so it's not the PKK matters it's the fact that is bound to carbon and why is it so slow to transfer protons to and from problems it's because those involved hydrogen bonds that sort of hydrogen bonds like this are not really good idea monitor mainly Coloma and there's not a lot of positive charge on the proton the attached to that's what Manfred Eigen show showed he shouldered by by the environment was an essential step in proton transfers and you don't get good hydrogen bonds when agent and part of it was going to
stop right there and talk more about addition distinguished orbital the text of protons and then
we come to also be will start talking about the a method to reactions reform wants to come
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Metadaten

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Titel Lecture 09. Addition to Sigma Star (σ*)
Serientitel Chem 201: Organic Reaction Mechanisms I
Teil 11
Anzahl der Teile 26
Autor Vranken, David Van
Lizenz CC-Namensnennung - Weitergabe unter gleichen Bedingungen 3.0 Unported:
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DOI 10.5446/19226
Herausgeber University of California Irvine (UCI)
Erscheinungsjahr 2012
Sprache Englisch

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Fachgebiet Chemie
Abstract UCI Chem 201 Organic Reaction Mechanisms I (Fall 2012) Lec 09. Organic Reaction Mechanism -- Addition to Sigma Star (σ*) Instructor: David Van Vranken, Ph.D. Description: Advanced treatment of basic mechanistic principles of modern organic chemistry. Topics include molecular orbital theory, orbital symmetry control of organic reactions, aromaticity, carbonium ion chemistry, free radical chemistry, the chemistry of carbenes and carbanions, photochemistry, electrophilic substitutions, aromatic chemistry.

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