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Lecture 11. Eliminations


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Title Lecture 11. Eliminations
Title of Series Chem 201: Organic Reaction Mechanisms I
Part Number 14
Number of Parts 26
Author Vranken, David Van
License CC Attribution - ShareAlike 3.0 Unported:
You are free to use, adapt and copy, distribute and transmit the work or content in adapted or unchanged form for any legal and non-commercial purpose as long as the work is attributed to the author in the manner specified by the author or licensor and the work or content is shared also in adapted form only under the conditions of this license.
DOI 10.5446/19229
Publisher University of California Irvine (UCI)
Release Date 2012
Language English

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Subject Area Chemistry
Abstract UCI Chem 201 Organic Reaction Mechanisms I (Fall 2012) Lec 11. Organic Reaction Mechanism -- Eliminations 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.
take treated trick there's some love candy floating around 6 simply the be the
passing the bag around the was gone that'll give you some some free calorie energy to keep your brains going down the path of calories referring to pursuing get older OK so we are removing on in this club forward to talk about a fundamental reaction another set of mechanisms the common to the introductory organic chemistry courses and specifically eliminations and really on it's a very it's a common set of reactions in which some sort of feel donate some sort of role to produce a pipe bonds and all the things that I'd like you could potentially apply to the reverse reactions and will do some of that later on but we're going to talk about 3 types of elimination mechanisms on all talk about he wants and wiry 1 reactions because you that deep Urals cargo cat that anyone elimination mechanism that was involve cobbled I on and they're fast because it's so easy to donate into an empty the orbital was the best except orbitals that we talk about look at the alternatives here is if we reversed the roles instead of having a really good except orbital hazardous lanes Cygnus star all I can we have a chlorine chloride Leading Group and we want to eliminate that out so if you have a not-so-good orbital donating it to you better have a good nucleophile that's anyone the elimination mechanism so non-binding long pairs especially for cover behind a very nucleophilic that's why that's fast not because you're except Wirral Sigma stars good it's because you're you're filled water is very nucleophilic and then last year in this sort of series here what you got not achieved orbital and you've got that's that's empty and but not general or worlds fill the CH behind all these are to be the worst of the 3 possible elimination mechanisms if you see an elimination you're trying to hypothesize what's the elimination mechanism do not jump to this is your 1st choice I'm saying that because I know you will have a tendency to jump to that is your 1st choice because it's the most obvious in focus of stock about these 3 types of elimination mechanisms ,comma I'll start off by talking about the 1 elimination mechanisms they all of cargo can irons and so that means we better have good leading groups that's when it's going to tends to be on those will tend to be prominent mechanisms for elimination and
not just good leading groups but you want to have a stable cobbled cannot and that's another clue that it probably proceeds through it in the 1 elimination
mechanism 1st to generate a couple can then in a 2nd step people proton and then finally another characteristic of these types of reactions are you we nuclear files right under those conditions if he had strong nucleophile you'd end up with an S 1 reaction so the it's typical of these types of reactions that you have this combination of factors going on so its talk about an example here but I guess 1 it as so theological significance maybe scientific significance as well this is a designer and I'm sure I would call it a designer Arnold an analog of from narcotic called Demerol Sowetan opioid looks just like morphine when it binds to the new opioid
receptors are men of course Demerol has a carbon near an oxygen over there and so somebody didn't know how to synthesize that they thought well just make this through green
yard addition to a towns and always like that who cares for the oxygen is there there it turns out that binds really well to opioid receptors just like morphine but the problem is that you're sloppy contest and the conditions are acidic you can see what's going to happen here is that you're totally set up for this generate cobbled Kanye I if you're not really particularly good at chemistry you throw into an acid or your proprietor chloride is contaminated with probiotic acid in your heating this up so the problem here is that while they were cranking out batches of this Demerol analog morphine analog of the others in a flat form they crested you run up and away
here is that so if you're cranking out batches of this what did happen is the branch of the badges they were cranking out were
contaminated with this the 1 elimination by product so you can see what happens here is you eliminate the city of a cargo can iron and then in the 2nd step not in the same status people of a proton from next door and so now they were giving out in Civil War history were selling batches or continue with this this is actually innocuous the problem is that the human body metabolites is this 1 of those rare metabolism here are you into a compound that who would have known this nobody would have known this on the causes destruction of the substantial Niagara area of the brain and so you end up with Parkinson's disease and it's not slow in onset Parkinson's like like age-related heart disease you instantly get Parkinson's disease and you don't recover from that and you just hear catatonic use this freeze
up in that area of the brain is too short that's turned out to be 1 of the number-one tools now and Parkinson's research because they can induce Perkins Parkinson's
disease in animal models on it all came from the 1 elimination mechanism OK so that's an example where you're totally set up have a stable Koble Cat that's that's not just tertiary depends elect must take a look at alternative elimination mechanism and probably the most common elimination mechanism that you will be exposed to if you do organic synthesis so more common in the new 1 mechanisms so that you won conjugate base refers to the fact that you're going to have some conjugate base in there you're going to pull a proton off and make a copy and iron so under basic conditions we've got strong bases and that's pretty typical of of organic chemistry nowadays we use powerful nuclear files like e-mail I told all this particular resonance structure the way we do lots of the chemistry on organic synthesis and so if you do and how little reaction who these would be typical
conditions up with sodium hydroxide Yukon duties at room temperature on you end up getting
a beta hydroxy Kerviel compound like this and this undergoes an elimination to give you any known under the conditions of the reaction it's reversible but thermodynamic favors this and the known and under the conditions of this reaction and sold with prominent about you making 1 CBE elimination mechanisms on is you have something that's capable of forming an acidic or something as people filming a Caribbean islands we have acidic sieges in there and if you were OK with making the Caribbean I'm here in the 1st step late well you already OK with making it here on the product the features of the about the same thermodynamic acidity and if anything it is got better an anemic asserting saying unimagined deeper alternating here to get the same kind of
late that you added them in initially in the starting material before the elder although there are pushing for the all but called Arnaldo
reaction will talk more about that was later OK so now you've got this great nucleophilic alone near here which is perfectly capable of pushing out and Allcock side leading group in order to give that's not and this is something that's very particular about curbing and on and on about how an ionic pairs they're perfectly capable of pushing up hydroxide if you somehow had some rule that hydroxide oxides could not act as leading groups look at this this reaction occurred at room temperature so Cox either perfectly capable leading groups will see more of that later so limited in example just to remind you that it's very common for actors leaving you may have thought only halogen sketches leading very frequently intermediates that look like this in
your reaction mechanisms this is nothing more than an hour than anyone see the elimination intermediate right you already know that Alcott sides connectors leading groups
and a minor along pairs nearby because you've done this over and over again when you when you've drawn out mechanisms for the hydrolysis investors under basic conditions focus that's the 1 reactions the 1 conjugate base there's the conjugate base and the conjugate acid is before you pull the proton off so every of highly basic conditions and you conform stabilized ,comma Van Lines you always go through on this sort of two-step mechanism and I'm going to urge you to I'm not to do this never not ever don't ever never Germany December like this never not ever Hey Hey I heating I don't even want a regular Arizona draw -dash heroes don't ever try to do this that and the few elimination mechanism if you do this you're implying that it's better to add to this Sigma star here Alderson all label that there's an orbital here for Sigma Stauffer ,comma carbon-oxygen you're saying it is better to donated their antibody moral stigma stock than it is to donate into pie for a and you know that's not true we talked about this in the 1st week of class it's easier to donated the star wobbles than it is to donated Sigma started making a late 1st and then you pick up hydroxide and 2nd day so resist the temptation to draw that is a one-step 2 elimination mechanism OK last the last mechanism is ready to if you can't form stable Koble Caroline's and you don't have strongly basic conditions and and the possibility for stabilize carbonized then you can start thinking about 1 the 2 elimination mechanisms attacked
you depleted all that stuff OK so long as that would take an
example from so he can form stable ,comma Kanye and Saudi Kennedy proved to meet in order to make some sort of carbon and you don't really have too many choices left security example of something that is definitely going through a need to mechanism there is a on a benzene sulfur leaving groups often into believing groups like toss mutilate and if you throw in a decent base here you could probably get away with traveling in this particular case they used Alcock side protesting TV topside on its slightly hindered based helps a slowdown in normal and otherwise also slow could you can feel tricked out associations and Neil Pendle center which really dissuades into reactions this position and see that this really has a whole lot of choice here on except to give you the elimination process so in this case the base for a handful of new dramas protons going downward because it'll just make me happier there we go know and you can make the Aral and on the carbon or you can make the arrow in the middle of the double bond and accept either wonderful just want you to be precise about where your showing air landing you can show that's that Meleiro making of forming a pipe for you joint attacking the same T-bond mortal here for the Slovan and all accept either of those 2 depictions OK so if can't cargo Caroline's easily or you can make carbon ions easily next to leading groups then you can start to come talk about Conservative lead to elimination infections in this case there is nothing on this
carbon atoms to stabilize the car behind so you have a car remedial or some some sort of accepting and from this stuff up here I mean this nothing conjugated to any there's
nothing there is nothing that would severely stabilizer and focus was look at the 2 in details on so you know it's not really that common of an elimination mechanism on if we really need to look at this and no because there's so many reactions that this helps us to think about so the going drawdown a sort of a conformational depiction here and I want to be very particular as I draw about this sort of substituted carbon-carbon system about the NT relationship the required interrelationship between the proton at you pull off in the leading group this is the idealized orientation to elimination reaction mechanism where this proton where this Proton carbon bond here is indeed the leading group and when I have like this there is bonding orbital here for this city and unjust label it so we're donating sickness .period reduce carbon ex-FAR and so this is indeed bonding orbital has a lot of on the or below was very big back there it's very easy to overlap with that and sold the electrons in these bonds are weekend when you when the base attacks and that makes it easy to to push that out if you look at at at the rates for reaction when this Proton is on
the Sewanee Writers outlets and interrelationship versus since it turns out that India's faster it's not infinitely faster but you
can put some numbers on that and I will put some numbers Bloomberg OK so here's to different orientations of the substrate and were talking about a system where there is no special staring incumbents to 1 relationship or the other and so usually when you look at the rate of the sea elimination adversity elimination it's at least a factor of 10 better to do than and at least I don't know many cases where it's not of these 10 times better to do this it intimidation had to
think about how would 1 think about this and we try to depict the antibody for this year found that an antibody
moral it's thinking back here and I'll try to draw some sort of phasing in here that Sigma staff the CX bonds and why is it that this condoning if you look at the canonical law but all 4 offerors signal bond like this 1 there is electron density on his backside and its smaller usually most of most of the electron the elect under spending most of their time in the middle between agencies but there is a lower on the back end here any time you draw a lone pair there's a little lower on the back and any time you draw canonical sort of representatives in the modern world a little over the back and so as you're putting electron density in here this starts to of electrons in there is overlap it's none 0 so it's not impossible to Busan eliminations they're just never is easy Santi elimination and so because of that I want you to be experts at seeing hinting relationship this is
critical not just for the lead to elimination reaction fought for a vast amount of organic chemistry and that's why I'm so picky picky picky
picky about how you draw cheers cycle the purpose of the could take a look at 2 different substrates horror 42 elimination reactions so this is a cycle hexane rang immortality Buell group that very very very much wants to be a Equatorial revolted TV will grow by at least a factor of a thousand does not want to be axial and here's to different isomers of the system trans isomers
a difficulties dies scary images of this cycle Axel bromide so if you elimination with with potassium team Utah
oxide which is a strong base so this will be to elimination that can make a stable core cat eyes says no we want there's nothing there stabilizer Ka-band I'm so it's not the 1 this is going to eliminate to lead to elimination mechanism and if you look at the relative rates free to elimination of the substrate is 500 times faster the reason why that's 500 times faster may not be obvious until we draw the protons that get eliminated there's 1 proton that's not interesting but there's a proton down here that axial fits perfectly into pure plenary at all times it's perfectly anti-terror to the carbon running on dictionary and we come over here and we look at the protons their data to the bromide here and neither of these protons
Isanti to the carbon bromine bond fund he emphasized the into relationships here so here's an antique relationship
between his Proton carbon bond and in the car and leading group on looks great that's perfect but we come over here this cotton-growing bonhomie look at what the anteater that it's these carbon-carbon bonds and you're not going to do to elimination by attacking those carbon-carbon bond right it's essential that you see these types of relationships because those would set you up for the nation OK so it unless you've got some other special reason why not to initiate as soon as the 2 elimination reactions are occurring on painted with energy relationship so you may or may not be familiar with the term stereo specificity selective reactions are reactions that prefer 1 they prefer to give you the Al-Qaeda
versus the reality where they prefer to give you a really throw dice terrorism were 3 Iestyn stereo specific means that 1
stereo chemistry of starting material give you 1 0 chemistry product and the other stereo chemistry started which opens new and different stereo isomers the product you can't know whether something is inferior specific continued test both stereo isomers of the study indicates that the only way that you can make a claim that something is scarier specific so it's wouldn't draw out ,comma scarier was the result of stereo specific elimination of antiprotons protons the to leading groups I'm trying to draw this in a way that you can clearly see with crystal clear to the besieged by using can adopt a confirmation that the antidote to that old tussle with only 1 proton unabated carbon here and so when you do any to elimination of this man in this case they did with the box then you're going to preserve this relationship here a relationship that you need to see to make sure you don't mess up when you drop the product were going to have a double bond in the product I and then he dropped the waiters in my notes here so and we have to decide there's another and there's a muffled Rubin hydrogen here and which 1 ends up where and what you need to see the see Spanaway here it is the answer to this carbon it's in starting material and it will be empty in the product the carbon this methyl is Conchita fan the other muscle-bound and I look over here and I see that this carbon bond advantages carbon stage bond it will still be empty to each other in the product and so it's important for you to see the ozeki relationships otherwise you might make a mistake drawing out 1 Eisner says the other if you take the other guys is that we know that stereo specific this these that this group took the time to try the other dies during misses the dies Deromer now and now the H on this stuff this
this Taxol toss Alexi carbon part right here the methyl analysis which teachers coming out toward you on me when we make it more obvious all
make it clear what's coming out toward you here here we go again this is the same reaction also lead to cancer now when we dropped the product here what are the relationships that get preserve seeded carbon here it's pointing forward that is the hard that they better be empty in the product In from eliminating out this this Proton tussle Oxy the pair of groups that I want those 2 methyl groups to stay in the and I look at my carbon fennel bond that better be Bianchi to carbon each on in the product so that's stereo specific reaction 1 stereo summer of history will gives you 1 steroid product and the other gives you a different stereo from you can't know whether something is still specific until your pet test both of the scary license in the reaction focus list included a stereo selective reaction another word I could also call this a stereo selected reaction because it prefers 1 scary each of those reactions hysteria selective any reaction did it favors 1 sterilized summer on is called stereo selected so whenever you take just long alkyl chains so you can imagine R 1 and R 2 year to be just big long alkyl chains and you have some sort of a leading group and use objectives to eat to elimination conditions so like
trampoline which is not so basic typically those or potassium TV topside at room temperature it's reasonable astronomers began to get at
this just alkyl groups secondary couple can find and stable Tonelli won elimination mechanism you if you don't have a car behind the lighting of Nottingham on artillery and economical I'm now all this can really only be any 2 mechanism they prefer to give Trans double bonds and the way to see why that is true is to look at and Newman projections that I stand in front of this sort of I stand over here and I looked down this carbon-carbon on what I see my starting materials may be the confirmations in the starting materials will tell me about the kinds of interactions in the transition state for elimination and that is true so if I stare down and bond I draw Newman projection for what I see in front of me here is the preferred confirmation I would want some sort of a confirmation which Manitoba I'm getting this right here there are 1 or 2 look like this this would be ideal I don't want these things bumping into each other I'm set of now so I so I can as a cold as Proton off the electrons in a bond push the comic strips and of course there is another Proton here I could eliminate many spin things around so that other Proton is now aligned in the there we go and I forgot to drop my last each year on the job located in adjust to different rotor murders of the of the same
starting job and just trying to get you to imagine what would happen if I spend this around so that green Proton year by it's not distinguishable than coloring green whatever device
by arrested that green Proton were in the to the leading group now there's an obvious problem with any transition state that resembles the starting material confirmation if you're transitions they resemble the and that's going to start to hurt in the transition state and that's why you favor these these trains olefins and the products annually to elimination on just a linear on just stretching out OK 1 of the hottest I think 1 of the hottest areas the most confusing areas all organic chemistry is the issue of competitive reactions between elimination substitution as saying 1 of my job it's a wider they start off with this in organic chemistry is so complex that the 2 until you have to worry about all kinds of a full lasting they should cover any any organic chemistry course I'm going to try to talk about the sticky issue of substitution versus
elimination of any time
you have a secondary alkyl he lied to me right or exit and those the interesting ones because they're typically have some stereo chemistry of the possibility for a stereo center close the interesting cases on if try to add some strong nucleophile strong liberals tend to be strong bases unfortunately and so you usually see mixtures of mixtures elimination substitution reaction you get mixtures of products and that's all for the sake a look at the example of some rates and Omega Pharma will draw very simple conclusion about the effects of temperature so I can't imagine anything simpler than this this is
about a simple you can get for a secondary alkyl bromide bicycle bromide we treat this a thought that we have 2 choices the dockside into
to reaction displace the bromide but that's kind of crowded and there's all these data protons 3 protons year free protons there that you can pick from and of course this reaction does below the gives you need to elimination and gives U.S. into elimination of sorry has into substitution reactions and I want to look at the the at the rates for formation of these as we vary the temperature so the rate constants that somebody took great pains to measure absolutely constants and don't worry about the units there from Pesek and just look at the relative numbers I'm looking at the number of media and thinking that slow 10 to the minus 6 from
all of the 2nd is a slow reaction and when I draw this out but the important point here
is also pretty similar but that's about the same injured going to get about a one-to-one mixture of elimination and substitution reaction so if you design your total synthesis of your natural product to rely this is your final step is going to get screwed unless you want mixtures of 2 different things I can think of why that would be good but that's what happens is we raise the temperature from about room temperature to about 120 or whatever about 100 degrees so maybe daughter reflux and toluene and there's other consequences to that but if we just look at the overall effect of Tandja now immediately which you can see it as a reaction to faster you should expect that of course reactions are faster as you raise the temperature molecules have more energy when they collide with each other for what you can say is that the passenger reaction didn't gain in speed as much as they need to this is
a general truth in organic chemistry and is not just the 2 were you want generally he favors
elimination over substitutions he favors elimination of a substitution so maybe want your reaction to be faster and you're trying to do a substitution be careful they're both going to get faster but the elimination will get even faster and raise the temperature so now you're looking at it like a 60 30 70 30 ratio these 2 Soviet trying to make out the next great you can increase the selected and so we just write this down so he generally favors favors elimination of over substitution of I come back to this especially for the 2 reaction isn't in tropical climates this age can only this age carbon bond can only push this out when it's auntie best is best able push sell when it entered the convent .period bond and this base will be most apt to attack the proton when it comes in exactly from the back side there's all this requirement for water in the transition state everything has to be aligned just perfectly and the chances of you having this confirmation or more likely when you increase the number of collisions that's the effect of temperature so you don't have as much of a conformational on entropy requirement for and the substitution reaction of those requirements are more stringent the entropy requirements are more stringent for elimination of wide benefits more from temperature from raising the temperature this is you can want to compare the 2 most common of leading groups because they're
not equal and you have a choice in organic chemistry what you make I'm going to give you a show your reaction here was just a long stretching out Kane With 18 gardens then at the end we have some sort of a leading group so they have an alcohol here you can choose to convert it into a bromide using PBR 3 three-year the appellate action will talk about later with Tractebel fighting or you can divert the alcohol into a toughly and so which 1 should you choose both depends on what you want to happen in these reactions to the choice between S 2 products lesson to substitution I'm not going to draw the whole formula the soldiers racy 16 received to 2 competitive reaction pathway so when and if you throw in potassium TV topside that with this index is equal to Bromide you don't get a whole lot of Edison to substitution here it's mostly elimination and of course that's volatile it comes off you never see that she might be reaction was so clean from is not so Walter seized 16 is a combination focused and then burst x equals possibly so what's a nucleophile same backbone same amount of hindrance essentially now we haven't I Pasolini group you get much more substitution much less elimination so why is there this difference between he
lied and possibly held by many measures you you might say that they have about the same leading Group abilities that's a pretty good generalization
reminded conflates are good leading groups for very different reasons so at the very beginning of this class we said that there are 3 kinds of factors that come into play as as reactants come together we said there's Coloma faxing related to charge the steric effects electrons don't want to occupy the same point in space and we said that there's interactions between filled and unfilled orbitals there's less take a look at how that explains this contrast in the outcomes of these reactions undergoing draw out a substrate where I have but of bromide in addition
to some sort of a proton and why should this kind of a system why should our global might be good for elimination reactions the reason why bromides are good elimination reactions it is when
attractive Super overemphasize this in some freakish way and we go there Sigma stars all phases some sort of freezing here for the Cygnus storm from a common bromine bonded common here Lijun Bonda along their weak and Sigma Star is a very very good is very easy for a nucleophile looking for some empty wobbled become overlap with this now there is some partial positive charge your bromine is also a lecturer negative but that's a relatively minor contributor here but this from my dominion of little partial minors so with a carbon halogen-bonding that is being Sigma star and a relatively small charge if you have iodine here iodine is about the same intellectual negativity is part of In fact if you
look at the original polling skills Poling said that carbon is more Elektra negative iodine so you can't be claiming that there's some charges that said it's helping you attack here in the case of Idaho ballots contrast this with a softening this could be a
Meza late red metal at half the weight of i've toluene there doesn't matter what matters is the softening now we got a decent Sigma store here this is mostly carbon-oxygen illegal socializing must always lets the anti-war bill nuclear file was looking for some durable overlap with and there you can overlap with role as good the big factor here is the significant amount of partial positive charge at the current oxygen especially with the sultanate there is much more electoral negative so now the
Alcott side will attack quickly here in the negatively charged nucleophile attack quickly here because the charge the partial
positive charge the Cologne lake effect is very big helps them with so newcomer where you look at Tulsa oscillate it's 99 favoring attack that discarded but when you have the brown-eyed here there's very little that's a partial positive about the carbon as powerful positive it's doable he's opposed the partial positive charge that the effect of an elected negative Adams create partial positive charge so now there's not a big contributor for the Alcott side minus onto B Cologne likely attractive through electrostatics to that Cardinal yet nitrite trial the troops you mean what what I predict for this elimination to eliminate out I would predict this
would be very poor because you know the bond is not particularly along since the bond is not long you don't expect a big Sigma star warble here it's not
particularly Elektra negative like of bilateral Adams I would expect it to to be very difficult to pull a proton off here and simultaneously attack here 14 U.S. into now there are many examples we do this but that's no this is more like anyone who see the elimination of Is this what you this switcheroo getting and that's very common mom still cyanide can act as a leading if you've got a good enough to break this longer there all the time always overlap and there's a better chance to pop to push the let's switch over I'm tired of protons and maybe you retire protons to discover something more interesting this was just imagine
for a moment that we put a medal here and I hadn't I wouldn't really call that a medal but those us sending metal sufficiently so whatever we were saying about protons you can imagine along nucleophilic bonds long bonds are more nucleophilic to tender silicon since 10 is better longer and more nucleophilic consumers city at either these medals here ,comma what you find is that it's very easy if you have some sort of a leading the bemusing probably hydroxyl is an example to overlap with the singer Mustafa a ,comma not wanted to push out the leading group these are particularly capable now these can be these are drawn is 1 step mechanism hours Arias as to step he wanted mechanisms we already talked about this this is not anything new we already talked about the fact that ,comma kind are vastly
stabilized by beta silicon abated In fact any long bonds will do this just take a look at some numbers I can't remember where they gave these numbers
before if you look at the relative rates for how easy it is to form cobbled Caroline's and then a 2nd step worry can be 2 steps you you can use this long bond took to donate over in form of bond like this sometimes you have to have a nucleophile adding to make that metal and 8 complex Stanley Silicon 8 on the that's the important point is that you can really favor want type reaction together beta metal bond has to go to the relative rates for these types of elimination mechanisms I they been measured it turns out that the in those long-term bonds are about the same as Mercury hammer and I think America is very similar things like palladium certain bonds are very long so metals transition metals In the periodic table comparable to 10 and those are about a million times faster then long silicon bonds so all of those right hand 6 here China's about tendon 6 better than silicon just as mentioned more anything that silicon can do 10 is better than and them fortunately it's also more toxic so 1 and that's again about 10 that that better than having a proton for an elimination so if you really want to eliminate you want to put a beta silicon nearby we have a proton on 1 side and silicon on the other Aqaba Kanye and it's the silicon it's going to eliminate that's that's I'm going to eliminate giving these types of a double bonds if you really want to control any 1 type elimination have silicon will hope you make the Carbol can and eliminate out OK so let's take a look at some Silicon reactions so again his great silicon is less
toxic and non-toxic I guess I would say so there is an
elimination process called the Pedersen authentication and there's 2 variants of this reaction or show you those 2 variants slashes the demanded we got some sort of beta silo alcohol and in this case I'll draw stereo chemistry here associated with the starting material I'm not going to draw the pages at this position women time Apple-style group alcohol beta to that and there's a particular stereo Commissioner to levels are coming forward instead of 141 back and so there's 2 different ways feed to eliminate out the silicon and the oxygen we a choice can treat with acid rain country with me you can either convert this into a leading group or you can convert that into a little while the people talk about a different variant of suspected of making this into a good leading to the animals have to use acid in this particular case the treated this was trifle overseas began hydride and insulated this alcohol so you can also do the FBI probe mentality under acidic conditions in this case where I'm showing as they make a better living by easily that I wish I had room here because I want to draw that Scotland's Silicon bond as long as there's not turn this into a better leading Group it's not a great leading groups try floral acetate is not like a tussling leaving on but what happens is that makes this good enough as a leading group that this carbon silicon bond can stabilize formation of a cargo O'Connor this will spontaneously popped out and give you a Vegas-style cobble can I'm highly substituted cargo cannot and let me see if I could on the draw like this
that's supposed to be playing and you get the idea and destroyed during the same shape so you get this super stabilized status
electrons in this long we are didn't draw along with his long nucleophilic of Bonner donating that NTP orbital and so as soon as the nucleophile comes in attacks and that I'm sorry I don't have room to drop the whole thing in here but all drop like this is a trifle acetate that got popped out is floating around it's very easy for these 2 attacks silicon silicon loves oxygen allows Elektra negative oxygen cylinders can come in and attacked the silicon and don't do as into it will initially former Silicon 8 with firebombs perfectly happy and then that Silicon With negatively charged silicon it's long and it's in integrated bond there will then pushovers not going to drop of those steps and so that's what gave you this in which this what's in which the CIS relationship but end preserving assists relationship between these 2 methyl groups here OK so that 1
version of the Peterson alternations there's a different version of the Peterson alternation in which you treat
this with a very strong base and you don't have it's covalently coordinating conference Utah not 20 years lithium counter answering 1 things potassium that don't bind strongly to oxygen and so when you do that I'm going to flip around this this central bond in my molecule I'm going to flip around when I draw this so that I can see the oxygen can adopt a confirmation in which it ceded to the silicon and so when I do that if I flip around this bond a twist around that Bonn cost after the final move that hydroxyl so pointing out that the move this with pointing back so it may look like there's a sleight of hand and this is just playing in my to the fact that the oxygen loves Silicon I just told you that
oxygen loves silicon and so we'll end up with this four-member murdering silicon 8 notice my use of language their
silicon a tells you there's a negative charge on silicon freak out over the 5 bonds to Silicon against silicon is not a 2nd rower began 6 farms I worry about that Silicon Labs oxygen so now all the bonds to silicon a nucleophilic into this whole contraption here can fall apart like this and so that will cut out a sidewalk sight on iron and now give you the Transylvanian get to choose with appears in all of a nation where the you get on the CIS isomer the trans isomer and Vegas-style poll to choose from and I really think about this 12th this goes back to the idea that if I drop this this may not use the same and color that I drop this signal orbital here there's also electron density on the back side here and that can overlap with the Sigma starring in demanding wonderful sing Mustafa the carbon-oxygen bond and so this canonical signal orbital for this new global carbon Vermont does have electron density on the back of the canal push that this is not a Paris parasite 2 plus 2 reaction analogous to the deals action there are lots of concerted reactions that are not parasite that this would be an example of 1 of the 1st OK so Tom loser 2 variants of the Peterson alternation you may never do appears in alternation in your life but you know I feel like it's very likely if you go in the synthesis that you'll end up using this type of elimination reactions and here's a more traditional example of an elimination and involves a silicon at home we will be said
complex molecules you're always looking for ways to use orthogonal protection strategies you don't want it where every single 1 of your 12 protecting groups all comes acid because often and you want take off 1 protecting European the others intact so sometimes will use a single detail protecting group chronicle silo and boxy carbon take off T stands for trainable silos and so if you treat this with the naked fluoride in so Tetra Beutel ammonium chloride really call this key battle brought out in this case for so that's a non coordinating countriesin detectable ammonium doesn't coordinator fluoride if anything coordinates the it'll suck the electrons it'll suck the nuclear Felicio like this is a source of naked fluoride and and so when you had the that fluoride comes in and attacks that silicon so yes oxygen of silicon chloride love silicon even more we talked about the bond strength of a silicon flooring long kind like the strength of a carbon-carbon triple by flooring loves silicon so you end up with this silicon 8 now all the bonds to silicon a nucleophilic and in this case the nucleophilic time to pop out this summer this column this car ban including 92 so by making a bond nucleophilic you can push this out injured and that allowed the news carbon-dioxide either on the work of a the reaction conditions and that's how you take off at a architect group without exposing your reaction to acidic conditions
the you may have occasion to want to make Granada regions that look like this but I can't make these and maybe it's obvious after talking about that take out why you can't make those the reason you can't make those is something called the board reaction or if you have a nucleophilic bond right here become nucleophilic Bettis and you're hoping that that's going to attack some private deal it's never going to get there because it's going to do this 1st while you're making your Granada rating is going to be decomposing on you this reaction to eliminate
ethylene so and this is true for any nucleophilic carbon metal bond so he did something like this in treaty was sodium and we'll talk about why don't make alkyl sodium is later on in the courts found that
in this case is that you could do green yard as well as magnesium if you convert this into a nucleophilic bond it will immediately snap open if you got a nucleophilic bond here that will immediately open up and maybe you want them to open up but in this case you would be able to make a green yard reagent some nucleophilic center here because it's simply going to pop that open OK we're going to see we've got just a little bit left to cover here for uniforms for elimination reactions to 1 will become Bakol finish up talking about these nucleophilic bonds the debate oxygen multiple viewpoints of stereo specificity instead of talking about nucleophilic carbon from carbon metal bonds will try to extend this to other types of bonds that can push a leading groups
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