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Lecture 07. Neighboring Groups Pt. II & Lec. 8. Solvation


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Title Lecture 07. Neighboring Groups Pt. II & Lec. 8. Solvation
Title of Series Chem 201: Organic Reaction Mechanisms I
Part Number 10
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 10.5446/19222
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 07 & Lec. 08. Organic Reaction Mechanism -- Neighboring Groups (Part 2) & Solvation 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.
OK we're going to continue with lecture 7 today and finishing up and we're also including in that lecture
8 which is really just a couple of things for me to say about solvation so when you look online for the notes lectures by combine them together there is no is no longer a link to a separate notes for lecturing OK so we were talking about neighboring group participation in ionization reactions and we had mentioned things like mustards where nitrogen known pairs can assist in the ionization of leading reflect chloride we talked about acetate groups spending lower with long and carbon heels making five-member rings when they participate in the ionization and so instead of talking about longer on a shift over at about double bonds
and when they can help groups to lionize away and I
want to urge you to look for Homo Allah will groups when you look at ionization reactions in here is an example of that but stake a look at the rates for S N 1 ionization the loss of this city of Aqaba paradigm I'm for these 2 types of Norborne all systems they look very similar the only difference is that this 1 has a double bond here that this position and when you look at the relative rates for S N 1 ionization to make a couple can find when you have a double bond over there the 2 3 positions of the Melbourne all systems on the the rate is the relative
rate and that looks a lot faster why is this huge difference in the rate for the
ionization of this toss Alexi group over here obviously it has to do with the double bond and this is purely example of something akin to neighboring group participation it sort of related to what we had over there with that nitrogen mustard except now the symbols of the Bible and the good draw some explicit participation out of draw a little bit of Arab wishing to see what would happen if I did use that double bond to help push out the leading group but what we see happening to attract draw some of those structures that you again and I hope you will immediately see what's going on here but to be something you're already familiar with now you can't tell from IL pushing on which bond and forming which carbon atom and from the violence can tell whether I'm coming here or here to arbitrarily I'm going to make the cargo can and this position and why is that a special program it's got that amazing cycle probable carbine all set up right here these bonds industry member during their strained their donating into that car will catalyze so it's a great couple
can should form stabilizing on energy or conversely this part bond is indeed helping to
push out that leading group and advise you 100 billion folding great acceleration and of course there's another resident structure could could draw that would put them the three-member anybody here and the couple can I'm on top so this is nothing more than forming a cycle probable carbonyl can the Major we didn't have a cycle propane in our starting material so maybe you didn't see that when you look at the problem said just assigned you all over the place I had these tricks embedded in here because I'm counting on the fact that you're not good at singing into your job is to try to see that stuff is like C Doku except instead of paying money to buy some local puzzling given you free puzzles to work on and we should be paying the money
1st 4 why
don't they used well this is a reaction were breaking the tussle Luxor groups that's no longer why don't all there's another resident structural why didn't I just don't have the time you can draw another resident structural this is bonded over here this 1 0 no because this oxygen is no longer bonded the carbon that's this is this is the cat that results from the ionization OK so what another seeker away that I've hidden this stuff in your problems among those I ask you to compare the fates of 2 different and related substrates This is not Banville Rutherford this is homo Bendel home-owning do about 1 extra
carbon and so instead of having al-Ali the Apollo al-Lail that means you have 1 extra Chicago this is not Denzel this is home Wenzel there's no carbon
atoms so straddle imagine and SN 1 ionization in the presence of formic acid is your solvents such a nucleophile under Solomon colossal losses and so let's imagine if we do a displacement reaction on this and we were going to do is we're going to imagine what would happen if I put a see 13 label right at that position right there is the benzoate position had ceases to label I don't know what I'll show you is that the end products here we to depend on the substituent upon this aromatic ring it is the will of the people of the world but what mean really you
get the same kind of fanatical substituted product number of cases but you don't know that anything interesting happened here until you didn't see 13 labeling
experiments if I label these with 13 the benzoate position the nitro group shows you that it's still there the benzoate position to move anywhere nothing special but when I looked at the at the substrate that group there what I find is that label is now scrambled its 50 percent see 13 label here and 50 per cent see 13 labeling up there so what's going on with this and just be clear that the 13 label at that position How can you get 50 % scrambling to position the strategy all this out I'm going to draw the substrate in this I'm going to try to imagine what we would see if I looked at that edge on infiltrated rather sort of three-dimensional and so it is a picture that aromatic
benzene ring from 1st to stand on the side of that in that John and I'll draw that's that side chains of stituent
sticking off here with the tussling leaving group so there that benzene ring inside on nearby look at them epoxy substituted system should be reminded that there is a resonance structure that I can draw in which I engage the lone pairs on the oxygen atom so the longer it lone pairs a nucleophilic we know that solid strong resonance structure this these electrons right here and I'll Donington and aromatic rings and I I think you've probably done a lot of these types of resonance structures in order to explain the rates of electrical work aromatic substitution why you favor with Paris substitution those are common concepts from from soft more organic chemistry so when I draw all the residents structure this hopefully it'll be totally clear why we're seeing this scrambling effect so now when I go that resonates structure what I can see is that there's a in that residents are true there's a "quotation mark ban located here at the position parent ,comma foxy group so here's a resident structure so you can see
him why offers them epoxy substitutes century but not for the nite substituted substrate you can expect this kind of
participation like that In other words the aromatic rings the electrons in the price system of the aromatic ring could help to push out this Leading Group and so the intermediate that you generate as this structure that at 1st looks insane and we call this particular ion not a realigned we call this often only a mile so well known kind of intermediate this is called often Noni online intermediate there's 26 to
27 kcal per mole ring training the three-member grain but this is really just another cycle properly ,comma vinyl it's that same kind of
cycle probable criminal I could take that positive charge and roll over here with a resonance structure and show that it's right next to those very nucleophilic circle probing boss and there's another resident structure where it's located right here on the other side next to the cycle broken by so that a super stabilize kind of structure and so that's why it's in particular will you have this and that the group here you really can easily formed at that table only mind which is really just a cycle proved combined can transcend our new nucleophile comes into force you have no way to control whether the nuclear follows attacking the label continuity unlabeled common because these look pretty similar those 2 carvings on the map is like isotope effect it would also work force that was among the problems of is the
OK so let's take a look at it think about I don't even know what through
space that most molecules are mostly empty space electrons the small so maybe everything is through space on and what I want to pay attention to where is but the effects of confirmation and medium-size rings and this is the idea we want considered well-aimed carbon hydrogen bonds on once poised I want to think about the ionization of Thai oscillates In a CD gas and so was guessed is the solvent and it's the nuclear this reaction from of course it's a boring transformation that these kinds of experiments really inform us about chemical structure and what I want to look at is how the rates vary as we vary the ring size but the change this ring size from 3 to 4 4 to 5 to 6 to 7 every kind ring size between what you find on is that for some ring sizes this becomes much faster and so the question is why should that be faster for those particular sizes of rings so the 11 member grains aren't quite as fast 12 member drains actually seemed pretty normal leftist more closer to a six-member what's special about this type should ring
size so in order to understand why you get this rate acceleration for these particular ring sizes we need to drop the confirmations of
these kinds of rings and it's closely related to reasoning for this kind of rate of ionization is closely related for why it is so hard to cycle eyes to make 8 9 and 10 and agree some going to start up by drawing what you would get if you had to chair rings used to each other call landmines system 10 carbon by cycling green system on the to cheers called Dekalim and the 1st thing that I know here with Declan although the Huygens the bridge because it helps me a better see the confirmation of this molecule the 1st thing I see is every and this is why chairs are so great every single carbon-carbon bond has a
staggered orientation so by drawing Newman infinite projection for any 1 of the bonds looking down the axis of any 1 of these bonds what I would is perfect
staggering between these bonds but that minimizes the amount of torsional straining the system so this title by cyclic system is is perfect but what happens if I don't have this carbon-carbon bond in the middle what happened about 10 carbon atoms and no bond in the middle of of it it will not be a comfortable situation so only if you want in this world that I want to drive bond middle and simply going to draw up again this this tackling ring system but I don't have a bond in the middle and I still have these hydrogen sticking straight up and straight down and everything else is perfectly staggered the question is where I was going to put these hydrogen atoms which now have to exist in the middle of the range the problem is they're trying to exist in the same locations in
space and this is the confirmation that minimizes the of
torsional strength and so now I wanna replaces carbon-carbon with 2 wages that's a problem and it's exactly this that makes it very hard to synthesize 10 immigrants suicidal creation we call those those bonding interactions between the 2 hydrogen is the goal those strands annular interactions in the trends and unions across the ring so they're bumping into each other and so if you take 1 of these groups down yeah you replace this with the task of and you look at it that the ionization of that group the Cobble can find that exists in this rain actually ends up being stabilized by the electrons in opposite ch by analysts these right
in each other's faces and the electrons in the ch ponder reaching across the
ring and donating entered into binding what they're there in the Cat Island and they're there in the starting materials and they're there in the transition state lowering the energy finalization of the tussle so having direct electron density and my my drawing doesn't do it justice you have to imagine the scene ways of of the into bonding orbital ready to overlap so you can see the effects even signal bonds helping to assist on pushing up leaving any kind of donation of Fillmore laurels and onto morals is a stabilizing interaction OK that's all we have to say about that up about the our neighborhood participation with long pairs with pipe bombs and ultimately if the position is just right even sing-alongs can have that kind of this year now I might confirmation doesn't do her justice that's something
maybe I put this review I put this other each going
downward Manhattan again I would my drawing doesn't do my drawn doesn't allow you to see which of the siege bond is poised at best about 6 OK so I had originally another lecture that just a couple of things to occur solvation it's not really a full lecture so on you can consider this to be luxury we just have a few things to say about solvation I don't know what to do with the rest of the world this is what
we will and so I want you to think about the S M 1 transition state for ionization of bromide from TV or bromide in the transition state the bond distance the distance between Brown and current getting longer featuring develop more partial positive charge this carbon partial negative charge carbon and in particular I want to look at the rates for this reaction as we change the solvent each but the relative rates actually the rate constants overlooking initially at chloroform then compare that to formic acid but and the
rate of reaction and formic acid which is a reasonably polar sold so signed a
relative rate of wanting chloroform for this ionization reaction it turns out that the rate informing gasses is substantially bigger and I want you to remember this difference in rates what if you could accelerate the Ph.D. by a factor of 200 thousand right away if you had way over the weekend for your reaction of finish and basically giving you a recipe for how to make your reactions faster there's something in here your reaction is you don't have to suffer with the reaction conditions you find in the literature because the basic concepts and apply them to how to make infested why is this so much faster but it has to do with this remember the school loans lowered this variant of the Cologne's loft where you say something about charge one-time charge to divided by the distance between the charge
but there's is also another factor in there by the productivity dielectric constant or
some version of that that has to do with the polarity of the solvent the dielectric constant for the solid how good is the sole and school meaning charges so the charges can see each other it takes energy a poem minus charge away from A-plus charge that's very very energetically costly to take a personal A-minus and pull them apart and if you could somehow decrease those charges couldn't see each other then you make it easier for the Chargers apart this could take a look at at some dielectric constants office and various solvents I'm going to give you a table where we were going to
compare it to the gas phase with the dielectric constant for the gas phase it's not 0 it's came easier because of its
hero in the divided it's 1 it's nothing can be lower than that so they think about non polar solvents that are similar to the gas phase hexane is a super non-color solve most things don't dissolve in hexane but it is more things there's all in hexane Zinni 1 anomalous although you might be inclined to use that there's other solvents that aren't quite as non better and better more commonly used eat there I don't believe there is a dielectric constant of only 4 ths is more color than Jeter Ted tried fearing an arms but still it's sort of in this range is wary but I I wouldn't call that an on-call all of it's not very poor I would refer to hack hexane there is non when somebody tells me non polar solvent those of the contents solvents use for chromatography 4 extractions that you think is something it's not allowed the temperatures slightly polar enough where I wouldn't call it on polling day ,comma Fain also has a dipole moment and it's a polar solvent the 2 chlorine carbon bonds have died was associated with them and that's got about the same dielectric constant bulk dielectric constant a teacher and I'm going to show you a big jump to some cop common solvents I would refer to these as polar solvents methanol Di Mathilde the CEO Night trial dynamical Fox side and they all have dielectric constant Ernest 38 of 40 range so numbers here 33 for methanol 37 at the scene nitrous 38 and I never remember the exact numbers phrase but I remember that the DMS so more polar than the rest of those as a group I would consider all these polar solvents Indiana so the most polar among but I try to think about polar solvents to run my organic reactions in
Dionisio was probably about the most polar solvent that I know of strong reactions
except for 1 is 1 of the solvent that I don't have on this list is the most polar water costs can it's totally in a different class here at 78 so wanted to run more reactions like it's in 1 reactions in water what most things .period soluble in water on the course water reacted as well it's a nucleophile and electrified so that's why you don't use water more often OK so another words if you change from the gas phase 2 Dimas so you could expect things to me how much faster for ionization not tender just 50 if you from 1 to 50 who can expect changes on the order of factor of 50 what's up with that that's a lot bigger than
the 30 or factor of 40 arena factor of 78 there must be something else involved here
it's related to solve with it's not just this is an important part of why formic acid is faster because dielectric constant thing can be the whole story so why else around I might that ionization me so much faster in formic acid that was 1 of the I'm going to redraw the transition state where this is pretty steep yield was starting to become flat Net bromide is walking away and starting to pick up its negative charge if you're in formic acid formic acid is very powerful and hydrogen hydrogen-bonding let me draw 1 of those and there's a long
hairs on here but also draw some of them it's just demand in some of those hired him on the Jordan a different
different-colored really gets emphasized a hydrogen bonds and if this is surrounded by former Gassett molecules that can form a very powerful hydrogen bonds that bromide in a way that makes it a better leading Group importantly it those hydrogen bonds stabilizing the transition state as this bromine is starting to walk away as it's starting to walk away with this negative charge those fools high-demand already there assisting they were there in the starting material there the transition state there definitely there when the br minuses floating all around on its own helping to stabilize the br minus age politics stabilize the transition state and what you have to to imagine is not just 1 hydrogen but I won't trouble formic acid molecule there which can imagine this being surrounded by high demands something had to sell
the effects of polar solvents are partially due to dielectric constant but there are
also due to very explicit effects involved interactions between field orbitals the landfill waddles like hydrogen bonds because I want to be cautious not overly cautious there's exceptions to this this idea that polar solvents leader rate accelerations course century action doesn't have a lot this degree of reliance on on dielectric This is what I'm really talking about here is less than 1 reactions to McArdle how's that for the most part of the who world for many issues make this be 333 Of course you can distill the exodus always baffled me still more liquid but it's a common source of try fluoride appearing to reaction and associates to give you a very powerful Lewis acid so there's a reversible dissociation of 143 trade but the effect of of going to a lot of what the effect of clarity on this reaction to sew up a lot of what we said is that if you have a polar solvent that it screens these charges as their starting to pull away but in this case you're not making my eyes you start with an eye on you go to something it's neutral and so here polar solvents will decrease the rate of this ionization polar solvents will decrease during rate of that ionization because the polar solvents will stabilize experimental and destabilize the more and more you go in the transition state
toward those 2 products sold in most cases ionization leads diamonds but there is a
case where you start with ions for something that's ironic and go to something to OK so let's look at some other a explicit tapes of solvent effects like these formation of these formation of those hydrogen bonds for the but at the time of the day it was 1 of the things we were going so th Evan and Eichler methane
have very similar dielectric constant just as a judge of solvent polarity but if you look
at ths it's totally different from that ,comma thank you when you run reactions in dike along with a minuet salts potassium carbonate sodium phosphate they're totally insoluble and indict former thing the sort of spin around at the bottom of of your reaction flask is this white solid it doesn't do anything but lithium bromide interjected totally soluble it dissolves like it's dissolved in water and you can't justify that based on dielectric constant the reason why that's true is because lithium is the 2nd row out of hand it once the electrons just like every other 2nd row out so as many solvent molecules that you can fit around there you will find them attached to teacher and a strong 2 molecules here attached to teacher if I were found to match the charges that I leave the charges of those dated bonds and if I want to have regularly structure 1
that to put a minus to 1 only 2 uncomfortable but on the few wanted leave the bonds is the advancement the charge so
lithium will happily take for ligands on it in order to reach the new-look up crystal structures within containing ions generally see for bonds to live in peace or importing things you see a electrons around with the lithium wants those electrons and we'll talk more about about lithium and organic lithium compounds in this desire but to have extra Bonds told him later so far let me do this leaders awareness of the board here because I think I'm going to be a little more than a month to look at 2 different destined to reactions and in order to
take alcohols and convert them into alkyl chlorides and we will sort the men
in taking this asymmetric alcohol is a sterigenics entering near that allows us to keep track of what's going on and if you treat this with vinyl chloride with Nestle sealed to deepen replace the hydroxy group with chloride probably will this this kind of reaction goes to a claw 0 sulfite Esther I'm not going to draw the arrow pushing for that I assume you've covered this at some point in the year some organic chemistry course as a long-term sulfur it's not important to me on a case you go through this school rose sulfite Esther and under the chloride floating around just displaced but now I'm going to your right here and so when you use final chloride on
secondary out calls you generally get inversion of configuration so what you get is the the chloride that's
got the opposite configuration because it displaces the leading group within version so this is just as simple as into reaction here the version of configuration to get the final outcome for us and I can knew that to make a green yard reagent do whatever you're going to do with Maloney contrasted this with the same starting material the same reagent the and for whatever reason he decided to use dialog saying for this reaction you get a different result it turns out that in this case the dioxide actually acts as a nucleophile that displaces the sulfite intermediate to give you an intermediate like this before the chloride can push out the coral sulfide from the back there is already a dialog molecule waiting right there and it displaces the causal flights to be acts
only intermediate and now in the chloride comes on and finally the Clyde makes its way to the other side of the molecule to go I got beat out by the Daxing solvent
Malik also sulfite comes in and when it displaces still innocent you reaction value went up with an overall retention of configuration because this process involved a double displacements the displaced once-in-a-century action with that same you displays the 2nd time at the chloride through the world result as the tension in the nation's focus so please this is effective solid it's not dielectric constant Solis actually participating in a reaction you have to look out for that in a lot of different reactions of and benzene if your stuff is soluble inventing random we're out of time common law now these time because benzene is toxic and the the focus of wire sold
effects great Sultan effects the region like chemists will not suddenly all the out of jobs and
I if want for solid affects just about every transition state and every intermediate we could grow into a computer and that the button and you have initial calculations on except for solid defense resolve and you have to know how the solvents are going to participate and draw them specifically in every conceivable configuration and nobody can do that so electronic structure calculations every time you see somebody say tho I calculated this was her tree Fox 631 stock the 3 Lidl and held easy to feel something like that when see electronic structure calculations there virtually never taking solvent into account and so these huge differences in reactivity agency was solid factors of hundreds of thousands in rate cuts will be absent from when you look at electronic structure calculations it with the with focus on at the end of our guest lectures 7 /slash 8 plant and that is the end of the material for the 1st exam so on when we come back on
Monday where to start talking about this place that is in addition to sing star orbitals and that will
not be on the exam that's on Wednesday on that's that's this is where we had to examine what material for getting you guys really
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