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Lecture 01. Arrow Pushing. Part 1.


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Title Lecture 01. Arrow Pushing. Part 1.
Title of Series Chem 201: Organic Reaction Mechanisms I
Part Number 1
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 10.5446/19217
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 01. Organic Reaction Mechanism -- Arrow Pushing -- Part 1 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.
I'm understated ranking I'm here had to proceed to organic chemistry 201 organic reaction mechanisms 1 at at the University of California at Irvine and this is of course that sponsored by the UCI open courseware consortia and let me start by explaining to the differences between chemistry till 1 in the 2nd quarter and this 2 core sequence organic reaction mechanisms Kim 202
in Cantua won the primary concern of this class is 0 pushing
we evaluated our curriculum about 10 years ago and realize that the courses that we were offering were not getting one-quarter students a good introduction to how to push heroes in organic reaction mechanisms and so we divided our course content into 2 different courses so the 1st that's this course which involves about 20 different topics covered over 2 dozen lecturers so covers Aero pushing and attended concerts like energy confirmation and structure in chemistry 202 which is not part of
this course we recover concepts like Paris psychotic reactions the deals all the reaction and mean reaction is signature rearrangements things
like that we also cover radical chemistry confirmation and stereo electronics and mechanistic tools like kinetics and kinetic isotope effects and when her friend and you relationships so these concepts are not covered in this quarter and this quarter will
focus on narrow pushing so and start off with a fundamental question in organic chemistry and that is what does it
mean when you draw mechanism so lonely startled by presenting you with a simple mechanism for simple reaction I must ask some basic questions about their reaction yeah but the but but but but but but but but but but but but 1 of the things that this is a hydration reaction of a carbonyl compounds on something this covered most often organic chemistry textbooks and
solitude due to offer a mechanism like this I think you can see that in the product we formed bonds between his
oxygen and hydrogen and bond between this oxygen and carbon atom and so if you were to draw mechanism is all that this oxygen atom needs to be attached and you need to break the pipelines and someone had or something like that and you know that you need a break in LH bond in the the starting material and when 100 something like this this mechanism is fundamentally incorrect and if I go to the literature of I go to the World Wide Web and I ask what's the mechanism for this kind of simple hydration reaction I will get every single possible conceivable answer for that by looking around and what people say even just the the 1st step of this multistep mechanism so if I go always onto the Web and I ask what's the mechanism for this reaction all find things like this as the starting point for this reaction you might think that if this is really a stepwise mechanism maybe I should use the oxygen on pears and attacked this ,comma carbon 1st I can find other places that say that maybe the mechanism involves a small amount of hydroxide on an iron present in solution until all drawl that mechanism like this there's other mechanisms that suggest that maybe I should activate the company by using the lone pairs on oxygen to pull a proton off of the water as the 1st step for mechanism and I can find other reactions that that suggest that maybe I ought to use a better acid to permit that carbon Neil maybe something like this small amount of Hydro in would be present in the reaction mixture and could prove some part of the company
so when I look all around I find all different kinds of mechanisms for a very simple reaction in Searchlight address that problem why is there so much diversity In an answer that on the very simple but as a mechanism for action but it was started with a simple question
unanswered said she knew all about it .period a simple question why why is it that if I ask any to organic chemists so that students bonafide organic chemists teachers instructors the people of great your exams the mechanism for a reaction I never seem to give the same answer console the problem is there is simply no agreement it seems that nobody can agree on what the mechanism why is it that when you look at the real answer that should draw a mechanism when you're answering a question and you look at the textbook you find that way you drew denied equal to what's in the textbook how frustrating is that and there's no explanation as to
why your mechanism is wrong and the textbook is correct why is it that if you go to a journal article you write a
mechanism for reaction and then you look but the published mechanism and mechanism doesn't match does that mean you're mechanism is wrong what does that mean that we never seem to find that kind of agreement and there is a simple reason for all these discrepancies and the answer to this this fundamental question of why there's so much diversity ,comma been so that there is no definition for what hero pushing mechanisms
organic chemists have never sat down and agreed on what does it mean to Goneril pushing mechanism and so
that's a question we have to confront the beginning of this course if we're all going to agree that there is a difference between the correct mechanism or a plausible mechanism and an incorrect mechanism 1 it's impossible we need to create a definition that works and that's consistent with what has been done in the past and what people should do in the future and solid start by
that school and give ourselves a definition of what a narrow pushing reaction but it the problem with this
but but so the person most
important feature of a narrow pushing mechanism and you have to satisfy this is that you needed to break the reaction down into
elementary reaction steps a good mechanism is a series of elementary reactions and that's critical and I'll explain what that means let redraw this hydration reaction so we can discuss these these aspects of an Arab pretty mechanism in terms of the common in terms of these 2 components of the definition so by draw out the on this reaction is hydration reaction and I ask What's the mechanism for this the key answer to this the key to the answer has to be a series of elementary reactions that the true mechanism for this reaction under typical conditions involves acid catalysis I to start this mechanism by drawing on acid and I'll abbreviate that with his depiction AGA let me draw a a little closer here and I'll use the lone pair here on oxygen to the programming that that hydrogen that protons attached to my acid which have symbolize with nature and someone I draw the next intermediate and that protons on his oxygen
with 3 bond stock satirical Lenox only myelin this and now that activated this this
car video card and I can now use the long hairs on my water nucleophile to attack the Kerviel so now have the 1st step in the 2nd step for this reaction is not created this new bonds to oxygen and now I need to do is to Deep Throat made to my nucleophile In order to in in order to create the final product here I can use the counter Ryan the conjugate base from my acid to release the proton so this is a correctly
drawn mechanisms so the important thing that I'd done here is that I have broken down into a series of elementary reactions stepped in Huntland know that these reactions differed elementary what's the
definition of an elementary reactions so 1st of all an elementary action is defined as a reaction as a single transition state each of these elementary actions in this mechanism as a single transition state there is a 2nd important property of each 1 of these elementary reactions that makes my mechanism and that is that they are microscopically reversible what this with these 2 things mean in practice is that
if I look at it a free energy diagram a reaction diagram for this reaction I should have a series the hills and
valleys that has To intermediates to intermediate and I mechanism and 3 on 3 would represent transition states so I draw on energy diagram for this it needs to have 3 transition states to match the 3 steps in my reaction the principle of microscopic reverse ability is not so important for Cantu 201 this course but what it means is microscopic versatility means of financed the mechanism for the reverse reaction it has to involve exactly the same intermediates that's the importance of microscopic reverse ability
OK so criterion number 1 elementary reactions that's what a mechanism this it's a series of elementary reactions criterion number 2 for correctly drawn mechanism who is
the current Maryland builders secondary to the fact that I broke it down into a series of elementary reactions so got Kerberos on each of these steps now the problem is How do we define what occurred here was supposed to be we may have drawn all the structures correctly because we know that we drew the Kerberos correctly and let's go and talk more about this I don't want to Kerberos meaning organic chemistry but but but but but
but but but but but but but but so help us understand what does an aerial means Kerberos we draw in organic reaction mechanisms me take what looks like a detour and stop and consider some distance dependent equations for comparing things in chemistry for comparing 1 transition state to another for comparing won confirmation to another for comparing the reactants to products in terms of their energies so there's 2 different types of love equations the typically used in I'm going to start off on which with 1 of the 1st 2 types of equations on that students are introduced to a general chemistry classes and it has to do with how you factor out the energy of non-binding interactions how do you estimate energy when any
22 molecules a species are interacting without forming bonds and you can break
this down a decomposed it into these mathematical terms and don't be put off by the math for now will talk about this in just 2 seconds were not going to use equations ,comma very much in this class of people OK so here's a well-known equation that used
for estimating the energy of any 2 things interacting it could be a substrate and act a protein it could be 2 substituent on a molecule in the interacting so there's 3
different terms 3 different things we need to think about when any 2 molecules are the 2 species come together I think you will recognize this 1st part of the equation the 1st term as a crew potential for good news right here ,comma so what this important about this there's 2 different charges and whenever 2 charges interact the distance between the matters the farther apart they are the less interaction between 2 charges humanity not recognize 2nd term but this is the vandals interaction and commonly quantified with something called the Lennard-Jones 612 potential there's 2 different distance depends Turner 1 of them leads to higher energies when you pushed 2 things very very close together 2 molecules to substituent they start to repel each other the energy gets high but when things start very far apart when they fly this should be ordered to the 6 when things are very for a pardon usually there is an attractive interaction energy gets lower these are
good equations for thinking about non-binding interactions for thinking about charges on the inside of a protein and I think about some sort of a lysine side chain has a positive charge and as
substrate is a negative charge this is a good equation for thinking about that or I want to think about the interaction of 2 methane molecules with each other there nothing about this this suggests a chemical reaction to me but the fact is that meant informs a liquid there must be some attractive interaction between methane molecules that doesn't make or break bonds but that causes the attraction and you compress the lending too much is something that's very hard to compress and that's this term taking over so this is a great equation for thinking of non-binding interactions so the equation in organic chemistry used to think about the interaction between different species and all right your binding energy but it turns out that this equation this set of equations were going to talk will also work for nonbinding interactions so the way that organic chemist factor out these different aspects of what leads to higher lower energy starts off on London the classical definition 1st if I go back and I
ask organic chemist from 20 to 30 years ago about what is that dominates the interaction between 2 molecules to species to functional groups they would typically answer all its
barracks or electronics stare exports electronics and I will urge you not to decompose interactions in this simplified way this is oversimplified and so is simply erase this this is a bad idea to decompose organic interactions into these 2 terms if you really want to understand the differences between 2 transition states between 2 reaction pathways between 2 conformations needed Bill just like this equation on top and decomposes interactions between different tribes sometimes start off by drawing something they should look familiar to you and that's this column's equation again molecules or even parts of molecules have a charge associated with them the 2nd part of this equation I'm going to be very vague about this about the form of this equation but it's distance dependent as they take to organic molecules and I push them together what to methyl groups among the communication together their interaction will serve to lead to a higher energy and finally the last part of this equation if you'll just have to hold on for a 2nd and and put up with my Mac but the and I'll explain what these terms mean in just a 2nd or what they should suggest to you OK so here's what I see when I see
this tournament messiness term what I away season it's a negative that suggests that it's an attractive force Elise Stone overall stabilization lowering energy we want
things like that we were trying to judge something to be favorable what I see here is I'm supposed to be something over something related to electric filed nucleophile that's the symbolism here and when I peek inside of this term when I see coefficients on at this place some over all the atoms in a nuclear bomb time some coefficient on all the atoms in the electrified and then there's this beta term this suggest warble overlap amendment in the denominator for this term there's a difference in energy is the energy of an electoral orbital and the energy of a nucleophile all this is about molecular orbital interactions and the structure of this time suggest that these are going to be favorable so just summarize it in simple terms here but these terms mean when I look at a chemical reaction I try to judge whether the their actions good what 3 things like me to think about I need to think about charges that's the kind of we just discussed that's clearly block the kilometer potential I need to think about spirits 22 things interact in Bonn really close to each other the closer I get the more costly and then finally there's this mysterious 3rd term here the relates to the interaction of field orbitals without influence morals in at the term
molecular orbital interactions every time you see 2 reagents reacting to parts of a molecule interacting you need to think about these 3 features so we have to decide what we're going to use arrow pushing to
represent the stood up by drawing out of reaction here and we can try to think about how the these different factors come into play dropped some sort of a nucleophile this lithium unilateral generally fear and O'Donnell electrified Alice's methyl iodide there's a lot of different possible reactions that I can imagine here .period with with the simple along but think about this lithium oxygen bond attacking this carbon and I think you can also see that the iodide is ready to act as a leading group so 1 possibility is that this would be an oxygen bonded tax carbon and in so doing it breaks the lithium oxygen bond as the oxygen gets closer and closer to card and if this really is the mechanism the lithium should get farther and farther away I should be breaking the the moxie and bond as we attack that's what that aerial another possible alternative would be to keep the lithium attached to the oxygen throughout the entire reaction mechanism instead of the lone pair on oxygen the tax a carbon this 2nd possibility the 2nd error that starts with a long but pair means that doctors within 1 does not break in this step and then there's a 3rd possibility that has nothing to do with the oxygen bonding to carbon it it has to do with this CC double-blind attacking informing upon ultimately to this discard atom at the end so the from an oxygen and carbon bond where former carbon-carbon bond if I want to break this down into its different components and judge which of these 3 possibilities is correct I have to resort to these 3 different terms here if I reduced charge as the meaning of my heroes and ultimately we need our arrows to the 1 and only 1 thing we can't make
it so that sometimes Darryl's mean charge and then sometimes are Kerberos means derricks and sometimes they need this molecular orbital because of the arrows can mean anything then they don't mean anything so let's decide what should we have our
was made that we look at the charge here most of the negative charge on oxygen I would predict it would make an oxygen carbon by by patented charges most of the negative charge on his oxygen on this new leaders on the oxygen but I pay attention hysterics I definitely would not want to make a bomb the carbon that's much more strictly hinted that his oxygen living longer the oxygen and finally but I pay attention like the wobble into action that's the aspect of this equation that would correctly predict that is this carbon-carbon bond forming reaction on the bottom which is correct as opposed to these incorrect predictions of the oxygen lithium ponder the oxygen long-period attacking alternately when you look at what will provide us with the best predictive power in organic chemistry its molecular orbital interactions this is where our arrow pushing her Kerberos should be taken to represent the
interaction of filled orbitals with unfilled orbitals as we're going to spend the rest of the quarter thinking about this and
learning how to use this equality between Kerberos a pushing and the interaction of field orbitals with unfilled orders among the real little bit meal from a book the dates back to the mid 1970 s is a book by by Ian Fleming called frontier orbitals an organic chemical reactions and since then there's been a new edition that has come out on that Scott
much more information but it's missing on some of the heart the really drove my generation
of organic chemists and the section in this book that discusses electrify like aromatic substitution and somewhere here in this discussion on how to read you this little sectional point out to what's important and what's important to organic chemists so in this discussion by Fleming says however we ought to be clear that this is a superficial argument which fortunately works currently area when used with a molecular orbital description of bonding work as well as they do simply because they illustrate the electron distribution in the frontier orbitals and for reaction kinetics it is the frontier orbital that is most important neither
seismic words and Salemme extract from that what you need to take home from that because it defined the costs what Fleming said was the
curly heroes or Kerberos mark at the end when they're used to represent the interaction of frontier orbitals unfilled orbitals with unfilled orbitals and if you really care about kinetics then you need to focus on those frontier orbitals and I can tell you that I am interested in kinetics and if you are taking this course that you were interested in kinetics too if you were interested in which reactions are fast and which reactions are slow if you're interested in which reactions are plausible and which reactions are impossible if you're interested interested in which mechanisms are right and which mechanisms are not right which reaction is mechanisms plausible which mechanisms impossible then you need to focus on frontier orbitals therefore we're going to that's why we're going to make a lot of this that that's what our curly heroes will represent and set down this central
canon of mechanistic euro pushing will define this so that now 182 people in this class you out Kerberos mechanism world drawing the same thing but at the end of
the it was 1 of the 1st day of the 1st half of the year of the Horse of the year was the the only 1 of the most on so the test
vigil applied every time to check whether your mechanisms are correct but the crate terraces is and you used Kerberos to depict the interaction of field molecular orbitals with unfilled orbitals and in particular With field frontier orbitals and unfilled frontier orbitals only give you an example that you just to suggest
you've probably been doing this whether you recognize it or not this isn't something new In fact you've probably been doing this come from your 1st day drawing our reaction mechanisms as a softness in organic chemistry course all of this and extending lake you could get by deep-rooted needing an album I could if I make this interact with some sort of an electoral filed in this case Iowa things the mechanism I would tend to draw would involve carbon-carbon bond formation between this carbon right here that as a doctor in and this react flight the electrified these correctly drawn arrows represent the interaction of filled orbitals with unfilled orders when I draw this out I can think about the structure in England as being the result of the interaction Of this non-binding on oxygen we have a label that we give it the society Alex and which means basically long pair non-binding centered on oxygen and it's donating enterprise star 2 There is an unfilled orbital here that's called store orbital between 2 cartons this likewise I have a feel normal announced this pie orbital between 2 carbon atoms that stone leading into a Cigna star were built between a common and so even without
these fancy looking labels here this mechanism is correct and I know that it's at least plausible because I had drawn out the
interaction of the field orbital within Bordeaux and the field orbital with an unfilled orders and so this is the intent of Arab Christian focus let me just remind you what our goal is for this class of Goldman this classes to break down reactions and Royal pushing mechanisms that of Cranberry Elementary every Arab pushing mechanism should be composed of a series of elementary reactions that reaction is that of a single transition state that's I know that it's elementary not multiple transition states and secondarily I had Kerberos to that that's series of elementary reactions and each 1 of those Kerberos represents the interaction of some sort of fill orbital with financial loss and that's the goal for
this class will come back for our next on sort of topic on the new 7 rules for mechanistic Carol pushing that allow you to make sure I couldn't that you're doing this correctly


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