Bestand wählen

Lecture 03. Molecular Orbital Theory (Pt. 3) & Energy Pt. 2

Zitierlink des Filmsegments
Embed Code

Automatisierte Medienanalyse

Erkannte Entitäten
OK I'll went out when we were talking about this 2nd topic about molecular orbitals
there was a slight on the forgot total oversight died shifted my notes around and so I wanted us to cover that before we go on back to this lecture on energies on
because it's an important part of part of our discussion of the interaction between filled orbital and warble and I want to bring you back to the ah explanation of the energetic consequences of
having warbles interactive or but also of the field interacting with orbitals the empty and I give this equation of it said that you can approximate the energy get that you get when all the laurels interact with laurels with a simple equation this says orbital overlap in the numerator and the difference between filled an empty orbital energies in the denominator and we said 1 in great detail about what happens is you raise the energy field orbitals nuclear files and you drop the energy of unfilled orbitals that the electrifies but I didn't see anything about this war will overlap I wanna stop and talk about that and the most important concept for us to consider is the importance of symmetry when we talk about this idea orbital overlap so let's talk about the interaction of canonical atomic orbitals or or hybrid orbitals but imagine's example we have 2 s orbitals interacting with the with each other for example to make a hydrogen molecules to wages as long as I have 2 s orbitals those these are 1 s orbitals interacting with each other I can imagine some combination where I have liked phases and that's the kind of interaction would lead to a hydrogen hydrogen bomb or you can imagine the interaction of 2 orbitals and what we have told mountaineers that there's 2 possible ways that this can interact 1 where we have
like phases and what we expect is this overlap areas should be even bigger in the products so the product molecules hydrogen
molecule has a lower energy orbital that looks like this that's constructive interaction between the 2 was fear I can Imagen interaction where they were legal pride bond and the symmetry of the pipe looks very big in the middle or we have constructive interaction on the other and hand and so the end result looks like this and I can even in a match in the direction of a P orbital with an S. orbital as long as I tilt this in this way so that everything is some sort of ends up with some sort of a positive constructive and guess in in terms of wave functions constructive interference 15 colored Inter well it's not a good here's what I intend to draw something that looks like this OK so that's those are examples of forming bonds on and in this case
I would end up with some sort of a signal bond again so graphically when Weagraff the energies as long as orbitals had the race symmetry I can produce 2 new orbitals up bonding
combination an endangered bonding combination and the bonding combination will be lower in energy because these interactions of productive so these are examples of things with the correct Dimitri interacting but the symmetry doesn't have to be correct but talk about an alternative situation where I tried to interact and as orbital with appeal orbital but I've got some aligned incorrectly so well expect out of this interaction if I look at the way I've chosen to face these as all get a constructive interaction here but I'll get an equally destructive bonding interactions hearing into interaction on the bottom part every takeout per mole of energy benefit the electron volts of energy benefit I get here I'll get
equally destructive interaction there so the end result would be no net change and that would
be an example of having to orbitals interact and I can depict the energetic consequences of the wrong symmetry here on simply by saying that there would be no interaction but the 2 new resulting orbitals I would get would have no essential difference so if you want to see some energy benefit you have to match the symmetry of the warbles the interactive so that the hatched part interacts only with hatched part or at least more parts and than on has part so I might get bonding interactions here but Dubai interactions there is no doubt energetic benefits between mixing of warbles focus of symmetry has to match in order to receive manager benefits and you get an equally useless interactions if I tried to make up the orbital interact edge on like this and there it's equally destructive any quick constructive
interactions and there's a simple rule that relates to this in organic chemistry and that has to do with by
cycling molecules and it's called Brecht's rule and let's talk about threats will it's basically just a localization for some sort of a statement of this idea of symmetry With time the kids and more the fact that 1 of the 2 with this so many drawn by cycling molecule This is a by cyclo 2 2 on ring system and so there's all kinds of molecules natural products and based regions that have this kind of by psychic structure and what this rule is saying is that if I
look at this bridgehead here that's the point of intersection between the rings what it says is I'm
going to have a hard time creating applied only that look a little better applied bond there but any reaction might try to generate that it's going to be incredibly unstable and potentially impossible to isolate any molecule that looks like this is what we refer to the regional office and use that reject right there that I worry is that on the wooden redraw this this pipe bonds it in terms of on the P warbles the combining together or they would have to combine together to make up I bonds like that so I'm going to redraw the spy psychic molecule this time I'm going to go hydrogen atom at the 2 positions of this of this by cycle octane system nor would I want to do is now that I can see all 3 of the things that are attached this carbon atoms I'm going to throw the in the P word but also would have to interact
or or the way the pure orbital would be aligned that said about that geometry so there's 1 P orbital never want to
combine that with the with appealable over here in order to make a bomb the problem as I look at the 3 things that are attached to this started carbon here's 1 hears 1 in here's 1 and almost in the plane of the board and what that means is that the P orbital but on this carbon atom has to be sticking almost straight out of the board and all phase that back into water like this and what you can see when I do that is those people orbitals orthogonal it doesn't matter that the closer together it doesn't matter that the that the both people were rules and had the potential in theory form a pipe but they have the wrong geometry and interact with the wrong symmetry you'll get no significant pie
binding better residents structure for this molecule right here would be have to radicals at those 2 carbons and natural way
you would expect it to react to 21 radical reactions were easily OK so rules an expression of this importance of symmetry on when to orbitals interact you won't get an energetic benefit unless you have significant overlap and that was part of this equation will relax over here again 1 last thing related to the importance of war overlap that would be In thank you thank you thank you being with and this was someone good draw just interaction of 2 pure on carbon atoms and I want you to try to imagine what difference does it make if those 2 carbon atoms are close together far apart but you have to imagine what happens is they bring these 2 carbon atoms closer together as a these 2 carbon
atoms closer together all expect to see more and more overlap between
those empty fuel and you only get that was 2nd row Adams with the pure orbitals are approximately the same size you only get stable 5 bonds with 2nd row foreign carbon nitrogen oxygen and flowering plants you talk about transition metals all that apply bonding stuff ,comma ceases to have that this phenomenal strength applied bonds among secular and legal and dropped some bond lengths for you if you go back and you look at punches in bunches and bunches of crystal structures Our 1st simple straightforward carbon-based compounds what you'll find is that typically to bond lengths for Alcazar about 1 another 1 . 4 6 sanctions as a good numbers there's not a huge degree of variation unless you strain the system in some way but I look at a simple Elkind there's more s character for those sp
hybridized carbon atoms in alkaline and substantially shorter bonds what that
means is that the P orbitals In an alkaline are closer together and overlap more effectively so what are the chemical consequences of that shorter bond length of the better overlap on the consequences for the pipeline's Elkind pylons are stronger and more stable so it's good and take an example of that is going to take a simple substrate molecule and will imagine treating this year with an epoxy rising agent medical proxy bends over gas and I expect you to know that proxy acids makeup oxides and there's 2 different pies systems that you could make an a Parkside with the product in this reaction the
major product in this reaction to results from
attack on the and has nothing to do with the fact that the Oxo Rainier Parkside would make from the kind it would be unstable made sure it would be unstable but you would not get 86 per cent yield of this product if this were reacting fast when it tells you is that at least 86 out of 100 molecules was approximating faster on the helping than on the al-Qaida and you can't argue that that's happening because of steric hindrance because this is less hindered you can't argue that it's easier to approach just 1 or 2 places here because the electron density alkaline is is symmetrical all around you can approach the all-conference any orientation you want there's no possible rationale for why this is true why this is the more nucleophilic agrees double
bonds on except the fact that this bond is weaker and more energetic so if
you look at it and SEC bond it's gotten energy units up here and if you look at the SEC pie bond in alkaline it's more stable it's lower in energy this is supposed to be a triple bond it's lower energy in its lessons with people know there's 2 pipe bombs in the Elkind it's still attacking less than the healthy looking so that all the importance of bond lengths and the more effective or what they little overlap in itself kind because those 2 carbon atoms are closer together and then make the bond more stable but also lifted with pockets so that's that missing slide that I had Our from lecture to what this sort of in your mind pace that that immunity was there on the lecture notes that I posted online on the ballots returned to this topic of energy
that we were discussing when we left on Friday and the gonna start
off with a very simple equation when I want to talk about the reactivity of methane on if you go over to Saudi Arabia at to Africa where drilling referred for crude oil at nite skies a lit up because when you pump crude oil out of the ground there's also methane gas coming out of with and it's too expensive to package up that methane gas and to ship it to other continents like our ,comma below we need that methane gas
and natural gas so they burn it office called flare-up and the Warriors it's so expensive to do she
began as it just takes a lot of infrastructure on about to ship gas 2 other continents at home the 1 thing that that people are trying very hard to do right now is to convert methane gas find ways to convert methane gas into liquids that can still serve fuels that are easy to shepherd the leadership liquids it's hard to ship gas and you can't do this at present you can't make simple oxidizing agent with methane and make liquids like methanol out of an e-mail methanol is a fantastic fuels for things like fuel cells and solar vices such a problem what's the problem with this equation here and what I'd like you to know how to do it's at least assassin in a simple way Is the problem free energy is it that this is not estimate dynamically possible for us to do any time I give you some reactions at like you have some idea about whether that's permanently reason was an axle from across as if you're talking about a reaction it's not Edsel Thurmond ,comma then there's no will .period talking about
actual here so our criterion that you know from
General chemistry is the free energy change transformation needs to be negative needs to be less than 0 otherwise you'll be trying to go uphill and unfortunately it's very very hard to know free energy when I draw out reactions like this I don't have a good life and you I some intuition and the problem is is this that free energy is composed of an the term that means things like bond strength strain on issues like that but there's other part of free energy that's entropic in nature and that's very hard to assess now fortunately if you're clever when you make comparisons of things comparisons of starting to offer products transition state transitions state as long as we keep things similar this week included ignore that entropic term the Delta St but I compared to
transition states that looks sort of similar except you just attacking different bonds of every actions 2 molecules reacting I get 2 molecules out as products it's kind of similar in terms of molecular tools on them really careful to compare similar things I can ignore this and Tropic part of free energy and we can just focus on solving this Delta each party and will be there is a simple way for you to estimate the ample changing reactions and I'm going to show you how to do it it's going to require some up-front effort on your part and I'll make sure you put in that effort
please that is so has long as actually very general but I'm going to show you how to apply has long to estimate Anteby changing reactions based on bond dissociation energies means so it's taken example and I'll try to explain what has long been in terms that relate to this methanol on this methane methanol equation such go back to this methanol of equation where we really love to know concur with the weather were asking too much of this
oxidation reactions and so I don't really know what the sample be change here is but I had a pretty good
idea for the energetic cost of breaking his oxygen oxygen bonded so I want to imagine that I don't know this number here but I can know this number what does it cost to break this hydroxyl hydroxyl bond to give 2 hydroxyl radicals and lost home already cleaned associate that they have a reasonable idea about what it costs to break a H bond to give to radicals and I also have a reasonable idea about what it takes to break this water major bond to give to radicals and I'm just going to continue on with this idea here and I hope I'm getting my pieces right and so what you can see here is I can imagine
some equation here I've broken this bonds to give these pieces and now I just have to imagine recombining these pieces I
believe I read on my pieces here years so amusing equals the that supposed to be an equal but you could see as I get the same pieces this way is a break the bonds over all the other way and we can construct a thermodynamic cycle from this another word of the magic that I take this piece of where this hydroxyl and make a bond you can say I'm going to get those that the methanol peace and the water piece on the other side ignore this equivalent sign and I just want to imagine that in 2 steps breaking bonds this right break and forming bonds it is I can imagine making those same products there even if that has nothing whatsoever to do with the actual reaction maybe I use of
platinum catalyst for this or maybe I just simply heated hard enough to happen doesn't matter as long as I know the ample be change here and the to be change there I can
simply add them together and I will get the exactly right at people this is what has lost says that the mechanism doesn't matter as long as you can create some sort of a thermodynamic cycle some equivalent to to get to your products it doesn't matter whether the mechanisms of the same the ample be changed will the correct so as long as we just memorize a few bond dissociation energies like this 1 and this 1 and this 1 and this 1 would simply add and subtract them and will come out with the right and but we changed so let's take a look at some bond energies and we're really looking for here if we want this to be a favorable process but we'd like to do if we if we want to get more energy out of forming bonds than the energy we have to put it to break bonds we wanna look for processes were we either break weak bonds that's cheap or processes were reform very stable
bonds because we get a lot out of that that's what we want to look for select review von
dissociation energies of the user numbers that I want you to memorize it and then you can do this at your leisure for any reaction I just summarize and this is so reactions tend to be favorable view break and bonds for form stable such use some bond dissociation energies and 1
it was but the ability to Montevideo definition bond dissociation energy is defined to be bond strength and both of those related to this equation or give you 1 example for an RH model could be C H 3 a cheery in ethane propane doesn't matter but by definition a bond dissociation energy off I use the word bond strength refers to a homo Oolitic process that gives you too radical by definition I didn't invent this definition I'm stuck with it just like you but
this is the definition of bond strength I cannot super important sold by
definition the ample be change associated with this but this reaction right here and that's a standard state free energy error-free Anteby change that's defined these bond associate Asian energy so why not MI harping on this idea that of bond strength because I'm going to refer to a strong bonds and weak bonds and 1 of the unfortunate consequences of that is that I'm going to find in a bond as a strong buy bond now you know as well as I do that protons exchange very rapidly in water and proton transfer reactions but the words strength implies that I'm not talking about proton transfers when I say the word bond strength I'm talking about home lit Association to give radicals and this is a very costly process the bond
dissociation energy from water is about 110 K calls from all that's considered a
strong bond and I'll give you some more numbers don't you put that into perspective so again when as soon as I say the word voluntary I'm not talking about on irons or bases or or acids and talking about radical reactions focus let's take a look at some some sample numbers were going to start off by looking at H bonds because those are very common animal look at bonds between carbon and other 2nd group Adams so start off by thinking about how canes like the bond dissociation energy there's nothing special there they're typically around 100 180 K cows from all high nineties it varies a little bit the larger the Bell case get on Fallujah those bonds get by a little bit but the around 100 for a typical ch in Qaim but look what happens when I change the hybridization I allow this carbon to contribute more as characters that signal bond it becomes more stable that becomes a stronger bond by 12 kcal per mole how many times more stable is the CH bonding is healthy this is OK Kallstrom from all that means at 12 kcal per mole difference unified divided by 1 . 4 how many factors of 10 that instability about 8 but tender dates but about 100 million times more stable because of the extra security modes can you can see what's going to happen as I contribute more as characters that ch bond I should expected to get even stronger I've never seen any reaction or something plucks in the and an al-Qaida anti-Japanese you with a carbon radical at the end of an alkaline both 102 strong nothing does that that's not to say you could prepare some al-Qaida radical some other way that you're not going to do about parking lot is a vacancy all ch bonds are not created equal pay attention to hybridization it's essential now last of farm another thing that will have an obvious effect and I think you know this 1 ch bonds is nearby pie systems so probably sometime back in you organic chemistry in undergraduate organic learned that you could brominated the benzoate position of molecules that because the city's bond is weak ,comma into but some numbers on that it's 85 K accounts all than it's very similar foreign Lilic ch as it is for events exceed each so residents can affect bond strengths Gillett's take a look at a few more of other common bonds and bond dissociation energies what's take a look at some bonds not to hydrogen but between 2nd row Adams carbon the carbon-carbon the oxygen is nothing special about a carbon-carbon bonds the physiological note is that it's weaker than a ch bond a process that leads 80 . socks that's why those bonds are hiring energy but if he didn't shoot carbon-based radicals at least the bond taken out of the water will help stabilize these radicals a seafood bond is weaker than a H bond if I look at SEC now there's to bonds in there you should expect that to be a much more stable pie bond because to dissociate that to to Corby's I guess ,comma doesn't cost exactly twice the energy but it's very expensive to simultaneously break both the segment of pipe bombs and if I look at kind then of course it becomes very expensive I wanna compare this took to carbon-oxygen bonds those are the main ones you'll think about her compare these with an organic chemistry Miranda start off by noting that there is nothing really significantly different between the bond energy between the strength of the carbon-carbon bond on the strength of the carbon-oxygen on what's important is when we look at the pie bond it is of course more expensive to break both of those bonds at once and I don't really wanna focus on these energies individually there is a set of numbers I want to extract out of this that I expect you to use over and over again and it's it's the individual energy just the pie bond here there's a signal bond here is a sigma bond here let's forget about that and just look at what's different looks different that extra pipe and I can create a number we think about that the energy of a CC high bond is the difference between those 2 numbers and it's about 64 K calls for more the obvious point here is that a I bond between 2 carbon at the 2nd bomber is substantially weaker then a signal carbon-carbon pipe bonds are weak you want to look for reactions that break carbon-carbon pylons and trade them for carbon credits in about reactions break carbon-carbon by bonds and trade them for complementing the bonds will tend to be favorable OK let's look at carbon oxygen Oh pipe bombs the strength of this at this time bond between carbon and oxygen is the difference here and now we're talking about 94 K calls from all that's stronger than a single mom the carbon-oxygen public bond that second-largest stronger than the first one was there this is considered a strong bond you want to look for reactions this form of carbon oxygen pipe bombs reactions that form carbonyl worlds tend to be from economically favorable is so when I look at reactions like that met their reaction and looking for reactions that break bonds between current and I'm looking there that wasn't an example of 1 but in general what I look for reactions that I want to be Thurmond intimately driven I'm looking for things that trade C pylons for steel pipe bombs and we'll talk about some of those examples later in the course of its those are examples of weak and strong bonds the single it's a of some single bonds that I would consider to be strong and I've already talked about 1 of those Manhattan OH bond is strong 110 K from all for a typical alcohol OH bond that would be very unusual treated any reaction plucks the hydrogen animals and that's not to say that there reactions that generally or 0 . but they don't proceed by plucking often each any chatter as I already mentioned another example of a strong single bond and there was this insanely strong ch bond being and alkaline and I've never heard of any reaction where anything can pluck this image fall off of the International Court of a thing that had not yet talked about Silicon Florida home by car crazy stability here 141 K calls per mole for the stability of a silicon flooring bond this is why use text review ammonium fluoride to the protects I'll theirs because you get this huge limit anemic driving force you also get kinetics speed associated with that process but it's hugely from economically driven form silicon foreign bonds and then the last common example of a super stable bond and I'm not even sure how to draw this on I can draw like this and it doesn't matter whether I draw this and I can never forget bond of phosphorus there's another resonant structure if you want you can draw like this as a way to help you rationalize the stability of the importance this bond is super stable if you have a reaction where you couldn't try fennel the there's a massive thermodynamic driving force for for somewhere in that process Peter generally try composting offside as the byproduct and the energy there again is 136 K calls from all hugely favorable selections like Nobu reactions of Appell reactions will talk about it
later on .period driven by the formation of appeal bonds against those examples of very strong bonds went to look for reactions that for most types of bonds still tended in fell prickly favorable and attended the summit manically flavored with negative delta genes but let's take a look at some weak single bonds let's keep our focus on single bonds right now the truth is and that we will this it this is this year it will be OK so carbon silicon by a single bonds tend to be week if you have a chance you'd like to trade up carbon I don't have the number of carbon silicon bond for an oxygen silicon bond or even better for Noreen silicon long that'll be cementing amply favorable with all the sponsor halogen if you take a simple alkyl iodide and you could under a Teal Celia will slowly see spot there to turn brown because your whole formalizing that weak bond the carbon item but will start the home lies in January I dont insightful Axel . gov knowledge bonds tend to be with bonds to halogen tend to be weak especially for allergens that are not 2nd rower Adams so builder examples of weak bonds you can expect those common kinds of bonds to break by instrument in Immokalee favorable reactions the notes would bet that the bonds were the 2 atoms of the same carbon-carbon bonds are sort of unique in this respect because I'm not super week but they're not strong on the disorder right in between but if you stay in the 2nd row where Adams are about the same size carbon nitrogen oxygen Fournier was 81 not amusing because specifically as saying that 81 an average total as you know he had substituent those numbers will change I think so it's going to take this specific set of part of molecules with same at what would have been designed Creasy Electra negativity tonight to unite to announce a weaker bond and I make it even more Elektra negative and it gets even worse she could see what's happening with this hydrogen peroxide methane reaction I looked at that time in half a 2nd I judge that that oughta be commanding amply favorable it's summer dynamically favorable for hydrogen peroxide to react chemically with nothing that's not to say that there's a kinetic pathway to do that without catalyst but any reaction of breaks and hydroxyl hydroxyl bond is going to tend to be 17 uniquely favorable that's a little ,comma bond that finally get on the fluorine chlorine and that's very weak so there is a sort of trend in getting weaker and weaker so the the reason why the flame off methane and converted to methanol using either but that is not a thermodynamic problem it's just that you can't stop it methanol antioxidant that they cannot oxidize methane will continue to oxidize it further became clear how to stop the oxidation of methane methanol is only continued to draw some of some of these some of these diatonic molecules you already know and the reason why you already know the East is because in soft organic chemistry you learn that you can hold lies these with light were he in order to generate radicals to do free-radical a calculation or free radical armed bands in college OK so let's has lost that's going on use these numbers and will apply this to the To this methanol equation so you can see how we use these numbers and how we add up those those bond Association energy and I think I released my equations all the right here a with me for of which it would have to and what I do is I want add up all the energies of the bonds broken and when had all the energies of the bonds formed so was the energy of this oxygen oxygen demand this is a movie In I'm wondering if we don't belong number because sometimes every now numbers for specific molecules and then other times of young numbers for industry thereupon types and it's 98 1st CAT Fund which is relatively strong so we add up all the energy costs us to break the these bonds will come up with a number of 149 from all and then we come over here and we look at the bonds that I not but that lying there so the O H bonds in water or just a generic OH bond who no so about 110 and for CEO bond but and someone we had these together we get 189 hits you can tell already the year generating more energy by forming bonds than you than you had to put it to break these bonds and so when we do our final calculations were going to say Delta H people bonds formed actually if we want this to turn out to have the right sign Bonds broken this is the and that's going to be all 149 minus 1 89 and is equal minus 40 minus 40 K Kallstrom from all down that means downhill and topically This is a favorable process that's and felt prickly favorable for you to do this reaction we don't know anything about how fast this is in fact it's not fast you could store hydrogen peroxide and methane as long as there's no catalyst that publicly speed of sold just some simple powerful numbers you can use any time you want to judge whether reactions look likely are like they ought to be from economically favorable and I put some problems on the problems that have had those together OK so the it's going to talk about energy from a different source it's becoming more and more common computers more and more often nowadays you're going to hear people refer to electronic structure calculations will say stuff like DFT the 3 archery Fox these are all numbers that are basically ample and let's talk about what those numbers mean and you can use these this law and and and bond energy calculation anywhere sitting in a pub drinking beers while you're listening for research seminar on but energies from calculations you're going to have to sit in front of a computer and it's not hard nowadays you destroy the structure and go Carol pop out to managing rules energies on Monday replaced this is not the only 1 in this so when you people who use terms like density functional theory or you see a very strange looking things like B 3 live and
somebody did some density functional theory calculations using the Bach's 3rd iteration of the Leiyang Power using the Los Alamos National lab space it doesn't matter what the letters of the general is the more letters you see here the more accurate the calculation it's so you see something like this and like that's not good this is generally true but is what these numbers mean the calculations are doing this the 3rd if you wanted to calculate the ample be changed and what you get out of the race you get out of these advantages and they're not going to say Delta H adult Uchida just gonna tell you it's an energy from these calculations that approximately the same as in ample the value means they did not take entropy into account they're just giving you like and can copy number and so I went to a computer and I drew these 4 molecules in each 1 of these molecules as an energy associated with the quarter has an energy associated with it it doesn't matter which bonds because that's not what the computer tells you just said you draw that structure and and I will tell you what the energies of and would tell you something like ,comma like this like the energy of the city of Amethi molecule is minus 25 . 2 to 2 . 5 7 2 3 8 2 2 K Kellstrom all fees to be being needed very large that's not bond that's for the whole nothing knowledge of what you do with that it doesn't mean anything by itself it's only when you add the energy of this in the energy of that that 2 starting materials and you compared to the energies of the 2 products added together that's the only way that those numbers can be useful to to you and this is another example of passes so what are they doing what you're doing with the computer is doing is it's taking the energies you get from all these nuclei on drop is the nuclei of all these carbon and hydrogen atoms but we have got to many it's assessing how much energy do I get by taking a bunch of nuclei plus a bunch of electrons and combining them together to make this molecule and of course you'll come up with a different number here because you get a different number of nuclei electrons to make methanol and then you can ask computer to tell me how much energy you get by making these types of things and then you can apply has that's why you can add the energy of this methanol energy water and subtract from the energy the starting interrelated together and come up with phenomenally accurate numbers for that now again it doesn't take any of the into account Missouri entropy on this into account but only that's OK OK so use numbers for electronic structure calculations in the same kind of way than that used this entry in the same way to use and company doesn't take entropy into account and I wanna talk about entropy before we leave today on because it it's it's a huge budget us it was in this term right here this it's really going to go on 2 place everything we talk about on secure footing OK so I want to draw a very simple interaction and what I'm trying to draw up is an idealized hydrogen you can use electronic structure calculations phenomenally accurate electronic structure calculations to calculate out the ideal hydrogen bond and if we compare that and then I can draw the water molecule in a computer and get the energy out that say Well what's the energy of his 2 water molecules added together energy of this and I add that to the energy that and what do I find out about what I get out of making a hydrogen bomb water what I find here is that the energy for a spike in you can measure this accurately and get directly adult age is at that summit in Immokalee favorable 4 . 6 K cows from all this hydrogen bond is in publicly favorable relative to just 2 water molecules floating in infinite distance in space that's no good the problem here is this I'm not so good at math what's the difference between those numbers like 1 . 7 or something like that what this tells you is that a hydrogen bond between 2 water molecules is thermodynamic Lee unfavorable it is to unfavorable 1st form this hydrogen line this is the problem with entropy and this is why you have to exercise immense caution when you use and overvalued what is this equation release saying here but this equation is really saying why it should be minus the of so here's what this equation is really saying where I get these numbers I wasn't really asking about the energy difference here I was asking about the end of energy difference between this idealized hydrogen hydrogen-bond and all other possibilities and there are many many many ways to make a non ideal hydrogen make it maybe I could have some sort of distorted bond where the angle here doesn't look so good or maybe I could have used to water molecules poking at each other with the oxygen Armenia can have something of hydrogen bond with this molecule was vented some not so good angle the number of ways to make them not perfect hydrogen-bond than on ideal hiding among is so laments there are so many ways to make something other than the perfect hydrogen that this becomes the modern manically unfavorable what you find is if you take it up box full of zillions of water molecules that only 1 out here's what this number tells you that only 1 out of 16 pairs of water molecules will be in his ideal hydrogen bonds state all the others will be in some other appeared organizations state so if you got all computer you draw they standing you conclude that all that's what water looks like you come up at the wrong result because you didn't take into account interest this is the problem with entropy it is very hard to draw every other possible configuration into a computer and get the energy of you don't have the time to do that nobody has the time to do that and that's what makes interesting book 1 last I'm going to give you a number to keep in mind this is a useful number tell you how much certainly do you should have a whenever you ignore entropy the there's a very famous calculation of the was published by Bill Jenks a famous physical organic chemist and just took some numbers on a piece of paper and sat down and did some rough calculations and what he was able so to demonstrate got of 1 of the most highly cited papers in the Proceedings of National Academy of Sciences so and I did not intend to do that intended to draw the deals older product here so this is a 4 plus 2 cycle addition reaction the point here this is not who cares about the fact from covering was the point is if you have any reaction were 2 molecules have to get closer and a long Orient correctly .period ballroom dancers not wanting any bond just getting close to forming bonds pairing up 1 mole of a plus-one being so that everything is paired up costs of huge amount of energy and has nothing to do with them will be what you find here is that this is of a favorable reaction from the BU you could have guessed that by looking at the back and we
just traded 2 pipe bombs
CC pylons further for single bonds just that bond analysis would have to leave told you the "quotation mark it's good the problem is it's that bringing to cycle credit lines close together not forming bonds no binding yet and is getting them together and getting 6 . 0 2 times 10 to the 20 3rd of these pairs ready to form bonds with just the right distance is just the right orientations costs this tells you about how how much it can cost you just to bring a molecule of a and a molecule be together it can cost you up to 40
kcal per mole how many factors at Tannadice
favorability about its tender the 10 unfavourable just to bring pairs of molecules together so every time I see a reaction like methane was hydrogen peroxide were 2 things have to come together I'm taking this into account now it can be less than that sometime this is a reaction as tropical component because this carbon has to get close to this carbon at the same time with this carbon has to get close to that 1 that has an entire and tropical plants than 2 things is just simply have to collide with each other so entropy can cost you as little as 0 but as much as this so there's this huge uncertainty somewhere between 0 and 14 giggles from all if we ignore entropy when we think about 2 things coming together in Tropic cost for intramolecular molecules were 2 things don't have to collide that's a lot cheaper to and tropical OK so managers use has long memorize
along dissociation energies I want you to know how to calculate ample be changes for
reactions on if you get energies from electronic structure calculations remember that ignores entity could you didn't draw every other possible candidate in and finally don't forget the central component ,comma because entity can really kill you when you bring 2 things together
Biologisches Material
Single electron transfer
Alkoholisches Getränk
Chemische Bindung
Hybridisierung <Chemie>
Siliciumorganische Verbindungen
Elektron <Legierung>
Fülle <Speise>
Elektrolytische Dissoziation
Funktionelle Gruppe
Setzen <Verfahrenstechnik>
Gangart <Erzlagerstätte>
Computational chemistry
Infrastruktur <Histologie>
Internationaler Freiname
Sterische Hinderung
Energiearmes Lebensmittel
Organische Verbindungen
Organischer Halbleiter
Monomolekulare Reaktion
Organischer Stoff
Transformation <Genetik>
Konkrement <Innere Medizin>
Chemische Verbindungen
Chemische Struktur
Systemische Therapie <Pharmakologie>
Chemischer Prozess


Formale Metadaten

Titel Lecture 03. Molecular Orbital Theory (Pt. 3) & Energy Pt. 2
Serientitel Chem 201: Organic Reaction Mechanisms I
Teil 05
Anzahl der Teile 26
Autor Vranken, David Van
Lizenz CC-Namensnennung - Weitergabe unter gleichen Bedingungen 3.0 Unported:
Sie dürfen das Werk bzw. den Inhalt zu jedem legalen und nicht-kommerziellen Zweck nutzen, verändern und in unveränderter oder veränderter Form vervielfältigen, verbreiten und öffentlich zugänglich machen, sofern Sie den Namen des Autors/Rechteinhabers in der von ihm festgelegten Weise nennen und das Werk bzw. diesen Inhalt auch in veränderter Form nur unter den Bedingungen dieser Lizenz weitergeben.
DOI 10.5446/19221
Herausgeber University of California Irvine (UCI)
Erscheinungsjahr 2012
Sprache Englisch

Inhaltliche Metadaten

Fachgebiet Chemie
Abstract UCI Chem 201 Organic Reaction Mechanisms I (Fall 2012) Lec 03. Organic Reaction Mechanism -- Molecular Orbital Theory (Part 3) & Energy -- Part 2 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.

Zugehöriges Material

Ähnliche Filme